Hi Karen,
You have gotten good advice so far.
The 'thermal stress' George is referring to, has to do with the thermal
expansion mismatch between the cermic chip and the PCB, and this in combination of
large solder fillets. The larger the fillets, the higher the stress. The
larger the pads, the larger the fillets. Also of course, the larger the delta-T,
the higher the stress. Thus, you will see more chip cracking with LF-solders.
The faster the heat transfer, both on heating and cooling, the higher the
thermal shock stress. On heating, wave soldering has the highest thermal shock.

Werner Engelmaier

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