Category: Guides

What kind of solder (rosin cored, etc. lead-free)? What is flux and when is it necessary?

As a starting place, for most small electronics soldering, 1/32 inch (.03) rosin-cored, 60/40 (tin-lead) or 63/37 solder should work fine. Rosin-cored lead-free is fine, too. Unless you have reason otherwise, don’t use “no-clean” solder–it’s very likely that you don’t need to clean the regular rosin-cored solder. The solder should be thin enough to prevent accidentally applying too much (and causing a solder bridge), but thick enough so that more doesn’t have to be gathered from the coil too often.

Flux core in wire-solder

Besides affecting your feed-rate and convenience, the solder thickness also relates to the amount of flux that is delivered. Flux is basically a weak acid that removes oxides so that solder can adhere to the metal, and is so essential to the soldering process that it’s built into the core of common wire-solder. It also helps the solder spread out (reduces surface tension), transfer heat, and acts as a protective blanket to keep oxygen away from the metal until solder displaces it.

For the most part, manufacturers include a sufficient amount of flux in the wire, but if you use an extremely thin wire there may not be enough to clean the joint OR the iron tip. Consider using a thicker gauge for cleaning the tip periodically if you’re using especially thin solder. Liquid flux is helpful for SMD soldering, too.

When picking a wire-solder, there are 4 features to decide on: flux type and amount (% weight), alloy (tin-lead, lead free, silver bearing, etc.), thickness and total amount (1oz, 1lb?).

  • Flux:Just what is flux, what kinds are there, and when do I need liquid flux?
  • Alloy:60/40, 63/37, tin-lead, lead-free, silver bearing, RoHS, eutectic, oh my…
  • Thickness and Amount:As a general guide, .032″ thick solder (21 gauge) should be suitable for through hole soldering and some surface mount soldering. For finer pitch surface mount devices, use .02″ or .015″, and if you’re soldering a lot of switch terminals, or tinning thick gauge wire you may want .05″. If you use .015″ solder consider having some thicker solder on hand to re-tin your tip, since the amount of flux in .015″ may not be enough to remove tip oxides. The picture below shows how the various thicknesses compare next to the standard .1″ spaced DIP pins.Various solder thicknessesExpand to see how .032″ and .015″ solder compare to a SOIC surface mount chip and fine pitch (.02″) device.




    How much solder do I really need? An ounce? A pound? How long will a pound last?

  • Solder Fumes:What is exactly in solder fumes? Am I safer using lead-free solder?

All about screws

This guide is written for hobbyists, prototype builders or engineers looking for some basic information and intuition like:

  • “How large of a screw/bolt do I need?”
  • “What types of screws are out there and what are the for?”
  • “What are washers for, and do lock-washers work?”
  • “How tight should a screw be, and how does that affect how much load it can handle?”
  • “Coarse vs. Fine thread?”

* If it really matters (risk to property or bodily harm), hire a professional engineer; there could be errors in this guide.

Contents

Terminology and basic identification

What’s the difference between a bolt and a screw? Most sources (like the Machinery’s Handbook) define screws and bolts in terms of how they are installed: if you turn the head it’s a screw, if you turn a nut it’s a bolt. A hex head cap screw and hex bolt may look identical. This guide refers to both interchangably.





Wood, Sheet Metal and Drywall Screws – differences, uses, and head types

  • Wood Screws: These have a coarser pitch (few threads per inch) than sheet metal or machine screws, and often have an unthreaded shank. The threadless shank allows the top piece of wood to be pulled flush against the under piece without getting caught on the threads. Some wood screws are tapered from tip to head, also. This site lists pre-drill sizes for various sized screws. Wood screws vs. sheet metal screws
  • Sheet Metal Screws: Usually threaded all the way to their head, these will work in wood, but wood screws shouldn’t be used in metal (this is based on hardware store employee advice, not experimental evidence). Most of these screws are self-tapping in that they only require a pre-drilled hole (pre-drill sizes), but some come with self-drilling (shown in above pic) or self-tapping tips. Here’s a large list of different types of tips, the more common ones appear to be A, AB (pointed) and B (no point). Here’s a good guide to the different point types and uses. See more pics of thread cutting screws here. Even more good pictures of different head types. 2 Drywall and 1 self-drilling sheet metal screw
  • Drywall Screws: The coarse thread version is meant to secure drywall to wood while the fine thread version is for attachment to metal studs (commonly used in office construction). The head-to-shaft junction is more curved than in a wood screw to prevent tearing of the dry-wall. These can also come with self-drilling tips.

Common Head Types for the Above Screws (and 3 machine screws)


Screwheads commonly found on wood and sheet metal screws

  • Slotted, Phillips and Square drives: The main drawback of slotted heads is that power driven screw drivers easily cam out. Phillips heads address this problem to a certain extent, but these were actually designed to cause the bit to cam out at a certain point to prevent over-tightening. There have been revisions of the original Phillips head, most notably the patented Pozidriv, which does not have rounded internal corners and won’t cause the driving bit to pop out. The square or Robertson drive is least likely to cam out and transfers the greatest amount of torque. The wiki Screw page and this one describe some other less commong drives.
  • Round vs. Pan head: A pan head is successor to the round head, and is slightly flatter with greater thickness near its circumference than the round head. This supplies more surface area for the driving bit to grip over the round head, especially for slotted or flat drivers.
  • Carriage Bolts: These have a square shank that sinks into and grips wood when a nut is tightened.
  • Flat and Oval Head: The most common type of head for wood, these heads end up flush or below the surface of the wood when installed. An oval head is similar, except that it has a decorative rounded top that remains above the surface.
  • Security Heads (tamper proof): These screws have heads that are either impossible to reverse or require a special driver to operate. Some other types include the spanner (two small holes), tri-wing (used on the Wii), and torx or square drives with pins protruding up in the center of the socket. Some even have sacrificial heads that can be broken off after installing the screw. Here and here are several pictures of these screws.
  • Hex Washer Head and Truss head: These screws have a built in washer to help distribute load to a wider area. A truss head (not shown) is a flatter and wider than a typical round or pan head and serves the same purpose. These are commonly found on license plates.





This fastener guide gives a great pictorial overview of just about every fastener type, and here is a good overview of possible head types..

This talks about different screw types and their uses, also a bit about washers and rivets.

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Machine Screws

Machine screws are generally stronger than wood screws, have finer threads and are made more precisely. They’re used with nuts or tapped holes. Several examples are shown below. Several machine screws

  • Socket Screws: While many hex cap screws may be found in vehicles, socket head screws are becoming more popular and have some space saving advantages over hex cap screws. Socket heads take up less space themselves and don’t require side room for wrenches. They also are usually made from stronger alloy steel vs. hex cap screws, but this depends on the grade and manufacturer.
  • Allen / Hex Socket vs Torx: Most socket head screws accept a hex or Allen wrench (6 sided), but Torx heads (loosely, a 6 pointed star) are also available. Torx sockets were originaly designed to prevent the driver from camming out, and can transfer more torque than a Phillips or slotted driver. They don’t require large amounts of pressure to keep the bit in the socket. Some people say that it’s faster to insert a bit into a Torx screw than a hex socket, which would be advantageous in manufacturing environments.
  • Button Head: This head is largely decorative and somewhat similar to a round head, but flatter.
  • Flat head (counter sunk): These require a pre-drilled counter sink, and are typically angled at 82 degrees (Unified thread), which, by the way, is not the angle created by most drill bits (118 degrees). Metric flat heads have 90 degree angles.
  • Shoulder Screws: These have precision ground shanks that remain above the head of a hole and provide a simple way to make an axel for a wheel. They are also used when something must be secured, but not clamped.
  • Set Screws: These are threaded along their entire length and are typically used to secure a shaft from rotating. They’re used in pulleys, sprockets, collars and knobs among other things.

