All,
     
     Stick with me on this...it gets kind of lengthy....
     
     About a year ago, I posted a problem to the Tech-net regarding
     solder fractures.  At the time I got a lot of information on the subject.
     Enough to help guide me in the right direction.
     
     The PWB is a Polyimide material, .070" thick with 12 layers.  All layers 
     are 1oz copper.  The PWB has a bonded stiffener, 6061-T6 aluminum @.040" 
     thick.  Board dimensions are 6" x 3.875".  
     Some components are placed over the stiffener, some are not.  The 
     assemblies are prepared with straight through lead termination's and are 
     wave soldered, some post assembly performed, cleaned, and inspected.  All 
     the assemblies exhibit good wetting and generally good solder 
     characteristics at this point.  
     
     The fractures all appear on gold plated, side brazed lead DIPs that are 
     sitting directly on the stiffener.  The fractures occur mostly at the lead 
     fillet, but occasionally happen near the pad fillet  Also worth noting is 
     that the fractures occur on the side of the leads (the thin portion of the 
     lead) with the more severe ones occurring near the ends of the DIPs and 
     lessening as you move toward the middle.  
     
     It turns out that because of the differences in Cte between the PWB and the 
     bonded stiffener, the board is warping during wave solder.  This warp is 
     basically soldered in place which places the solder under constant loading 
     as the board tries to return to it's room temperature dimensions.  The side 
     brazed DIPs do not provide sufficient stress relief and eventually the 
     solder fatigues and fractures at the weakest point.
     
     Get to the point...what is your question you ask.  Is there a way to 
     eliminate this warp during wave solder or will attaching another material 
     to the PWB only make things worse??  No we can't get rid of the side brazed 
     DIPs because they are a military part only manufactured in that 
     configuration. 
     
     Any help you can provide will be greatly appreciated.
     
     Steve McBride
     Frontier Electronic Systems
     P.O. Box 1023 Stillwater Ok.74076
     
     [log in to unmask]
     (405) 624-5281

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