This is related to the surface energy of the solder resist. High surface energy is good, low surface energy is bad.
For example a non stick frying pan is low surface energy, not easy to paint!
There are many different types of solder resists and the surface energy varies with all of them. Over curing solder resist tends to lower the surface energy as silicones can be forced to the surface. I suggest that you get a set of dyne pens, these are inks of a known surface tension. The point at which they wet will indicate the surface energy of the board.
As a guide 36 to 38 dynes is average, a no clean assembled board would be about 32 dynes and anything less than this bad.
These levels are what we (HumiSeal) consider good, bad and average for conformal coating purposes. You have probably moved PCB manufacturer or are now using a different solder resist to the one you are used to.
Wednesday, 20th January 2010