Dick,
I refer to the region at the intersect of rigid and flex as the dead zone for vias.  Some of the worst effects I've seen are thermal cycled vias straddling this line.  Between terminated glass ends, excessive uncontrolled adhesive, C-Stage, B-Stage, manual lay-up, flexure stress... did I miss anything?  Oh yeah the stress happens in the center vertical of the PTH; well half of it...  Rigid Flex design 101 needs to start off with "DON'T PUT VIAS HERE", I believe it is the single most important issue on rigid flex.
Are PTHs near the rigid edge suspect (not near the flex intersect)?  Hand solder a couple of those, I'd wager they take the heat.   I enjoy a good hand soldering bashing as much as the next SJR person but I suspect you've got a design/layout problem.  If the design "can't" be changed get the fab house to back off the coverlay out of the rigid area a bit.  

That interface area is worthy of a very close look.  If you have actual numbers I'd be happy to review.  The 2223 added a paragraph stating to keep vias out of this area put they made it a bit large. 
And while soldering in that area don't slam any doors, the sudden vibe could be harmful.

Brad

----- Original Message ----- 
  From: Richard D. Krug<mailto:[log in to unmask]> 
  To: [log in to unmask]<mailto:[log in to unmask]> 
  Sent: Monday, July 26, 2010 10:59 AM
  Subject: [TN] Hand Soldering on Polyimide Rigid-Flex Assemblies


  We assemble limited quantities of polyimide rigid-flex assemblies.  Due to moisture absorption by the polyimide substrate, we perform a bake prior to mass soldering processes;  either reflow, wave or single point selective soldering.
  We have recently found some vias near hand soldered locations on the rigid portion of the substrate that are open.  Cross sections reveal the opens are at the interface between the rigid substrate and the flexible substrate.  We have not specified a bake prior to hand soldering components on the rigid portion of the rigid-flex assembly.  We are using Sn63Pb37 solder.

  Is the open caused by TCE differences between the rigid and flexible substrates or by moisture turning to steam?
  Is it necessary to bake polyimide rigid-flex substrates prior to hand soldering?
  Is this inherent in all polyimide rigid-flex substrates or do we have a weakly bonded substrate?

  Dick Krug, CSMTPE
  SMT Process Engineer
  Sparton Corporation
  30167 Power Line Road
  Brooksville, FL  34602-8299
  p (352) 540-4012  (Internal Ext. 2012)
  [log in to unmask]<mailto:[log in to unmask]>


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