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Date: | Fri, 12 May 2006 15:22:24 EDT |
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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
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