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May 2006

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
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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