All,

 Where I work, we have some PCB's that have been populated by a contract
manufacturer. Actually some were
 populated by one CM and others were populated by a different CM.  But
the phenomenon has been seen on all
 boards, so it isn't just one CM's process that is causing the problem.=20
We have checked with the CM's about
 their process, and Samtec approved of both of the solder reflow
profiles as well as the solder paste
 application process.Anyway, each of the boards use a surface mount
connector from Samtec.  When these=20
 boards go through our production testing, which includes several cycles
of hot (85C) and cold (-40C)=20
 temperatures, we find that the solder bond between the PCB pad and the
connector pin fails, which in turn
 causes the unit under test to fail.

 In researching and brainstorming to try to understand the problem and
to come up with ways that would
 improve the reliability of the soldered connections, several questions
arose that have not been brought up
 before.  At least not here anyway.

 The question is " What would the solder thickness between the pad and
the component lead be of a given=20
 surface mount component"?

 We know that the solder stencil is 6 mils thick.  For arguments sake,
say we have a solder pad that is 50
 mils by 25 mils.  We know that when the solder paste is applied, it
will cover the whole pad and will be 6
 mils thick.  We also know that the shrinkage rate of the solder paste
on reflow is about 40% (by volume).
 And we know that the pick and place machine puts the part into the
paste slightly so as to hold it into
 position while the board goes through the reflow process.  And finally,
we know that some of the volume of
 solder will cover parts of the lead edges and sides to form fillets on
virtually all four exposed sides of
 the pin. =20

 With all of that said, how would one go about calculating, or guessing,
how much solder would tend to be
 between the pad and the pin? =20

 I know there are lots of knowledgable people on this list, so I
volunteered to try to find out the answer.
 Any help anyone could give regarding this question would be greatly
appreciated.
=20

 Thanks and Regards,

 Jim



--=20
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