In general terms, guidelines for moisture sensitive devices say that you
shouldn't put a component through reflow after it has reached its
exposure limit with respect to moisture absorption.
When using an automated hot air rework tool to remove and replace an SMT
component, there is a risk of damaging the neighboring components if
they have absorbed enough moisture and if they get enough heat. For this
reason, heat shielding is used for protection and assemblies are baked
prior to repair.
Assuming that a component has surpassed its exposure limit and has
enough moisture content to be damaged when put through a reflow cycle,
what minimum conditions (max. temp., ramp)  would be required to cause
moisture damage (i.e. delamination or popcorning)?
If neighboring components are thermally profiled during the rework cycle
and it is shown that they do not receive enough heat to cause moisture
damage, product baking and heat shielding may not be required.
Any ideas?
Yves Trudell
Quality System Engineering
Nortel, Wireless Networks Calgary

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