Robert Baer wrote in news:z2XJF.379072$ snipped-for-privacy@fx47.iad:
If one is referring to HV isolation, ANYTHING between those nodes is a setup for a creepage trail. That is why an "air gap" is so desireable.
So, the contaminant can be any part of the process, but yes, flux remnants or rinsed dust that got trapped even.
I can even remember using oval pads to increase the space between the nodes (rid of those corners too)