Here’s a great doc from tessco that talks about different screw types and their applications, grade and strength information, and screw material guidance.

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Thread Types: Most common types and when to use which


The most common thread types are the inch-based Unified coarse / fine (UNC/UNF) and metric coarse / fine. Other types and their purposes are described at the end of this section.

Coarse or Fine? To oversimplify, use a coarse thread unless you’re tapping into sheet metal. The differences are:

  • Coarse theads have fewer threads per inch than fine threads.
  • Coarse threads are more common, and more shops will have coarse taps.
  • Coarse threads are less likely to cross-thread, or jam because the screw is inserted at an angle. They’re also faster to install.
  • Screws with fine threads are slightly stronger. This is because the smaller fine threads take up less of the available area. See the loading charts below to see typical differences between the tensile strength of a fine vs coarse thread. A 1/4 UNF thread is about 14% stronger than its UNC counterpart.
  • Coarse threads are slightly stronger (against stripping) per length of engagement than finer threads (see thread strength section below). This may be surprising given the almost universal recommendation that fine threads be used in sheet metal and other thin-walled materials. If coarse threads are stronger and there is less than optimal thread engagement length available, wouldn’t it better to use the stronger threads?
  • Coarse threads are more tolerant to slight damage or corrosion than fine threads since they have more room for error.
  • Fine threads provide finer adjustment since they advance less per rotation than coarse threads.
  • The metric coarse thread is actually between UN coarse and fine thread, and the metric fine thread is finer than the UNF threads. Blake’s “What Every Engineer Should Know About Threaded Fasteners: Materials and Design,” recommends not using fine metric threads.





Thread specification – How threads are notated / designated:

example Unified thread designation:

1/4-20 UNC-2A

  • 1/4 – the nominal diameter, also the major / largest diameter
  • -20 – the number of threads per inch
  • UNC – UNC = Unified Coarse, UNF = Unified Fine. You may also see UNRC or UNRF. These refer to an external Unified Rounded thread (there is no internal rounded thread). UNRC’s and UNRF’s are interchangeable with their non-R counterparts. The only difference is that the vallies (roots) of external R threads have a mandatory rounded shape, whereas with the UNC and UNF threads the roundness is optional.
  • -2A – This represents the tolerance / fit of the thread. There are 6 common options, 1A, 2A, 3A, 1B, 2B, and 3B. A=external, B=internal. 1 is the loosest fit, 3 is the most precise and tightest fit with potentially zero clearance. If the tolerance isn’t specified, chances are it’s the more common 2A or 2B designation. 1 is hardly used, and only in cases where frequent re-assembly is needed or the threads need to work even with significant damage. Class 3 have slightly greater stripping resistance, and are common in the aerospace industry.

example ISO Metric thread designation:

M6 x 1 -4g6g or M6-6g

  • M6– M is for metric, 6 is the major diameter and nominal size in mm
  • x 1 – Pitch. Note that this is different from how Unified threads are specified. UN threads write the number of threads per inch after the nomimal size, whereas metric designations write 1 / threads_per_inch after the nominal size. If this is absent, coarse pitch is assumed.
  • -4g6g – This is the tolerance / fit class. The number refers to a manufacturing tolerance window, higher numbers are “sloppier.” The letter places that tolerance window relative to the ideal thread. Capital letters refer to internal threads, lowercase external. An h/H has the least amount of allowance–ie, there could be no clearance. g/G and lower represent more allowance. The two numberLetter pairs apply to pitch grade/tolerance and major diameter grade/tolerance for external threads, pitch and minor diameter for internal threads. When only one pair is present (as in M6-6g) it applies to both pitch and major/minor dia. 6g/6H is approximately equivalent to 2A/2B, 4h6h/4H5H is approximately equivalent to 3A/3B, although 4g6g/6H is usually used, which provides a little clearance over 3A/3B.

* a -LH at the end of either inch or metric threads indicates Left Handed threading.
* a (22) or other number at the end refers to the ANSI series of threads.

Some history and info about other threads:

In 1949, Canada, the United Kingdom, and the US agreed to a Unified thread that is largely the same as the American National thread that came before it, and screws from both systems are interchangeable. The new Unified system bascially added more manufacturing tolerances and tweaked some other ones. See ANSI/ASME B1.1 -1989 (R2001) for details.

Metric threads are specified in ANSI B1.13M-1982 (R1995), which is very nearly equivalent to the original ISO 68 specification.

Camera mount threads: These are generally a coarser old standard called the “Whitworth” 1/4 inch diameter and 20 threads per inch.

UNJ or MJ: These threads are used in situations where fasteners must withstand high fatigue stress, predominantely the aerospace industry. The basic difference between UNJ and UN is a larger root radius. Avoidance of sharp corners is critical for fatigue resistance. The root is given a large enough radius that it could potentially interfere with a typical UN internal thread, so there are both external and internal UNJ threads (and MJ’s). According to Blake, it is statistically highly likely that an external UNJ thread will fit a regular internal UN thread.

Standards: In general, the geometry is definied by ANSI, ASME and the ISO, while material strength properties are defined by ASTM, IFI, SAE and ISO.

Cut vs. Rolled Threads: This refers to how threads are manufactured. Rolled threads are stronger than cut/ground threads because they are strain hardened when they are made, and the internal grains of the metal are not cut. The Unified standard doesn’t demand that the roots (vallies) of external threads be rounded, but just about all fasteners less than 1″ come that way because their threads are rolled (see Blake book ref. above), and rolling produces rounded roots.

Constant pitch series: This refers to the many series of threads where the pitch does not increase with diameter. The 8-UN (8 threads / inch) series is apparently very popular above 1″ diameter fasteners. Typically these are used for adjustment devices and not fasteners.

Extra fine threads and minature screws: The Machinery’s Handbook has listings of dimensions for really small screws.

Power Screws and ACME Threads: Power screws’ job is to translate rotational motion into linear motion. Because of this, efficiency is a concern, and the 60 degree thread profile in standard fasteners is unsuited. The most efficient thread would be square with 90 degree angles, but this is difficult to manufacture, so the ACME thread is used (this has a 15.5 degree angle between root floor and tooth wall). Why is square more efficient? None of its force goes into pushing outward, whereas a 60 degree thread has a substantial force component away from the axial direction of the screw.

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Sizing and Strength: How big a screw is needed?


At first thought, sizing a screw for a given load would seem to be a simple matter. If you need to hold 100 lbs, find a screw that can hold 100 lbs before it yields…

But things are not so simple. If a screw can withstand 100 lbs of force before yielding, it is recommended for a number of reasons (discussed in the next section) that it be tightened to about 80 lbs of tension / clamping force just for installation alone. Does that mean that it can only withstand another 20 lbs of external load before it yields? Why would we tighten a screw so much if we’re using up the majority of its strength just to hold it in place? It turns out that only a portion of the external load is seen by the bolt, a rough estimate is about 1/3, but this depends on lots of things.

