Bob,
My feeling is mixed on interposers in use with BGAs, I like interposer on flat preferably gold lands hence how do you use that with a BGA.... For me re-balling is required as the leaded assembly reflow temp profile is not high enough to properly reflow your no-lead balls.  I like leaded assembly, I like unleaded assembly, I like matte tin finish leads in leaded and unleaded assembly however the BGA ball is not a finish -it is a volume of material- and it's engineered collapse must be how it was intended.  
Rough reflow numbers 215 for leaded and 260 for unleaded.
One point is that matte tin in a leaded assembly wants to have 100% coverage of the matte tin finish; the concern in long life apps being the whisker.  The BGA refloat is not a whisker issue... although I don't want to start chasing my tail here...

Boston Brad
  ----- Original Message ----- 
  From: Bob Landman<mailto:[log in to unmask]> 
  To: [log in to unmask]<mailto:[log in to unmask]> 
  Sent: Friday, July 09, 2010 10:05 AM
  Subject: [TN] Interposer for SAC BGA


  I have to use a new FPGA from Lattice and they are not offering it in SnPb, only SAC balls.  This is a fpBGA 23mm sq with 484 balls in a square array with no special heatsink pads.

  Environment is -40 to +85C, mild vibration and shock, outside environment in electrical NEMA enclosures.

  SnPb is a must - I refuse to use lead free process for our products.

  Two choices:

  Interposer board (Bill Rollins just told me about this option 

  Reballing

  Comments please on your preference.

  Bob Landman


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