ok thanks James ... though i AM curious what happens to the metallurgy (composition) of solder over the years ... and if impurities etc migrate though to create 'cold' joints over time
AFAIK, the culprits are
1. Cyclic stress causing fatigue and cracking
2. Corrosion
If these are old enough to be leaded solder, it's somewhat less susceptible to fatigue, but exhibits creep, in which case static stress is a problem, too.
Also, is this a 1-layer board, or more? PTH?
Single side solder, no PTH, is very weak: the solder fillet bears the entire component lead strain, with little or no support from the hole. It's very common to see cold solder joints on such equipment.
While there is an intermetallic layer (Cu-Sn species) on the surface of leads and pads, it is usually thin and strong, and not changing over time. Thickness of the layer depends on time*temp during soldering, which must be kept low for this reason, but -- much more significantly -- also because everything else involved (PCB and component resins) breaks down quickly at those temperatures.
A graphic illustration of intermetallics might be disassembling copper water piping, if you've ever had to do that. The joints are usually pretty tightly fitting to begin with, filled with lead-free solder (tin with a few percent antimony), and heated with a hand torch. Disassembly can be tricky. The interface layer is sticky and squeaky in texture. If you aren't pulling and twisting straight on axis, it may not move at all, and it's easy to miss the melting point and keep on heating the joint. Which grows the interface layer, only making the joint tighter and tighter. An overheated joint can be effectively diffusion-brazed together and actually impossible to disassemble!
Again, this isn't significant in normal PCB assemblies, even with very fine pitch components.
Gold plating can be significant, but that's a matter for another thread
Tim