We found the TSOPs we use have a high tin content solder plate or solder 
coat ( 85/15) .
The appearance of the solder is reminiscent of "solder pest", allotropic tin 
formations at low temperatures, often encounered before the addition of 
antimony in the alloy was common.
I wonder what the effects of the different tin alloys are where the two flow 
together and wonder if this causes the lines you see.
These lines look like cracks beginning to form so we pull on the leads and 
find they are usually not cracks, just some kind of boundary.
When we do encounter actual cracks, they are pretty obvious.
 ----------
From: TechNet-request
To: technet
Cc: ELOUIS
Subject: Re: cracked solder joints with alloy 42 tsops
Date: Wednesday, October 09, 1996 8:11AM

     Hi Ed -

     A question- you said "..had stress lines develop....after convection
     reflow" - is that after some type of stress testing (thermal, etc.) or
     literally right after the reflow process. If you are having "stressed"
     solder joints right out of the reflow process I suspect that you have
     a process problem or a board/layout design problem. The TechNet
     traffic has shown that there are quite a few electronic manufacturers
     that have/are experiencing cracking problems with Alloy 42 TSOPs as
     part of the stress screening.


     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator
_________________________________
Subject: cracked solder joints with alloy 42 tsops
Author:  [log in to unmask] at ccmgw1
Date:    10/8/96 2:21 PM


   I would like to send or fill out over the phone a questioneer for all
users of tsops having alloy 42 leads who have experienced problems with
stressed or cracked solder joints on pwas. We had stress lines  develop in
solder joints after convection reflow when we had bulbous solder joints.
Reducing stencil apertures to (19.7 mil pitch) 10 mil width with 50% length
using a 6 mil thick stencil rather than an 8 mil stencil gave solder joints
with heel fillets and a 50% toe fillet that survived the reflow process but
they showed stress lines after wave soldering. Hand solder repair of cracked 

joints appeared to remedy the situation.

   E.Louis
   Lockheed Martin Corp.
   11 Federal St.
    OPS-2C
    Camden,N>J> 08102
    phone#:609-338-3508
    fax #: 609-338-2575

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