Here’s a rough guide for picking a screw or bolt for a given load:

Start off with the load that needs to be held in tension, call this F. If you have a shear (sideways) load, you should design so that friction or dowell pins will bear the load and not the bolt, but if this isn’t an option note that shear strength is 60% of tensile strength in many steels.

We’ll use a safety factor of 2.5, so the design load is now 2.5F. Now we need to select a screw with enough strength so that it can withstand the combined external load and pre-load from tightening. Assuming that 80% of the bolt’s proof strength is being used up in preload, that leaves 20% to handle 1/3 of the external load. Or in other words, we’re looking for a bolt where 60% of its proof strength is greater than the load.

Let’s try an example: What size grade 2 bolt is necessary to hold 100 lbs? The proof strength of Grade 2 bolts between .24 and .75 inches is 55 ksi (thousand pounds per square inch), and 60% of this is 34.2 ksi. So, we’re looking for a bolt with a tensile area greater than our load (2.5*100 lbs) divided by 33 ksi, or .0076 square inches. A #6 UNC should work. For perspective the diameter of a #6 screw is .138″, (1/8 = .125″). If this seems small, keep in mind that the ultimate strength (breaking strength) of a Grade 2 bolt is 74 ksi, so a #6 screw could theoretically hold 672 lbs in pure tension. If you’re wondering why bolts you see in cars and weight machines are so large, it’s partly to guard against loosening and fatigue failure in addition to safety factors.

What about changing loads? According to this Unbrako whitepaper on the Fastener Act, over 85% of failures are due to fatigue and not a simple overloading situation. Think about breaking a paper clip, which is easier: bending it back and forth or out-right pulling it apart? If you have an oscillating load and want a joint to last forever, the best advice we can offer is to multiply the anticipated load by 10 or more, and even this may not be sufficient. Steel can handle about half of its ultimate strength in an alternating load, but add in the pre-load stress and something called a “Stress Concentration Factor” due to the threads and the problem gets more complicated quickly. Here’s a good explanation of these effects along with a lot of other great screw information.

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Installation: How tight should a screw be? (very)


In the tables below, we use 80% of proof strength as an estimate for the amount of stress on a correctly tightened bolt just due to its installed clamping force, without any external loads. The Machinery’s handbook recommends 50-80% of ultimate tensile strength, or 75-90% of yield or proof strength (75% for reusable joints, 90% for permanent). Some screws are even tightened to yield. It would seem that this level of installation tightness leaves very little strength left over to handle external loads… why do this?

  • The tighter the screw, the more friction there is to resist loosening.
  • Screws are very likely to come loose if any relative motion occurs between the threads. Static friction is substantially higher than sliding friction, so once movement in one direction starts, it becomes much easier for un-screwing motion to happen, also. A high clamping force creates more friction between the clamped surfaces and therefore reduces the chances they will slide relative to each other.
  • As a followup, bolts are less strong against shear (sideways) loads, so more clamping force and friction helps reduce these loads, also.
  • Fatigue loading:This is possibly the most common reason sited for extremely high initial tightness. As derived above, as long as the external load doesn’t cause separation in the joint, only a portion of it is actually seen by the bolt. This is especially helpful in cases with alternating loads because the lifespan depends heavily on the magnitude of the changing load, not just its average. In many cases it’s better to have a higher average stress and lower alternating load, and this is the tradeoff that a high pre-load supplies.One subtle point is that the percentage of the load seen by the bolt only depends on the relative stiffness of the joint and bolt, not on the amount of preload. As long as joint separation does not occur, additional tightening preload does not buy any additional fatigue protection. Large intial preloads still pretect against joint separation, but unless you know otherwise, it’s probably not worthwhile to tighten right up near yield.
  • A tighter initial clamping force can actually increases the joint stiffness slightly by flattening somewhat uneven joint surfaces. If only a small pre-load is applied, the joint members might be resting on hills on their surfaces which could get worn down or compressed over time.





How to achieve the required tightness

According to the Machinery’s handbook, tightening by feel is only +-35% accurate, and using a torque wrench only improves the accuracy to about +-25%. These uncertainties are massive, and give good reason not to tighten too close to yield, or too loose, and also to design a joint so that it will still work with a huge span of possible pre-loads. A method called “turn-of-nut” can supposedly get within +-10%, but this relies heavily on a reliable starting point from which to start counting turns (see Machinery’s handbook).

If the application is critical (and you are not relying on this guide), ultrasonic sensing of the bolt length or drilling a hole down the middle and attaching strain gages will achieve much higher accuracy. Friends in the navy have told us that workers will sometimes heat a very large bolt during installation to utilize its cooling stretch in achieving proper preload.

To add two more variables, additional torsional loads are present during installation, although these usually dissipate shortly afterwords. It can also be assumed that, due to a variety of factors (surface smoothness, uneven loads, thread deformation), some 10% of preload will be lost.

Why is a torque wrench so imprecise? Friction. Some 80+% of applied torque goes into defeating friction, leaving little for actually stretching the bolt. What’s worse is that this friction is highly unpredictable and depends heavily on the materials involved and any lubrication that may be present. Most fasteners have a small amount of oil present from the manufacturer to prevent rust.

From the machinery’s handbook, the following equation can be used to approximate the required torque for a given preload:

Torque = K x preload x nominal_diameter

where K is a friction constant that depends on material and lubrication

material K
mild-steel, 1/4 – 1 inch dia. .2
nonplated black finish .3
zinc-plated .2
lubricated .18
cadmium-plated .16

Just to get a flavor, using K=.2 and 80% of 120 ksi proof strength for a SAE Grade 8 1/4 bolt, the above formula gives a seating torque of about 150 in-lbs, or about 13 lbs at the end of a foot long wrench.

Another method recommended by the Machinery’s Handbook is to measure the torque necessary to break a test bolt, and then use 50-60% of this value. This will supposedly be 60-70% of yield.

Pencom offers a chart with recommended torques for various grades and sizes of screws. Another chart from the Elgin fastener Group. Joseph has a great page about bolt tension and even did his own experiment to verify the variability of tightening by hand.

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Grade and Strength Information


There are numerous standards that fasteners are manufactured to, and those standards describe everything from material chemistry to surface finish to heat treatment. The most relevant numbers are “Proof Stress,” “Yield Stress” and “Tensile / Ultimate Stress.” Tensile Strength is how much stress the material can withstand before finally ripping apart. Yield Stress is the amount of stress that a material can undergo before permanently stretching. Proof stress is similar to Yield stress except that it is slightly less (about 90%), and only applies to fasteners. The thread geometry causes them to yield slightly before the Yield stress level of the material, so Proof Stress can be thought of the true yield–in other words, the fastener will behave like a spring below that stress level.

So which of these numbers should be used? While there are many arguments for tightening a screw past its yield point (for instance), from this author’s viewpoint, if an external load yields a screw, and if that load is ever removed, the screw will now be permanently stretched and loose. Therefore, we recommend designing so that the combined internal and external loads stay below the proof stress to avoid any possibility of yielding. If proof stress is unknown, 85% of Yield stress can be used as an approximation. The ultimate or tensile stress is sometimes designed to, but we do not know when this acceptable or not. Also, the ultimate stress is used in designing joints for alternating loads, but this is beyond our scope.

