Many coatings and adhesives for electronic assemblies have similar
cure requirements. I would not be concerned by the time at elevated
temperature, rather I would be concerned with the rise rate from
Ambient to 90 and from 90 to 160C. Try to keep rise rate to less than
2 degrees C per second. I've developed processes for customers (we
manufacture curing systems) that have cure requirements as high as
150C for up to 50 minutes. In the Reflow world thermal profilers are
almost always used to develop a process but in the curing world this
tool is often overlooked. I suggest using a thermal profiler to verify
the rise rate seen by your assembly.

On Fri, Sep 10, 2010 at 12:15 PM, Temkin, Gregg
<[log in to unmask]> wrote:
> I have been given a new design which has an underfilled leaded BGA on an SnPb soldered assembly (however many passive components are PbF).
>
> The underfill has a snap cure of 160C for 7 minutes after a 90C pre-heat and dispense.  The PCB is FR-4 with a 170Tg min and the assembly is double sided SMT with selective soldered thru hole connectors.  It's is a Class 3 assembly.
>
> From my understanding, components are generally not harmed by a few well controlled SMT cycles consisting of a soak period in the 150-180C range, followed by a ramp-up to 210-235C for reflow.   But in total, they dwell about a third of the time of the snap cure.  Is the snap cure comparatively benign or harmful or are there too many depends?
>
> Appreciate any comments.
>
> Thanks Gregg
>
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