Choosing Solder

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Choosing Solder

Choosing the right solder for your electronics project can be a bit daunting for many beginners and rather confusing even for seasoned veterans. My goal with this page is to provide some clarity for you, the hobbyist, so you can make an informed choice.

To start at the beginning: You want to use solder intended for use in electronics – not plumbing solder. In plumbing you apply the flux with a brush and the solder itself has no flux in it. This is not useful for electronics. Plumbing flux is way too acidic for electronics use and is also incredibly messy.

Flux

The purpose of flux is to clean the solder joint as the solder is applied, thereby allowing the solder to flow, resulting in a good and void-free solder joint. The flux also changes the surface tension, which increases the solder's adhesion the metal in the solder joint. The solder used for electronics has the flux embedded in it and the wisp of smoke that is emitted during the soldering process is caused by the flux boiling off. Prolonged exposure to flux fumes is a health hazard. The health risk is likely smaller for the hobbyist performing soldering on occasion. Still it is good practice to set up a small fan to blow the flux fumes away from the work area while soldering.

There are three different kinds of flux available for electronics soldering. The main difference is the difficulty involved with removing the flux.

Water soluble. The main advantage of water soluble flux is that it is relatively easy to remove. Rinse the circuit under warm running water and agitate with a soft bristle brush if necessary. An ultrasonic cleaner can be used as well. Follow up with a rinse in de-ionized (DI) or steam distilled water. The main drawback of this flux type is that it has to be removed. Rosin-based. Traditionally, flux used in electronics solder has been based on pine tree rosin. It is available in three 'flavours': non-activated (R), mildly activated (RMA), and activated (RA), the latter being the most acidic of the three. The residues from rosin-based flux are mildly corrosive and should be removed after soldering. Note that the RMA solder has been formulated such that the cleaning, while recommended, can be omitted. RMA is also the most common type of rosin-based solder. Rosin-based flux can be removed with isopropyl or isopropanol alcohol followed by a DI water rinse. Some agitation with a soft bristle brush is usually necessary. No-clean. As the name indicates, no-clean flux is formulated such that cleaning is not necessary. Some argue that even though no-clean flux does not require cleaning, the flux should be removed anyway. Unfortunately, no-clean flux is very difficult to remove, requiring the use of flux cleaners containing acetone, hexane, and other harsh solvents.

If you would like to minimize the solder inventory in your toolbox, I suggest purchasing solder with RMA flux. Alternatively, I suggest using water-soluble flux for circuits that can be easily cleaned and no-clean flux in situations where cleaning is difficult or impossible.

Note that many PCB rework materials, such as de-soldering braid (Solder-Wick, for example) contain flux. Ensure that your various sources of flux are compatible, i.e. if you are soldering with RMA flux, make sure to use Solder-Wick with RMA flux for solder removal.

Flux Cleaners

The trouble with flux residue is that it's hydrophilic, i.e. attracts water. This means any flux residue on the PCB will cause significant leakage currents on a wet day. You may have a circuit that works well in a dry climate but fails in a coastal climate. The combination of water and flux residue is also corrosive and can cause your circuits to fail over time. As noted above, the exceptions are the residues left behind by no-clean flux, which are not corrosive, and those from RMA flux which are only very mildly corrosive, thereby allowing the cleaning step to be omitted.

Flux removers come in varying degrees of aggressiveness ranging from light duty to heavy duty. The light duty flux removers tend to mostly be isopropyl or isopropanol based, whereas the heavy duty flux removers include acetone, hexane, and other rather nasty solvents. These cleaners are extremely flammable and should only be used in well-ventilated areas. I strongly advise that you read the Material Safety Data Sheet (MSDS) before using any of these flux removers. In addition to the personal safety, note that some of the flux removers dissolve plastics, so be careful.

Personally, I like Chemtronics Flux-Off No Clean Plus, which you can get from Mouser in the US. It does not ship via airmail due to flammability. It's a relatively aggressive flux remover and does tend to leave a dull residue on the PCB. This residue can be removed with a water rinse. MG Chemicals (and many others) make flux removers as well.

Any assembled board I send out to a customer will have the flux removed.

Solder

There are two overarching groups of solder used in electronics: Lead-based and lead-free, the latter being dominant in electronics production today due to the environmental concerns with the disposal of electronic products.

Lead-free solder does not have the best reputation, in part due to technical issues with the soldering process. Most lead free solder alloys melt at a higher temperature (about 220-250 ºC) than tin/lead solder (about 180-190 ºC). Thus, changing from leaded to lead-free solder will require a change in soldering iron tip temperature. The typical tip temperature for leaded soldering is 320-370 ºC (600-700 ºF). For lead-free the temperature needs to be increased to 370-425 ºC (700-800 ºF). In addition to the higher tip temperature, the dwell time needs to be increased. A solder joint can be completed with lead-based solder in less than a second. Using lead-free solder, this time needs to be extended to avoid cold solder joints.