Several organizations publish standards for fasteners. For inch/english, this includes SAE, ASTM, ANSI, ASME and others, although the most commonly used are the SAE “Grades.”(standard J429). The most common metric specifications are published by the ISO. (ANSI metric specs agree with ISO for all practical purposes–Machinery’s Handbook)





Common Inch / Imperial SAE Grades: (all values in ksi or 1000 lbs / square inch)

Head
Marking		 Grade Diameter (in) Proof Strength Yield Strength Tensile (Ultimate)
Strength
SAE grade 2 bolt marking 2 1/4 to 3/4 55 57 74
3/4 to 1-1/2 33 36 60
SAE grade 5 bolt marking 5 1/4 to 1 85 92 120
1 to 1-1/2 74 81 105
SAE grade 8 bolt marking 8 1/4 to 1-1/2 120 130 150

Socket Head Cap Screws made from alloy steel are typically manufactured to a higher strength than SAE Grade 8: 180 ksi tensile strength for fasteners up to 1/2 inch, 170 ksi for larger sizes (ASTM A574, p. G-34).

For many more head markings and their corresponding specifications, see here.

Metric ISO Marking

Metric fasteners are marked with two numbers separated by a decimal point, like 10.9. The 10 is 1/100 of tensile strength in MPa, and the .9 represents the ratio of yield to tensile strength. So 10.9 represents a tensile strength of 1000 MPa and yield of 900 MPa. Some strengths are stronger than this method shows, see table 10 on this page. Other references for this table: here and here.

Grade size range proof
strength (MPa)		 approx yield
strength (MPa)
grade dec x tensile*		 tensile
strength (MPa)		 approx equiv.
to SAE grade:
4.8 M1.6-M16 310 336 420 SAE 2
8.8 < M16 580 640 800 SAE 5
M16-M76 600 660 830
10.9 > M5 830 940 1040 SAE 8
12.9 M1.6-M100 970 1100 1220 ASTM-A574
alloy socket
screws

*these value aren’t necessarily from the standards, they’re calculated as described above.

Tensile stress areas and acceptable load estimates for various grades

For applications where there is any chance of bodily or property harm, don’t rely on our external load estimates–they are intended to give a rough approximation of what screws of various grades can hold in non-critical applications, and are based on the following assumptions:

 

  • We use the proof strength as the maximum stress that should be endured from the combined internal (original tightening) and external loads.
  • If proof load isn’t specified in the above tables, we use 85% of yield
  • It is assumed that the joint is twice as stiff as the bolt, which implies that 1/3 of the external load is seen by the bolt, and the other 2/3 goes into reducing clamping load. The forumla explained above and used below is 60% * proof * tensile area / 1.0 (safety factor). We recommend using a 2.5 safety factor for non-critical / costly applications–ie, divide the numbers below by 2.5. For joints clamping aluminum, plastic, gaskets or other softer material it’s safer to assume that 100% of external load is seen by the fastener (multiply by 20% instead of 60%).
  • tensile stress area:Tests have shown that the average of the minor and pitch diameters approximates the effective area of a fastener. The Machinery’s handbook has a different formula for bolts with tensile strengths over 100ksi, but due to some doubt about its origins, we don’t use it.
  • As far as we can tell, SAE Grades apply only to bolts at least 1/4″ in diameter. Any unmarked machine screws smaller than that are probably Grade 2; we show the higher Grades for reference only on those sizes. Alloy steel socket head cap screws will most likely have a greater strength than SAE Grade 8 unless their manufacturer says otherwise.
  • We assume shear loads and torsional loads from tightening are zero.
  • For alloy socket screws, yield strength is 180 ksi until 1/2″ and 170 ksi for larger diameters. We use 85% of these values to approximate proof strength.

Inch tensile areas and loads (in lbs), both fine and coarse thread

size –
threads / in		 dec. major
diameter (in)		 tensile
stress area
square inches		 Grade 2
(proof strength:
<=3/4″: 55 ksi
>3/4″: 33 ksi)		 Grade 5
(proof strength:
85 ksi)		 Grade 8
(proof strength:
120 ksi)		 alloy socket
head (ASTM A574)
<=1/2″: 153 ksi
>1/2″: 144.5 ksi
#0-80 .0600 .00180 59.4 91.8 129.6 165.24
#2-56 .086 .00370 122.1 188.7 266.4 339.66
#2-64 .086 .00394 130.02 200.94 283.68 361.692
#4-40 .112 .00604 199.32 308.04 434.88 554.472
#4-48 .112 .00661 218.13 337.11 475.92 606.798
#6-32 .138 .00909 299.97 463.59 654.48 834.462
#6-40 .138 .01015 334.95 517.65 730.8 931.77
#8-32 .164 .0140 462 714 1008 1285.2
#8-36 .164 .01474 486.42 751.74 1061.28 1353.132
#10-24 .190 .0175 577.5 892.5 1260 1606.5
#10-32 .190 .0200 660 1020 1440 1836
1/4-20 .250 .0318 1049.4 1621.8 2289.6 2919.24
1/4-28 .250 .0364 1201.2 1856.4 2620.8 3341.52
5/16-18 .3125 .0524 1729.2 2672.4 3772.8 4810.32
5/16-24 .3125 .0580 1914 2958 4176 5324.4
3/8-16 .375 .0775 2557.5 3952.5 5580 7114.5
3/8-24 .375 .0878 2897.4 4477.8 6321.6 8060.04
7/16-14 .4375 .1063 3507.9 5421.3 7653.6 9758.34
7/16-20 .4375 .1187 3917.1 6053.7 8546.4 10896.66
1/2-13 .5 .1419 4682.7 7236.9 10216.8 13026.42
1/2-20 .5 .1599 5276.7 8154.9 11512.8 14678.82
9/16-12 .5625 .182 6006 9282 13104 15779.4
9/16-18 .5625 .203 6699 10353 14616 17600.1
5/8-11 .625 .226 7458 11526 16272 19594.2
5/8-18 .625 .256 8448 13056 18432 22195.2
3/4-10 .75 .334 6613.2 17034 24048 28957.8
3/4-16 .75 .373 7385.4 19023 26856 32339.1
7/8-9 .875 .462 9147.6 23562 33264 40055.4
7/8-14 .875 .509 10078.2 25959 36648 44130.3
1-8 1.0 .606 11998.8 30906 43632 52540.2
1-12 1.0 .663 13127.4 33813 47736 57482.1

alternative load carrying recommendations: here.

Metric coarse thread tensile stress areas and estimated loads (in N)

Fine pitch information and more can be found here. Formula for tensile stress area: pi/4* (Nominal_Diameter-.938194*pitch)^2

Formula for load: 60% * tensile area * proof stress / (safety_factor = 1.0)

size x
pitch		 tensile
stress area
square mm		 Grade 4.8
(proof strength:
M1.6-M16: 310 MPa)		 Grade 8.8
(proof strength:
< M16: 580 MPa
>= M16: 600 Mpa)		 Grade 10.9
(proof strength:
> M5: 120 MPa)		 Grade 12.9
(proof strength:
970 MPa)
2x.4 2.0732 386 N 721 N n/a 1207 N
2.5x.45 3.3908 631 1180 n/a 1973
3x.5 5.0308 936 1751 n/a 2928
4x.7 8.7787 1633 3055 n/a 5109
5x.8 14.183 2638 4936 n/a 8255
6×1 20.123 3743 7003 10021 N 11712
8×1.25 36.609 6809 12740 18231 21306
10×1.5 57.99 10786 20181 28879 33750
12×1.75 84.267 15674 29325 41965 49043
16×2 156.67 29141 56401 78022 91182
20×2.5 244.79 n/a 88124 121905 142468
24×3 352.5 n/a 126900 175545 205155

Nut and tapped hole strength – How much thread engagement is needed?

