Selecting the best circuit board solder is a process that can depend on a handful of different factors. The most important consideration is the type of soldering you are doing, since different techniques call for specific types of solder. For example, reflow or wave soldering usually require different materials that soldering with a soldiering iron. There are, however, some alloys that are well suited to both applications.
When looking for the best circuit board solder, the first determination you will need to make is what type of soldering you plan on doing. If you are reflow or wave soldering, you will need to look for a circuit board solder that lacks any flux. This is due to the fact that the flux is typically applied by itself prior to the application of the solder. Wave and reflow soldering is also done with solder paste, and the presence of lead can sometimes lead to the formation of metal foam, as well as having potential environmental concerns.
Soldering with an iron is typically easiest if you choose a product that has a rosin-core. The inclusion of the flux inside the solder makes it easier to handle, which can typically help a job go faster. Another important factor to consider is the alloy the solder is made from, though any alloy of tin and lead that contains over 60% tin is typically good enough for electrical work.
After you have determined whether you need solid or paste circuit board solder, you will need to take a number of other factors into consideration. Even though the general rule is to choose a solder that contains over 60% tin, some applications call for specific alloys. If you are working with a circuit board where the solder will come into contact with gold or silver circuit traces or components, you will need to choose a solder that is not overly reactive with those metals.
The presence of tin in solder can actually dissolve both gold and silver. Circuit board solder that contains indium is typically a good choice if it will come into contact with gold, since indium does not react with gold the way tin does. Adding silver to an alloy of tin and lead can help reduce adverse reactions to silver circuit board components, though tin-free alloys can be used in those cases as well.