Solder is a mixture of relatively low melting point metals, designed to melt at a lower temperature than metals like copper. This allows you to join components, providing a strong chemical bond that is electrically conductive. So what is solder made of? It’s made of a special mixture of metal. Frequently it also contains a core with chemical additives to make it easier to work with.
The precise formulation of both the solder alloy and the core varies depending on the application. But, generally speaking, solder is made of an alloy of low melting point metals.
Traditionally, solder alloys contained lead. Lead has a pretty low melting point, and when you mix it with other metals, you can get the melting point lower than the temperature of boiling water.
The formulation varies but generally uses numbers to indicate the mix. 60 / 40 solder contains 60 percent lead and 40 percent tin. It melts at the relatively low temperature of 376 degrees Fahrenheit, or 191 degrees Celsius.
Another popular formulation was 63/37, because it had a lower melting point. This alloy melts at a temperature of 361 degrees Fahrenheit, or 183 degrees Celsius. This makes it much easier to work with.
Lower melting points are possible by adding trace amounts of other metals to the tin and lead, but not optimal. Consumer electronics can get hot enough to melt the joints. This makes lower melting points undesirable. This is why baking video cards, game console motherboards, or SSDs sometimes fixes them. Getting the melting point much lower than that of 63/37 isn’t really desirable.
The EU ban of 2006
In 2006, the EU issued a directive banning leaded solder from most consumer electronics, due to health concerns. The rest of the world phased leaded solder out of commercial electronics manufacturing as a result.
A hobbyist doing repair work or making something for their own use may legally continue to use leaded solder, but commercial products today contain lead free electronics solder.
When you see a tag on consumer electronics stating that the product contains chemicals known to the state of California to cause health hazards such as cancer, that is a sign that the product contained leaded solder and was manufactured during that transition period. Of course there are other industrial chemicals that can pose health hazards, so that warning doesn’t necessarily refer strictly to lead. But it can be an indicator.
Phasing out lead
Lead-free solder for electronics generally has a higher melting point and is more difficult to work with than the old-fashioned leaded solder. Modern industrial processes can deal with the difference. And we’ve now had approximately two decades to figure out how to adjust. Theoretically, lead free solder is prone to tin whiskers developing over time, bridging traces unintentionally. That said, news stories from the 2006 time frame predicting and apocalypse of failing electronics seem to have gotten it wrong.
That’s not to say consumer electronics from the 2006 timeframe are issue-free. But the problems with that era came from capacitor plague and excessive heat. Not tin whiskers.
Lead-free alloys generally contain metals like tin, Silver, Copper, Antimony, Bismuth, Cobalt, Nickel, Indium, Zinc, Germanium, and rare earth elements. These alloys generally melt at around 422 degrees Fahrenheit or 217 degrees Celsius. The difference of around 15 percent may not seem like much, but having worked with all three, I notice it. If you need to work with these modern formulas, a soldering station with good temperature control is much easier.
It is not a good idea to mix alloys when repairing electronics. If the device dates to before 2006, use a lead-bearing alloy. And if it dates to 2006 or later, use a modern, lead-free alloy. If you cannot use the same general type of alloy as the manufacturer did, you will need to remove the component completely, clean the factory solder from the component and the PCB pads, then resolder the component.
Rosin core solder and flux core solder have a rosin or flux core, respectively. Either one can aid in removing the oxidized layer on copper PCB traces and electronic components to make the joining process easier and more reliable, to avoid cold solder joints. It’s still a good idea to use flux and not just rely on the flux in the solder.
And in either case, you need to clean off the flux residue after you finish the job. Over the long term, flux is corrosive and can damage the connection. Cleaning off the flux doesn’t just make the PCB look neater. It also increases the long term reliability. Flux residue comes off easily with alcohol or mineral spirits. Most people use 91% or 99% isopropyl alcohol. Using 91 percent concentration is fine. If you don’t have optimal storage conditions, any 99% isopropyl alcohol you buy will become 95 or 91 percent by absorbing moisture from the air around it.
The formulation for plumbing solder tends to be different. Lead was banned from plumbing solder much earlier than from electronics solder, because it carries water meant for human consumption. This ban occurred in 1986.
It is possible to find people who deny this, just like there is a large population who insists that the earth is flat, but that doesn’t mean either of those groups is right. Don’t use leaded solder for plumbing applications.
The other difference, besides the lead, is the core. Plumbing solder has acid cores, rather than rosin. The difference in formulation is because of the difficulty of soldering copper pipes versus soldering tiny circuit traces on a circuit board. Pipes absorb a tremendous amount of heat. Acid core solders for plumbing applications contain additives to make it stick more readily to copper pipe, at the expense of being slightly corrosive. The corrosion doesn’t affect the ability of a pipe to carry water, but it does interfere with electrical conductivity. So you don’t want to mix the two. If you use plumbing solder on electronics, don’t expect it to work forever. And if you try to use electronics solder on your pipes, expect a hard time. Even if you use lead free electronics solder.
Does solder contain silver?
There is such thing as silver solder, and it does indeed contain some amount of silver. Silver solder is led free, and it happens that silver is one of the metals that can mix with other metals to make a usable solder when lead isn’t an option. A mixture of tin, silver, and copper is one common alloy.
However, even though almost all solder generally is silver colored, that doesn’t mean all solder contains silver if it’s not explicitly marketed as silver solder, it doesn’t contain silver.
The big advantage of 63/37 solder is not its slightly lower melting point, but the fact that it is a eutectic alloy. That is, it goes directly from solid to liquid without an intermediate “plastic” phase. That property makes it easier to create high quality solder joints. If you are going to work with solder that contains lead, I highly recommend that alloy or its close relatives that contain silver such as 62/36/2.
Tin-lead alloys with silver added are typically referred to as silver-bearing solder. That’s to distinguish them from the soldering alloys with a high silver content that are used in jewelry; those have a much higher melting point and are not suitable for electronics. Silver-bearing solder is recommended for soldering silver-plated components, and it can also be used for general soldering; it’s a joy to work with but it’s more expensive than 63/37.
The most common lead-free solder alloy, SAC305, contains silver. The full formula is 96.5% tin (Sn), 3% silver (Ag), and 0.5% copper (Cu). The name SAC305 comes from its composition; the tin percentage is not called out. Adding silver to a lead-free alloy improves its handling properties and cuts down on the tendency to form whiskers. If you work at home with lead-free solder, the solder you get will either be SAC305 or a variant that adds a small amount of antimony; SparkFun sells the latter as their Special Blend. (Some stores may try to sell you lead-free solder that only contains tin and copper; don’t buy that, it’s miserable to work with.) Some manufacturers have switched to a silver-free alloy that uses small amounts of nickel and germanium to reduce costs (silver is expensive) but I haven’t heard of home builders using it.
There are low-temperature alloys containing bismuth that are usually only found in the form of solder paste. (The alloy is too brittle to easily draw as wire.) These are either 42% tin and 58% bismuth, or variants that add silver to the mix; the version with silver is less brittle. Bismuth solder is handy for surface mount rework because it allows you to put in a new component without disturbing surrounding parts; set your hot air gun hot enough to melt the bismuth solder but not the surrounding SAC305 joints. Bismuth solder has a melting point of 138C, which is usually high enough to avoid accidental melting, but it should NEVER be used on a board that contains lead solder or board tinning; a tin-lead-bismuth alloy can have a melting point that is well below 100C.