To all you tech types,
     
     I imagine this question has been seen before, but here goes....
     
     We are experiencing some problems with cracked solder joints on an 
     assembly.
     
     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 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 assemblies are then sent to our test department where 
     they are subjected to 10 thermal cycles of -54 Deg. C to +95 Deg. C at a 
     rate of 15 Deg. C. per minute (customers request).  The 
     assemblies are inspected and that is when the fractures appear.
          
     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.  
     
     My question is - What is causing the fractures and what can be done to 
     eliminate it?  I am pretty sure of the correct answer, so don't beat me up 
     too much on this one, I had to ask.  I need reassurance.
     
     Thanks in advance for your help!!!
     
     Steve McBride
     Manufacturing Engineer,
     Frontier Engineering Inc.
     (405) 624-6821

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