Solder contamination

Hi we are currently experiencing a problem with solder contamination of the gold contact pads on our pcbs following the reflow process, possibly through sputtering. We have been advise that this can be caused due to the pcbs being humid or due to the stencil not being completely clean. We've excluded the latter and will be tempering future boards prior to the run. Can anyone indicate any other possible causes of this contamination as our reject rate is too high.

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