I have been running small batches of production over a year without a problem. Today, I found a problem that is never happened before. I have changed the PCB to gold plated in my recent batch and I see mold build up at a certain location 20 out of 500 units. I am unable to understand the root cause.
The production environment is humid (it is in Asia) but for mold to build up more things are needed especially sugar or some other type of food which I believe hard to come by in such environment.
What would be the cause of such build up? Is gold plate more prone to organic exposure?
I look under the microscope and now I think it is corrosion. Here is the photo.
What process mistake would cause a problem like this?