| Also the preheating of the board is done to help evaporate any water/alcohol that is in the flux - water is a big problem in No-clean processes. From our experience, a temp range around 200f - no more than 220f is ideal for the bigger boards, but temp needs to decreas as the board size gets smaller. If the the temps are TOO hot, it will evaporate the flux and cause no solders on your smt parts. | The substrate problems are pretty much a thing of the past with smt spun boards. Trial and Error is still the only proven way to go when dealing with these types of issues. | You're sure right about "overtemping" the flux. However, there are still and always issues concerning doing it to PCB substrates - especially pure SMT types.
In pure through hole, component holes were supported by definition. In those SMT, none are. Therefore, when a board gets too much thermal stress or shock, it may delaminate or hole walls may come apart.
Z axis expansion is the primary culprit in hole wall failure followed by delamination through holes thus providing electrical opens. The opens may close during bed of nails testing causing suspicion elsewhere.
Copper plating ductility is an ongoing serious issue with respect to about 50 ppm/degree C expansion rates in the Z axis for most board materials. Poor designs, poor supplier evaluation (concerning copper plating quality as tensile and elongation characteristics), poor material selections, poor MLB constructions and process management, and poor thermal management during assembly processes all provide paths to substrate destruction and hole wall failures. This is very true for SMT today and tomorrow when faced with expansive thermal excursions found during wave soldering operations.
There's so much more, but for now enjoy,
Earl Moon
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