If a screw / bolt fails because the threads strip, it can be hard to detect both during installation and later because the threads will still have some grip on the screw. If the bolt breaks, however, it will be completely loose, be easy to detect and remove, and usually fail during installation when additional torsional loads are present (torsional loads usually dissipate within minutes after tightening if you’re wondering why we didn’t take them into account before). Because of this, fasteners are designed to fail in the bolt, not the threads, so most nuts are more than adequate–just make sure you use a similar grade of nut compared to the screw.

How much thread engagement is needed in a tapped hole, then? According to “Fundamentals of Machine Component Design”, 3rd addition, by Juvinall and Marshek, p. 413, if the bolt and nut are of similar material the thread stripping stength will equal the bolt tensile strenth when the nut is .47 * diameter. Standard nuts are 7/8 of a diameter, for comparison.

Interestingly, more than a third of the load is held by the first thread in a nut according to this. As the bolt tightens, its threads stretch and the nut’s threads compress, which reduces force on the far threads.

The .47*dia calculation above takes this imbalance into account, but it will certainly be different for other material combinations. This offers some formula (also found in machinery’s handbook) for calculating the shear area of threads, but it’s uncertain how one would apply that formula given the imbalanced thread load. The Machinery’s handbook suggests at least 3 threads of engagement. We recommend 1 diameter depth for steel and 1.5-2 diameters for aluminum. The referenced formulas may at least provide a rough estimate for sheet metal, where thread engagement is limited. Unbrako’s Engineering guide has several charts showing experimental testing of various sized holes. According to their guide, formulas have performed poorly at predicting thread strength.

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Washers – what are they for? Do lock washers really lock? Do lock nuts? + other locking methods


Washers help distribute load and prevent the screw head from digging into the joint material. If the surface of the joint isn’t smooth, it’s more likely that the screw will compress higher spots over time and come loose. Also, if the surface is damaged by the screw or nut, it can lead to problems with future re-installation. It’s important to use a washer that’s hard enough for the given screw. For instance, be sure to use a hardened washer for high-strength screws and bolts (Grade 8 and socket head cap screws).

Locking Devices: There are several different types that ostensibly help to keep a joint from loosening. Their effectiveness depends on the application and is somewhat debateable.

 

Split lock washer, Internal tooth lock washer, Fender washer

  • Split lock washer: These have two features that supposedly prevent loosening: a spring action and an edge that digs into the screw on reversal. According to this threadit seems that everyone from NASA to British defense to the US navy thinks split lock washers are useless. Some of the rationale include the fact that the spring force of the washer is only around 5% of the force from the stretched bolt, and that the edge cannot dig into anything when it flattens out. Yet, millions of these washers are used every year, so one would think they’re not completely useless.According to the “Handbook of Bolts and Bolted Joints,” by Bickford, p. 243, the lock washer undergoes additional deformation after it flattens with a spring rate more comparable to that of the bolt. This extra springyness is helpful for preventing fatigue failure, but it’s unlikely that it helps prevent loosening.These washers are probably most effective in joints where the recommended tightness cannot be achieved, such as soft metal, plastic or wood joints. In these cases, the washer would likely not be entirely flat and would indeed dig into the screw surfaces.
  • Toothed washers: These have small teeth that dig into adjacent screw and joint material. The consensus seems to be that these are more effective than split lock washers, but will (and must) cause damage to adjacent surfaces, which may affect repeated installation.
  • Belleville washers: These are cone shaped (not shown) washers are used more as a precision spring than a locking device. They can be stacked to increase their combined spring rate (see the wiki). Their spring rates are substantially higher than split lock washers. They may provide some protention in high vibration or temperature changes. Wavy washers have a similar purpose.
  • Sems: These are screws with freely spinning captured washers–washers that are permanently attached. See some pictures here.
  • Fender washers: These have a much wider outer diameter than typical washers and are useful on softer materials.
  • Loctite: This is actually the prefered method for securing screws against vibration. Loctite is like a glue that hardens when oxygen is removed. The most common type can be removed by heating the joint.
  • Castle Nuts: These have slots that accept a cotter pin that goes through a drilled hole in the bolt.
  • Lock Wires: Bolts heads with holes are strung together with wire so that they cannot turn relative to each other. Used for vibration resistance and as an anti-tampering device.
  • Lock Nuts: The most common type is a nut with a nylon insert. These are very effective (certainly more so than lock-washers), but may not work for reassembly. There are also nuts called “prevailing-torque” locknuts. These have warped threads or tapered features that apply increased friction on the bolt.

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Material Selection


There’s a vast amount of different materials, platings and coatings to choose from. This talks about many of these options, we’ll focus on some of the more common ones here. The material selection is influenced by required strength, temperature, resistance to corrorsion, joint materials, and cost.

 

Zinc plated, Stainless steel, Galvanized, Black, Oxide Screws

  • Zinc plating: Because of steel’s tendency to rust, you’ll never get bare steel fasteners. The most common covering is zinc, but this won’t stand up to outdoor conditions.
  • Black Oxide: Most common on socket head cap screws and other machines screws. This provides very mild protection against corrosion and usually has an oil film added for additional protection.
  • Hot-Dipped Galvanized: For outdoor use, this provides the best (common) protection next to stainless steel.
  • Stainless Steel and aluminum: These materials are inherently resistant to corrosion because they form a tough oxide layer when exposed to oxygen. Note that the strength of stainless is much less than alloy steel, and even less so for aluminum.
  • Galvanic Corrosion: When placed in contact along with an electrolite (like humid air), certain metals form little batteries that corrode each other. It happens when the metals are substantially electrochemically different from each other. For instance, brass and zinc plating wouldn’t be a good choice. More informaion.

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Additional Resources and References

  • “Unbrako’s Engineering Guide“
  • “ref 1” : “Fundamentals of Machine Component Design,” by Robert C. Juvinall, Kurt M. Marchek
  • The Machinery’s Handbook, 27th ed.
  • “Handbook of Bolts and Bolted Joints,” by John Herbert Bickford, Sayed Nassar
  • “What Every Engineer Should Know About Threaded Fasteners: Materials and Design,” by Alexander Blake
  • Here’s a great doc from tessco that talks about different screw types and their applications, grade and strength information, and screw material guidance.
  • A massive guide about material selection with links at the bottom to just about everything else you’d want to know about screws, including dimensions, installation torques, heat treatment, thread creation, etc.
  • An interesting article claiming that some bolts are not meant to be tightened to proof load:
  • A substatial glossary of bolt terms.

contents ^

Comments:

flag
Dec 10 2007, admin said:

Feedback and corrections are greatly appreciated.

Sep 09 2008, D Henderson (unregistered) said:

A very useful page, thankyou.

