Bob Jacubec asked:

     What is the proper storage of SMT ICs;  are there any guidelines or 
     standards to use?  If moisture absorption is a problem with plastic 
     parts, is the standard practice to bake assemblies or parts prior to 
     reflow?  What are the baking requirements?  
     
      Our in-house stores personnel won't break down the sealed outer packs 
      as supplied by the IC manufacturers (they'll issue ICs in quantities 
     as supplied by the IC manufacturers, approx.  100 or 200 parts each).  
     As a result we have opened partial packs of parts remaining in the 
     manufacturing area.  Thanks.

************************************************************

Our typical build is 5 to 10 boards of a given design.  We assemble one,
(first article), and a couple weeks later finish the build.  The logic
behind this process is to prevent us from populating boards until the design
is proven.  This means we are ALWAYS in the position of storing parts that
were originally supplied in sealed bags.

I have heard as many opinions on how long parts can be stored without 
baking prior to assembly as there are people I have talked with.  Since I
recently "Popcorned" several thousand dollars worth of parts by relying on
these opinions and my personal prior experience instead of real knowledge, I
STRONGLY recommend that you learn a little about package molding materials
and die attach methods.  Armed with this information you can be sure you
are getting adequate information from your IC suppliers to help you determine
which parts are most likely to need baking and which parts will be more
tolerant.  The general notion that only THIN Quads, TSOPs and the like require
baking is only part of the story.  Leadframe plating and overmold materials
play a big role in just how resistant your parts may be to moisture absorption
induced delamination and cracking during reflow.

I was recently assisted by a gentleman named Gans Ganeshan, formerly of 
Motorola's Advanced Packaging Development Center.  There are many papers which 
discuss this issue in the 43rd and 44th ECTC Proceedings. You may find the 
following two papers especially helpful.

Gans Ganeshan and Howard M. Berg: Model and Analysis for Solder Reflow 
Cracking Phenmomenon in SMT Packages. _Proc. 43rd. ECTC*_, 1993, pp. 653-660

Gary L. Lewis, et al: Role of Materials Evolution in VLSI Plastic Packages in
Improving Reflow Soldering Performance. _Proc. 44th. ECTC_, 1994, pp. 177-185

*(ECTC) IEEE Electronic Components and Technology Conference

Gary P.
---
                                  Gary D. Peterson
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