Mike Buetow asked:

>We have a member who is specifying a large number of a soon-to-be
>discontinued plastic DIP IC. These parts will then be stored up to two
>years at their facility.

We have the same situation. We solder old DIP's all the time. Look at your
date codes - even if you are getting "new" parts via disty, they could
easily be that old.

>His concern is with handling of these parts, particularly how to store
>them (vapor barrier bags, for example?). He is well aware of the
>precautions necessary for SMDs, but doesn't know whether these apply to
>DIPs too. Any advice?

Solderability would be my main concern, although not a major one. Keep your
storage room controlled with a good A/C system. Low humidity is best for
retarding many oxidation processes, although that may not correlate well to
ESD concerns. About 40% RH is ideal. With conventional filled epoxy mold
compounds in DIP parts, the concerns of moisture absorption by the package
are of little importance, IMO. Do NOT bake the parts at the point of
assembly, it would be more likely to adversely affect solderability of the
lead finish than to reduce any risk of package damage.

Most people don't look too hard at their storage areas, and there can be
problems. HVAC should have good flow, replacement ventilation, free air
movement in the racks, and controlled humidity. Outgassing of various
materials which can affect the activity of metals (from even the mold
compound, for instance) is almost impossible to prevent.

One potential problem is storage in corrugated (kraft paper) containers,
which usually contain a significant amount of sulfur. In the presence of
water vapor, you get gaseous sulfur oxides - these can cause long term
effects on copper via diffusion to the base metal and attack of the finish.
You may find it useful to keep the parts in a closed cabinet which has
dessicant in bags and anti-oxidant materials (also sold in bags) to slow
the affect of vapor problems. If the parts can be sealed in vapor barrier
bags with dessicant, this may be useful.

If you de-trash those cardboard boxes, store the parts in their tubes in a
reasonable controlled climate, I predict minimal problems for typical DIP
lead finishes in your time frame, even without unusual preventitive
measures. I don't believe I can recall ever seeing a DIP "popcorn" in my
whole career.

Then again, if your philosopy is that you can occlude oxygen and solder
without the benefit of chemical oxide reduction, you may find it hard to
solder parts untouched by human hands, removed from hermetic dry nitrogen
chambers 5 minutes before assembly.

(i.e., use a nice potent flux when you solder, your sins will be forgiven).


YMMV, but good luck,

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