Many possible causes, some of which have been mentioned by others.

However, I would bring up vesication, which is the formation of myriad 
tiny-to-small blisters under the solder mask or conformal coating, 
usually due to a hygroscopic contamination under the coating. This pulls 
water vapour through the coating, the resultant pressure lifting the 
coating off the board. I wrote about this pp 138-140 if you happen to be 
able to lay your hands on a copy of my book. In some cases, if each 
vesicle is very small, it may look like the coating has a milky appearance.

Brian
Ralph Richart wrote:
> IPC A-600, section 2.9.4 refers to Blisters/Delamination in Soldermask.
> 
> I am looking for clarification as to what these actually are?  How do they manifest themselves?  I can't say I've ever seen one, so I am trying to know what one would look like if I hade seen one.
> 
> thanks
> 
> Ralph
> 
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