Hi Kevin & All, > 1. If you follow J-STD-001, reference section 4.9.1 - Preheating. > Basically, you must preheat the boards for machine soldering. This is to > reduce the thermal shock to the PCB when it contacts molten solder, such as > in wave or selective soldering. An exception is automated SMT. We do not > pre-heat boards for this process, as the pre-heated board will cause the > solder paste to soften when it is screened on, and the reflow profile > hopefully alleviates the thermal shock by maintaining a proper ramp. And > while not required by J-STD-001, pre-heating the PCBs can also be very > beneficial for hand soldering operations, particularly on thicker backplanes > with copper ground planes. > A: Actually, the reason for preheating is not so much to reduce thermal shock, but to have the smallest practically possible temperature gradients through the PCB. This keeps warping to a minimum. The real thermal shock danger is on cooling—too fast and you fracture SJs due to PCB/Component warping. > 2. Pre-baking between various stages would depend on the environment and > duration the assembly is subjected to between processes, and also any > intermediate processes. For example, if you are assembling in a very humid > environment, pre-baking can help drive out moisture absorbed by the > assembly. If you clean the PCBs between processes, and the board is not > dried sufficiently, there will also be absorbed moisture. > A: Moisture absorption is even more of a problem with the higher LF-soldering temperatures. The 35°C T-increase causes vapor pressures to double. Werner