The lead-free solders are a problem, too. A lot of the techs trained
on lead-containing solders don't know how to use the mostly tin
replacements. Another problem with the newer ones is the formation of
tin whiskers, which I believe is a crystallization process over time,
leading to the degradation of the joint.
The cold solders vary in quality, the best are silver-powder
containing epoxies. Needless to say, most are worse, and many
manufacturers do not make the design changes necessary to accommodate
the different characteristics of cold solder. Ergo, defective
parts.-Jitney