We are building assemblies 90% SMT, 10% TH. Fine pitch and BGA devices, etc. The customer is complaining about cold solder joints (SMT) after about 1 year of use, scattered thru-out the assembly, not consistant areas, we haven't actually seen a failed assembly yet, this is what the customer is telling us. The assemblies are visually inspected during manufacturing with no problems seen. The assemblies are used in equipment that vibrates some what and tempertures from 20f to around 125f. The PCB's are ENIG, double sided and we use SnPb solder. Are there some standards to apply to make this assembly "more" rugged? More "robust", last longer? Board finishes, paste, processes,etc?
On the other hand, as you say, it's really tough to assess a situation with such a small amount of information. Based on what you described, your ENIG boards could have 'black pad' as a possible explanation as good as the next. If that's the case, ruggedizing the board wouldn't change very much.
Do you have any input as to the vibrations seen on the end-use device? Types of device(s) that have solder joint failure? How many thermal cycles the PCB goes through before failure? There are several areas in production of the board that could be the inducing the problem. Is this infant mortality (new boards) or have these boards been in the field for a while? How are the boards mounted? Do they have vibration isolation? Too many questions to list here. My company is listed on this site...Contact me and we can discuss the details. I AM a consultant, but would be willing to discuss initially free of charge.