Why is 60/40 Sn/Pb bad? Have had Kester 60/40 as general purpose solder for years.

Why eutectic solder? AFAIK, the only difference is the single melt/freeze point at 183 degrees C vs a melt range of 183-192 C.

It is the plastic state that leads to cold-solders. The smallest bit of movement during the cooling period (and for 9 degrees C, that can be several seconds) can and will lead to failures as soon as immediately, but more typically over time. Further, many will be invisible. prior to that failure. Further, these sorts of problems tend to include a lot of heat at the bad joint, leading to the potential for cascade failures. Using a true eutectic solder eliminates the concern.

Peter Wieck
Melrose Park, PA