How many thermal cycles for rigid PCB before possible latent field failures

We typically expose our product to 1 to 4 thermal cycles (SMT reflow, Wave, Selective solder) depending on product. Unfortunately, like most of us we occasionally have to rework a small # of these which (at least in a localized area) are exposed to an additional thermal cycle. At some point, even if no physical damage is evident, one would think some degradation is occurring. Was wondering if anyone else has worked thru this. I have yet to ask this question of our PCB suppliers. Any input is appreciated. Could not find a previous thread that addressed this specifically.

Charlie

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