Michael Branning asked:

>My company is setting up a small surface mount line. We have been building
>thru-hole boards with one PQFP package per board (IC only available in SMT)
>for some time with very good success. We've been using hot air (Leister)
>rework tools to mount the SMT parts by hand. However, we are in the process
>of designing some full SMT boards (one sided).
>
>We've just purchased and started using a Lovell convection oven and a small
>manual screen printer. Up till now, we generally have ignored the
>humidity/exposure warnings on the IC packaging. Is the only effect we must
>be concerned with "popcorning"?

Popcorning is a term with indistinct meaning. It refers to a variety of
problems caused by rapid heating of parts which have absorbed moisture.

>Does the package always dramatically fail?

No. The part will not go "pow", it may look OK, but the die attachment or
some other internal element may fail with little outwardly visible
indication.

>Or can problems be created that are invisible to the naked eye?

Yes. You may see slight bulging of the molded surface, especially at the
belly of the part. After assembly and soldering, this would be "invisible".
If the die moves relative to the leadframe and die attachment pad, damage
is possible from a wirebond failure.

>If the parts
>must be baked, what equipment would be suitable for small production runs?
>(50-100 boards)

Any old oven will do. Bake at a temp above 100 degrees C 8 hrs _min_ and
try to use the parts quickly after cooling to room temp. Put the parts into
sealed bags with dessicant and moisture indicator labels. Your disty source
should be sending these parts in such packages.

>Sorry to sound like a rookie

You can quickly adopt an authoritative tone and use buzzwords to avoid
sounding like a rookie. If you do this without inquiring and learning, you
won't sound like a rookie, but you'll look even worse. We can't see you,
but others can.

Our experience is that the thin profile TQFP and TSOP packages are much
worse than conventional QFP's. IMO, this is because the overmold plastic is
much thinner, allowing moisture to penetrate to the level of the die, and
also because the overmold is weaker and less capable of maintaining
integrity.

If your storage is less than 60% RH and you keep the parts in sealed bags,
you may very rarely see the problem - especially if the parts are
conventional QFP's with bodies 3 mm thick.


Your mileage may vary. Good luck!


regards,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com


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