Leaded solder

Health hazard: Leaded solder contains lead (DUH!). If ingested, lead accumulates in the fatty tissues in the body, including the myelin sheath surrounding the nerve fibres in the brain. This can lead to brain damage, in particular in infants and small children. This is mostly an issue with lead casting where the lead is heated to near its boiling point. The temperatures used in soldering are much lower. The main risk of lead exposure is from the lead that rubs off from the solder onto your fingers. Please make sure that you do not eat or drink while soldering. Wash your hands thoroughly when you are done soldering.

There are three commonly used lead-based alloys for electronic soldering:

60/40 (Sn/Pb). The main advantage of 60/40 solder is cost, hence, most older equipment was assembled using this type of solder. The main drawback of this alloy is that it has a 5 ºC plastic region. 60/40 solder becomes plastic (pliable but not quite melted) at 183 ºC and melts at 188 ºC. The solder transitions through the same plastic region as it cools down and if the joint is disturbed or moved as the solder transitions through the plastic region, a cold solder joint is formed. This can make hand-soldering a frustrating experience, in particular for the beginner. As long as the solder joint remains still until the solder has fully solidified, the plastic region has no practical implications on the solder joints. 63/37 (Sn/Pb). 63/37 solder is the eutectic alloy, meaning that it goes directly from solid to liquid without plasticity. 63/37 solder melts at 183 ºC. This type of solder is slightly more expensive than 60/40 but the absence of a plastic region makes it nicer to work with and more beginner-friendly. Joints made with this solder alloy will appear shinier than those made with 60/40 solder. That is purely a cosmetic effect. 62/36/2 (Sn/Pb/Ag). 62/36/2 "silver" solder is gaining popularity in audio circles - probably because it's more expensive and contains silver. For soldering on copper wires and circuit boards, there is no evidence that "silver" solder should be superior to regular 60/40 or 63/37 solder. However, if you are soldering to silver wire, including some mica caps and silver-on-steel RF cables, you may want to use "silver" solder. This is because regular Sn/Pb solder will dissolve the silver over time. The silver in the 62/36/2 prevents this from happening.

In terms of conductivity, the three types are within a few percent of each other. The tensile strength of 62/36/2 solder is about twice that of 60/40, but whether that actually translates into mechanically stronger solder joints depends on the joint geometry.

Lead-Free Solder

The development of a good lead-free solder alloy has been a challenge, and some of the better alloys are only available in the form of solder paste. The first lead-free alloy introduced was the SAC305 (96.5/3/0.5 - Sn/Ag/Cu). Joints made with this alloy are dull and grainy in appearance, thus, indistinguishable from cold (failed) solder joints made with 60/40 solder. I suggest shying away from this alloy.

The two more user friendly alloys of lead-free solder are:

AIM Sn100C. This alloy is almost 100 % tin. It contains 0.7 % copper, 0.05 % nickel, ≤0.01 % germanium. The remaining approx. 99.25 % is tin. It is an eutectic alloy with a melting point of 227 ºC. As this solder is the only game in town for lead-free wire solder, it is rather expensive at over twice the cost of 63/37 leaded solder. 95/5 (Sn/Ag). The performance of 95/5 solder is very similar to 60/40 leaded solder, which is very attractive. This alloy has a rather large plastic region. It enters plasticity at 221 ºC and melts at 254 ºC. It is not as useful for the hobbyist as it is only available in paste form.

It is not recommended to mix leaded and lead-free solder. Thus, ensure that soldering tips are used only for either leaded or lead-free solder. A tip tinned with leaded solder can be used for lead-free soldering after 4-5 thorough clean/re-tin cycles, though, it is strongly recommended that you pick one solder type for the tip and stick with it. Some R&D labs have a separate soldering bench set up for lead-free solder to avoid cross contamination.

In general, solder alloys should not be mixed. Keeping the solder chemistry clean ensures that only the alloys the solder manufacturer intended to form actually do form when the solder cools.

Diameter

Choosing a diameter of solder wire that is appropriate for the task at hand can be a considerable help in the soldering work. Small diameter solder makes applying a small amount of solder much easier. This is very handy for soldering surface mounted components. For larger components, such as leaded components or connectors, using small diameter solder requires a significant length of solder to be fed to the joint, which extends soldering time and risk of overheating the components.

For work that involves surface mounted devices, I prefer 0.5 mm diameter solder. For leaded parts and connectors, I use 0.7 mm diameter solder. For most electronics work, solder in the range of 0.4 - 1.0 mm in diameter will work well. If you perform a lot of work on surface mounted devices, aim for the lower end of this range.

Shelf Life

Yes. Really! Solder has an expiration date. For the alloys mentioned above, it is recommended that solder wire is used within three years of manufacturing. That said, I am just now finishing the roll of 0.7 mm 60/40 RMA flux core solder I started in the late 1980ies and the solder joints I am making today perform as well as ever.

Do observe the shelf life on solder paste, however. Solder paste consists of small beads of solder suspended in flux. Over time, the flux will oxidize rendering it ineffective. The result is that the solder won't flow correctly and it becomes very difficult to get a good solder joint. The shelf life of solder paste is about six months. By refrigerating the solder paste, the shelf life can be extended to about a year. It should go without saying, but please don't store the solder in the fridge you use for food!

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