Jan 03 2009, Mark G. (unregistered) said:

This is a very well written article and I have some information I thought might be helpful. The SAE grade marking for number one is the same as for number two. If you use stainless hardware (bolts, nuts, washers) they will not rust for an extremely long time, even in a salt water environment, unless if they are in contact with carbon steel. Then they will rust. If you have to do any cutting or filing of stainless hardware, clean it with a non ferrous wire wheel or brush after the work is done. There can be enough metal transfer from a file to your stainless piece to cause rust to star. Galvanized hardware is a good alternative to stainless but be aware that the process of making something galvanized requires that it is dipped in a molten metal vat (hot dipping). The coating can increase the size the item in thousandth. Also the galvanized coating on a bolt can make the threads thicker causing binding and seizing of the nut. I have snapped many bolts because of galvanizing. I you need a item to be corrosion resistant and you do not want to buy stainless or galvanized components you can use buy a product called Cold-Galvo or Cold Galvanizing spray. It has many of the same proprieties as hot dipped hardware. Though not as effective as hot dipped, it is easy to apply and dries fast. One note of caution about Cold-Galvo and other such sprays; lots of them contain highly dangerous materials such as lead, be very careful when using them and have the necessary respiratory protection. My thoughts about lock washer; they are fine for use in low to no vibration applications. I have worked on many of an engine and found many high grade lock washers snapped in half. Three things; if you are going to use a lock washer but do not want the material to be damaged put a flat washer between the lock washer and the material being secured. Second, if you want an alternative to the lock washer try using lock tight. It is anaerobic drying glue. It will only dry between the threads of the nut and bolt and it is very strong. Lastly, if you need to have a locking nut and bolt use a lock nut instead. They cost a little bit more but are well worth it. My preference is a nylon lock nut. It has a nylon insert that locks the bolt. The other kind that I know of is a pinched lock nut (I don’t know if that is the right name for it), I have only seen these in aluminum but it looks like a standard nut that is slightly squished, the aircraft industry used them a lot. A note on lock nuts; once a lock nut is used and then loosened, the locking ability is reduced. On last thought about nuts and bolts; it whatever you are doing will need to be undone at some future point and time consider applying an anti-seize compound to the threads of the bolt. Your local auto parts store has tubes of anti-seize but it is of a low grade and works only so-so, but the tubes only costs from $3 to $5. If you can afford it buy an industrial grade. A bottle of industrial grade anti-seize about the size of a bottle of Tylenol can cost you up words of $45, but a little goes a very, very long way. Anti-seize has a melting temperature of 1400 F to 1800 F so it is great for engine work. Most of the anti-seize compounds I have seen are copper or graphite based. I hope my ranting has helped.

Jan 03 2009, scott (admin) said:

Awesome info Mark! That’s interesting about steel shavings enabling rust to start from filing a SS screw.

I’ve heard that an easy way to loosen loctite is to hit the bolt with a flame for a minute. I think they sell high-temp loctite, too, though.

One other “locking nut” I’ve read about is one where the threads are slightly off-pitch, or maybe just the last thread… you end up with definite yielding. If things are tightened correctly, though, this happens a little bit in the last couple threads of a normal nut anyway.

References and links to other good guides and resources

Desoldering

The three most common inexpensive ways to remove solder include a “solder sucker”, solder wick, and an iron with an attached desoldering bulb.

Desoldering tools

 

 

Surface mount desoldering: Surface mount chips are especially hard to desolder because it’s very difficult to completely remove all the solder pin by pin, and avoid overheating the board and lifting a pad. Professional shops use expensive hot air guns or special tips (shown below) to heat all the joints at once.

Desoldering a SOIC

There are fortunately a few cheap ways to desolder surface mount chips.

  • ChipQuik provides an interesting solder that when melted over existing joints produces a new low-melting point alloy (under 200 °F) with a much longer solidification time. The longer solidification time enables you to melt all the joints at once and then flick off the chip.
  • There are many guides for making a DIY hot air gun with the RadioShack® desoldering iron and a fish tank air pump. Engadget has one of the better how-to guides. Here and here are some more plus the supposed original.

Protection

You may want to add what’s called a “conformal coating” to the connections to keep dust and moisture away. This is basically a clear coating that conforms to the surface of your parts, and there are many different kinds. In some military applications, they actually embed circuits in a solid chunk of epoxy or silicone to safeguard them. Note that if the circuit needs to be repaired, these coatings can be very difficult to remove.

Various encapsulation materials

Clean Up

Most manufacturers will clean off residues from any flux that isn’t labeled “no-clean,” despite flux datasheets like Kester’s that say even some of the more active fluxes do not need to be cleaned. For short life-span hobby projects, it probably doesn’t matter unless you’re using a solder/flux labeled “organic” or “water-soluble”–these fluxes leave behind very aggressive acids that will quickly eat away circuits. Cleaning may be necessary if you’re applying a protective coating that won’t adhere to flux residues. Finally, some rosin residues are tacky and may attract dust that can short a circuit.

The fact that a flux is made from rosin doesn’t tell you much about how strong it is or whether it should be cleaned. What matters is how concentrated the mix is and how much acidic (halides) activators were added. RadioShack® doesn’t supply any information on the flux in their standard rosin-cored solder, but it’s probably weak enough that the residues do not need to be cleaned off.

Isopropyl alcohol works decently on rosin-based residues, but clean shortly after soldering because the residues quickly harden. Use water for water-soluble fluxes. This pump containing bottle dispenses a little alcohol when you push down on the top with a brush, and keeps the rest from evaporating. If you are going to clean, make sure you wipe up the remnants with a lint-free cloth–don’t just spread them around the board with a brush and alcohol.

Cleaning flux residues

Cleaning with acid brush

Heat and Solder the Joint

  • Heat the joint: Place the iron tip so that it touches both the component lead and pad–the goal is to get as much surface area contact between the iron tip and joint as possible. Almost no heat will travel through the point.

    Tip placement

  • Make heat bridge: Add a small amount of solder between the tip and the work–heat transfers much faster through the liquid solder than dry surface contact. This is why a tip that won’t “wet” is so difficult to use. Pressing hard should not be necessary. This step may not be necessary if there’s enough solder already on the tip from tinning it after cleaning.

    Solder heat bridge

  • Apply solder to opposite side: Apply solder to the parts, not the iron. By doing this, you ensure the parts are hot enough for the solder to “wet” and bond with them. Also, solder will run towards the heat source, so applying solder opposite from the iron helps to spread it out and cover the joint.

    For larger joints, rather than dumping in all the solder quickly, continuously pulse in small amounts to keep a fresh supply of active flux available.

    Add solder opposite to iron

  • Time: The joint should take about 2-5 seconds total time for standard 60/40, 63/37 lead based solder and a non- no-clean flux, and up to 7 seconds for lead-free solder. Lead-free solder just takes longer to “wet” the metal.

     

    In general, the goal is to make the joint as quickly as possible. Longer times can char and damage the board, lift pads, overheat components, burn off and polymerize flux (making it harder to remove), and finally lead to a more brittle joint. Solder doesn’t just freeze on a joint, tin in the solder dissolves and chemically reacts with copper in the connection to form a new bonding material, called an “intermetallic layer”. While this layer is what makes an excellent thermal and electrical bond, it is also extremely brittle; a doubling of its thickness reduces joint tensile strength by half (ref 1). Since this layer grows faster with higher temperatures, joints should be made using the coolest temperature and shortest soldering time possible. This layer is also why re-heating joints has been shown to weaken them. Having said all this, I have to admit that I don’t know just how long is too long for projects that don’t need to operate for 30 years with 100% reliability. After 10 seconds there’s a good chance the flux has been used up.

