Homebrew PCBs

At 2:52 pm ((PDT)) Thu Apr 12, 2007, stuart.winsor.lists wrote:

>Usual scenario is the solder joints between the PCB and the valve-holder,
>because of the heat and the cycling, start to go crystaline. They become
>high resistance and "dry" [snip]

At 3:24 pm ((PDT)) Thu Apr 12, 2007, stuart.winsor.lists wrote:

>[snip] but there is also the weight and leverage caused by valves. Not
>forgetting the forces involved in getting them in and out of the sockets
>periodicaly to replace them.

In your experience and opinion, do the joints usually fail
1. through the body of the solder or
2. alongside the solder tag?

1. suggests to me either
a) simple work hardening and stress fracture, possibly
b) combined with recrystallization near the melting point,
while 2. hints at
c) brittleness though formation of an intermetallic compound
with something dissolved from the tags.

A high-temperature solder might reduce (a) and (b) by
being stronger and more stable at the working temperature
while a solder doped with small amounts of other metals
might reduce (b) and (c).

There are a fair number of online articles on solder metallurgy,
some produced by metals suppliers such as Johnson Matthey
and some propelled by the recent RoHS drive to lead-free solder.

Whether it's worth the bother of seeking something different from
what's already on the workbench, I can't advise ;-)