Hello Loz,
Blue residues on circuit boards can be caused by copper corrosion, component body deterioration, marking permanently, and adhesive break down. In this case, a more subtle mechanism appears to be possible. This is a no clean application, so cleaning, and the causes listed above are unlikely candidates.
Many �low residue� no clean fluxes are deactivated by subliming the activator (adipic acid) in the soldering process. Sublimation is defined as changing from a solid to a gas and vise versa without going thru the liquid state. As the activator sublimes to a gas, it can recondense as a solid on the solder joint as the board exits the wave leaving microscopic crystals of the activator on the newly formed solder joints. Very small crystals can diffract light and give a blue color. This can come and go due to air currents, activator concentration, conveyor speeds, preheat temp, etc.
I hope this helps,
Steve Stach Austin American Technology www.aat-corp.com (+1) 512-335-6400
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