  • Remove solder, then iron: Pull the iron out fairly quickly to avoid leaving a solder spike.

    Remove solder, then iron

  • Good and bad joint gallery:The solder should smoothly ramp to meet surfaces and be shiny in appearance if it’s lead-based. Lead-free solder will have a duller and grainier surface, but will still be a good joint as long as there are no signs of non-wetting. The important thing to look for is any solder that looks like it didn’t cling to a surface, or is just sitting on top or next to a surface.Gallery of joints:

    A NASA gallery of every possible joint / board defect you could ever imagine. Here’s another great gallery of defects.

    Great comparison pictures between lead-based and lead-free joints. p. 34

Clean and tin the Tip

Oxidized tip

  • Regular cleaning = easier soldering and longer tip life:The iron tip’s ability to transfer heat is drastically reduced when it gets covered in oxides and burnt flux residues. Not only does heat not transfer as well through this debris, but the contaminants also prevent solder from wetting or sticking to the tip. Most heat transfer actually goes through a fluid solder “heat bridge” that lies between the iron tip and components, so an iron tip that repels solder will be very ineffective.The longer oxides and charcoaled flux residues remain on the tip, the harder they become to remove, so it’s a good idea to clean the tip every time you pick up the iron.damp sponge with hole Wiping the iron on an edge of a hole cut into a sponge can help to remove oxides easier, and also allows waste to fall away. A dry cleaner can also be used. It consists of soft metal shavings that are coated with flux. You clean by thrusting the iron into the shaving a few times. By avoiding the thermal shock of touching a damp sponge, these cleaners help to increase tip life, and in our opinion, do a better, faster job.

    cleaning with dry gold curls

    Usually touching the tip with rosin-cored solder will supply enough flux so that oxides can be removed with a damp sponge. If this isn’t sufficient, you can purchase “tip tinners and cleaners” that are a mixture of solder paste and flux. The flux is oftentimes stronger (more activated) to help remove oxides.

    tip tinner/cleaner

    Finally, when that doesn’t work, special polishing bars to can be used to salvage extremely bad tips. Another last resort is to gently rub the oxides off with an emery cloth or soft steel brush. Cover the tip immediately with solder after cleaning to prevent further oxidation. Never file the tip to clean it or form a different shape. The tips are mostly copper with a protective iron plating, and once that plating is pierced, the tip will die quickly. Copper is used because it’s an excellent heat conductor, but if exposed to solder, it will quickly dissolve into the solder.

    Weller polishing bar

    Tinning the tip

    Tin the tip: Add a small amount of solder back onto the tip. This helps to protect the newly cleaned and exposed tip, and also helps to transfer heat to components.

Prepare the Work

Corroded pin repels solder

    • Start with clean components: Flux can remove small amounts of oxides, but will be of little help for heavy oxidation, grease, oil or dirt. Notice how the solder in the adjacent picture has been repelled by the heavily oxidized pin. It may be necessary to lightly use steel wool or fine grit sand paper to remove especially bad oxides. Some people say that you should not do this because it creates scratches that can promote future oxidation… sand at your own risk. Use Silicon Carbide sandpaper (black) as opposed to Garnet (brown, for woodworking) sandpaper because the Garnet paper will shatter and become embedded in the metal. An effective and gentle alternative is to use a pink eraser, especially for copper traces.
    • Clamp your work:PanaVise makes a popular clamp that accepts several different attachments for holding different sized circuit boards. It’s by far the most popular clamp and is also very sturdy. Having the work held in place is especially helpful for desoldering when it’s necessary to push or pull a bit. The alligator hands are a cheap alternative.
      Serveral types of clamps

 

    • Wire preparation:Tin stranded wires so they don’t “bird-cage,” or bend out from their original lay. Expand for instructions on the correct way to strip a wire manually, use an automatic stripper, and tin wire. 

       

      Examples os tinning and stripping wire

    • Insert, clinch and trim components: First, make bends beforeinserting the components. Avoid stressing the connection between lead and component by bracing the lead with pliers while bending. Pliers with serrated tips aren’t used in high-reliability production because the grooves can create nicks in the leads that eventually cause a break after a lot of vibration and thermal changes. Round nose pliers make it easy to make any sized radius.Unless the component has a metal casing or needs clearance for air flow to keep cool, insert it until it’s flush with the board. This doesn’t apply to some transistors, and also capacitors that have plastic coverings that need to be kept out of the solder joint. Clinch or bend out the leads so the component is held in place during soldering, and finally trim the leads to about the radius of the pad. Trim the leads before soldering since doing so afterwards can shock and crack the joint. Wearing safety glasses for this process is not at all excessive–those leads can get you. Everything else about proper component installation: NASA guide.

      Insert, clinch and trim leads

 

  • Add heat sink: Some semi-conductors (some transistors and diodes) are especially heat sensitive. This clip acts a heat shunt to keep the transistor protected.

    Heat sink on transistor

Select a Soldering Iron

A 25 or 30 Watt iron should suffice for most small electronics work.

 

    1. Most soldering “guns” are vastly overpowered for electronics soldering and can easily overheat components or expose them to harmful voltages. However, some people cleverly use them to solder multiple leads on surface mount devices. Soldering “guns” are for plumbing and much heavier duty applications, and are usually over 100 Watts. The “guns” work by passing high currents through the tips, and these currents can generate voltages that damage electronic components. Also, magnetic fields from guns with transformers can damage some electronics.By forming the heating element in the shape of of the chip, a soldering gun can be used to heat many leads simultaneously.

 

    1. How much wattage do you need for a particular application and how does wattage relate to tip temperature?

 

A loose analogy: Imagine a car tire has a leak, but you’re trying to keep it inflated by pumping air into the tire at the same time it’s escaping out the leak. The bigger the leak, the more air you have to pump into it to keep the pressure up. If the tire pressure represents tip temperature and the air lost through the leak represents heat lost through the tip, then wattage represents the maximum amount of air your pump could supply. Once more air escapes through the leak than your pump can replace, the tire pressure (or tip temperature) starts to drop.

 

If you had a very small leak and a huge pump (say a 100 Watt iron equivalent), you might be afraid that the pump would cause the tire to explode since so much more air is going in and so little going out. But if you have a nozzle to regulate the pump’s air, you could only allow just the right amount of air in to replace what’s lost through the leak. This is how “temperature controlled” soldering irons work. As long as you aren’t losing more heat out of the tip than the iron can replace (up to its rated wattage), it will automatically regulate just the right amount of heat into the tip to maintain the same temperature.

 



However, typical plug-in irons have no such regulation. A 15 Watt iron always delivers 15 Watts of heat to the tip, and the tip temperature stops increasing only when 15 Watts of heat escape through the air. When the tip touches a part, its temperature drops, and if the part you’re soldering can dissipate enough heat, the temperature will keep dropping until it won’t melt solder any more. After the iron is pulled away from the joint, the temperature will climb again. There is some amount of natural regulation: as the tip gets hotter, it dissipates more heat, and as it gets cooler, it dissipates less.

