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From [log in to unmask] Fri Jan 24 09: |
04:09 1997 |
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<"9OQQj.0.9tM.rQ9wo"@ipc> |
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Hi Steve -
No, you are not oversimplifying the problem. It's just that getting
the "rework" portion of the recipe right is not very simple. A fair
share of the industry looks at solderability problems as having two
main source causes: oxidation and intermetallic growth. Chris Hunt at
NPL has some really good info on intermetallic growth and growth
prediction equations - check out the 96 SMI proceedings for his paper.
The area I see lots of misconceptions is how to restore an oxidized
solder finish. If you had a thick solder finish that is just plain
oxidized without any copper/tin intermetallic complications then a
controlled process reflow with the right flux chemistry will make
things solderable again. If you have a thin solder finish that has
converted (via diffusion) itself into a copper/tin intermetallic you
need to use a very aggressive flux to attack the intermetallic oxide
and then build up/deposit a reasonable thickness of solder (say 60-120
microinches). If you just re-process the thin solder scenario without
increasing the solder thickness the copper/tin intermetallic just
reforms and oxidizes again -now you are back at square one. If I would
quickly (within a few days) process these thin solder scenario boards
in my assembly line with the right flux chemistry I would have
adequate yields - solderability goes down hill if the boards sit
around.
That's why you will hear some camps say a reflow will restore an
oxidized board (the JIT type of processes) or hear some camps say a
reflow won't restore a board (the long storage time processes). There
are some new restoration/fluxless solder technologies that are in the
work too (e.g. ROSA, PADS etc.) that may help everyone in the future
if they prove viable.
Dave Hillman
Rockwell Collins
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: Re: "Tinning house"
Author: [log in to unmask] at ccmgw1
Date: 1/23/97 11:35 PM
Dave,
I have been following your discussion on solderability (root causes of
excessive oxidation) and am lead to believe that if porous/thin tin leads are
"reworked" properly by these tinning houses we could avoid the many inventory
problems (storage time, slow moving parts, bagging with desicant, etc.) and
still maintain or improve solder connection reliability.
The eventual/ultimate solution being to get the OEM's with poor tinning
corrected or off the approved supplier list.
Am I over simplifying with this conclusion?
Steve Ross
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