The year for major ABS control unit failures was '98 X70s, not so bad before or after. At a dealer, we don't send them out, we get new units (or so they appear) and there is now a core charge on them so they "might be" rebuilt but I can't be certain. It's possible that Volvo is just getting them back to inspect them too.
There is a clear, rubbery gel coating on the inside of everyone I've cut opened and it's there for waterproofing. It's my opinion that re-soldering the board is just a guess as to what's really needed and doing so would not be an easy task for anyone with basic home type soldering equipment.

I ask "what year" here because IF it's a '96 850, THAT year had another very well known wiring problem, was a service bulletin on how to fix 4 bad crimp connections under the fuse box. '96s rarely need new ABS control units. If the ABS engages at slow speeds on flat dry surfaces (no wheel slip), it's most likely in need of the wiring cure and it's too detailed to describe the 12 page paper SB here. It was an old paper SB, not current in VEN.