 

Usually, the bigger the component the more heat it can absorb and dissipate, so the general rule is that you need more wattage for larger parts. If you’re just soldering small resistors and ICs, 15 Watts will probably suffice, but you may have to wait a bit in between joints for the tip to recover. If you’re soldering larger components, especially ones with heat sinks (like voltage regulators), or doing a lot of soldering, you’ll probably want a 25 or 30 Watt iron. For soldering larger things like 10 gauge copper wire, motor casings, or large heat sinks, you may need upwards of a 50 Watt iron or more. The following video shows what happens to tip temperature as 15, 25, and 40 Watt irons solder various sizes of wires and components. For cheap irons, higher wattage does indeed mean higher temperatures!

 

  1. What Watts, What? A short article about how much wattage is needed. From the article: “Power doesn’t do it. Temperature control does. All you need is enough power to keep the tip hot. Anything more than that is a waste.”

 

 

 

  • What is the difference between cheap RadioShack® irons and more expensive ones like Wellers®? What do $100+ and $400+ soldering “stations” have over the cheaper kinds that plug straight into the wall? expand Among the irons that plug straight into a wall and don’t have a separate station, the dirt cheap kinds will work satisfactorily for many applications. From personal experience, the tips on RadioShack® irons often come loose and sometimes can be impossible to remove. The irons can also get uncomfortably hot to hold after several hours of use. The more professional Weller (or other) lines are made for longer, continuous use and have insulation on the handles that keeps them cooler. They can also take a wider variety of tips.
    Soldering iron “stations” usually provide some control over the heat being supplied to the iron tip. Ones that are temperature controlled automatically control the amount of heat delivered to the tip so that it remains at a set temperature. In every iron, when the tip touches a component, some heat is lost and the temperature drops. One measure of quality is the time needed for the tip to regain its temperature. A nice feature of many soldering stations is that the tip heats up in seconds after you turn it on.
    Many stations also allow you to hot-swap the iron tip, which can be very helpful if you’re alternating between surface mount joints and larger components.
  • If standard tin-lead solder melts below 400 °F (and lead free below 500 °F), why do most soldering irons have tip temperatures between 600 and 800 °F? Just what is the right soldering temperature? expand The basic reason that tips are so much hotter than solder’s melting point is because that difference helps to transfer heat faster to the joint. What is the “correct” temperature is a debatable topic, but a common rule of thumb is to start off at 600 °F and increase from there until acceptable results are achieved. Typical Kester (a solder manufacturer) datasheets recommend 600-700 °F for lead-based solder, and 700-800 °F for lead-free solder. “No-clean” or “low solids” fluxes will burn off before a joint can be made with higher temperatures, so low temperatures (below 700) may be essential for these fluxes.
    From Kester’s hand-soldering knowledge base: “When hand soldering with a rosin flux such as the Kester #44 or the # 285 the recommended iron tip temperature is 750°F. If you are soldering with a low residue no clean solder such as the #245or # 275 we recommend a tip temperature of 600-650°F.
    What are acceptable results? The goal is to heat up the parts enough so that solder will adhere to them and form a good bond. The higher the iron temperature, the faster it will heat up the parts, so why not set it extremely high to work faster?
    Besides the obvious increased risk of overheating components and the board, higher temperatures cause the iron tip to oxidize faster and can significantly reduce its life. Some claim a 10 °C rise reduces tip life by half (ref p.33). For occasional use, though, tip life may not be much of a factor, especially if the tip is kept covered with solder at all times.
  • Tip size and shape: a basic guide is to pick a tip that’s slightly smaller than the pad you’re soldering to. From there, you want a tip with a large thermal mass and short stroke (why?)  In most soldering irons, the tip is not actually the heater, but sits in between your work and the heater. You can think of it like a heat bucket that empties into your work and gets filled again by the heater. Typically touching a component empties heat out of the tip much faster than the iron can replace it, and if you have a small bucket (tip), the temperature will quickly drop to an ineffective level.
    Especially if you have a small wattage iron (15 Watts or less), the temperature will drop before you can heat up a larger part, or you’ll have to wait a bit in between joints for the tip temperature to recover. With a bigger bucket (tip), you can handle larger joints with smaller wattage, but eventually you’ll need to step up the wattage.
    The “stroke”, or length of the tip should be minimized to get the heater closer to the work; it takes some time for heat to transfer through the tip. This is balanced with the need to get into tight places where you need a longer tip.



  • What do common tip shapes look like and what applications are they best for?
    three_tip_shapes.jpg
    Screwdriver, spade, and conical are some of the more common tip shapes. Personal preference is the biggest factor when choosing a tip, but the goal is to get as much surface area contact between the tip and work as possible. Chisel and spade tips have more surface area at their ends, and also “hold” solder at their tips more readily than conical tips, which have a tendency to draw solder away. Even for fine pitch surface mount soldering, having a small flat at the end can be helpful.
    plato_tip_catalog_page.jpg
  • There are myriad other tip shapes and sizes. The picture to the right shows one Plato catalog page of many. Some other non-standard shapes include a knife-blade (useful for fine pitch leads) and a surface mount desoldering tip.desolder_surface_resistor.jpg

    knife_and_soic_tip.jpg
    desolder_soic.jpg

    To preserve tip life, the number one thing you can do is reduce the tip temperature (if your iron allows this). After that, ALWAYS keep a layer of solder on the tip to prevent the tip itself from oxidizing, and clean it in between uses. Put a glob of solder whenever you put it back in the stand, and before you turn it off. When heating up a new tip for the first time, hold solder against it so the tip can be covered as soon as the iron gets hot enough.
    The longer flux residues and oxides are left on the tip, the harder they are to clean off. They also can drastically reduce the tip’s ability to heat up a part, and prevent solder from “wetting” the tip. Regular cleaning of the tip before use is one of the best ways to prolong tip life and make soldering easier. It’s important that solder “wet” or cling to the surface of the iron–without solder in between the tip and work the tip’s ability to heat is drastically reduced.

  • What about gas powered irons and the Cold Heat® iron that is supposedly touchable 1 sec. after use?  Butane (and other gas) powered irons are mainly used in situations where electrical power isn’t available. Weller sells some battery powered irons as well.
    Everyday Practical Electronics gives a pretty damning review of the Cold Heat iron here, in addition to having one of the better how-to guides out there. To summarize, the Cold Heat® iron has a forked end that you must bridge with the work or solder to turn on the iron, so it can be hard to hold it in a place that keeps it on and also effectively heats the part. Many people complain about pushing harder to make a good connection and then having the brittle tips break. Running power through your work to heat it may not be the greatest idea with some parts. Finally, the iron doesn’t get hot enough for a lot of jobs, or cool enough to do anything like throwing it in your pocket right after use. But for something that’s portable and cordless, heats up and down in under a few seconds, maybe it’s worth the price ($20).
    Weller’s battery powered ($20) iron doesn’t have a forked end and supposedly heats up in under 15 seconds, but I don’t know about cool-down time.

 

Cold Heat Iron

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CuriousInventor launched in late 2006 (pre-arduino era!) as a place to enable hobbyists, students, and musicians to create their own technology. We sold open-source kits and tools, and offered numerous guides & videos on things like soldering, metal working, screws, electronics, and more. 

The store is now mostly empty, but we’ve kept the product pages and guides up since they have useful information. Many of our guides and videos still rank on the first page of google searches and have been seen millions of times. Content on this site and the CuriousInventor YouTube channel produced by Scott Driscoll.

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