Hi:
I have seen this problem before. It is caused by improper curing of solder mask by the fab manufacturer. When you run the board thru' wave solder, and then wash the board, you see the problem of white residue. However, when you apply heat, you can reduce them. Sometimes these are like white spots, often giving the impression of contamination. Good thing is apart from cosmetics, electrically it has no effect.
Problem is certainly not assembly but that of fab house. But getting them to admit is another issue. You may be able to obtain some studies about it from the manufacturer of the solder mask. One way to avoid this problem partially, is to bake the board yourself before loading, thus speeding up the curing action. The solder mask is epoxy based compound, and improper mixture or handling can cause those spots or blemishes.
If board house do not want to take responsiblity of the poor process, get another.
Good luck!!!
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