Always 63/37

There are other viable formulas of solder, but 63/37 is just easier to work with and it's performance is very good overall.

​​​​​​It tends to cost less than an equivalent amount of other formulas that perform as well or a bit better.

I've used various formulations through the years, and found a few that work well, but none are as well balanced, relatively low cost, easy to work with, or all of those combined.