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February 2000

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Subject:
From:
"Louis, Edwin @ CSE" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 29 Feb 2000 12:54:17 -0500
Content-Type:
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After some problems here with cracked solder joints on Alloy 42 leaded TSOP
devices, our stress analysis expert did some calcxulations And found that
the stress were above yield strength for the SN63 solder but not by much. We
sent some of the devices back to the manufacturer for their testing and they
found solder joint pull forces necessary to break the lead/solder bond to
be between  0.33 Kg and 0.78 Kg. The former is 0.726 lbs  and the latter is
1.716 lbs. We are talking about fine pitch 19 mil.
Some one in  the assembly business said that between 1.0 and 1.5 lbs pull is
expected. These alloy 42 leads had a tin/lead (high lead composition)
plating which was not reflowed. My experience shows has been that tin/lead
plating has to be reflowed in order to be
a protective film. This type of plating is very porous and the lead oxidizes
easily. I hear reports from some quarters that alloy 42
TSOPs are used successfully and others say never use alloy 42 TSOPs they
don't work. We encountered a manufacturer who used identical TSOPs as we
used but did not complain about any problems. The difference was that they
assembled the parts as soon as they arrived. We did not. Our solder joints
showed cracks at the heal hours after soldering. Reflow of our stressed
joints
manually, using RMA flux, alleviated the problem. I am thinking here that
the alloy 42 TSOP problem is caused partly by solderability
and partly by coefficient of expansion mismatch. I have heard that some
people are changing to palladium or palladium/silver
plating to overcome solderability problems. Can some one tell me where to
learn about these platings and are they worthwhile
pursuing.

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