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February 1997

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Thu, 20 Feb 97 12:02:58 EST
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     Ted,
     Count 1 vote for using high Tg materials on any design meeting the 
     following criteria:  
     - 50+% overall resin content
     - 0.093+" thickness
     - Qty: 10,000+ vias .0135" dia. or smaller
     
     My PTV reliability simulation shows a little over 2x MTBF for 170 Tg 
     mat'l vs. 140 Tg (all other factors being equal).  It's difficult to 
     predict actual board failure probability - it's a messy Weibull 
     distribution...  But, as I'm sure you know, the resin's CTE increases 
     by 15-20x above it's Tg which imparts added tensile stress to the 
     barrel for the duration of the reflow profile, solder wave, etc.  This 
     corresponds to a little more elastic strain on the copper barrel, but 
     considerably less plastic strain (i.e., deformation, which if severe 
     enough, causes fracture).  All formulas in this simulation program are 
     based on IPC-TR-579 - a great resource.   
     
     Joe Felts
     PC World, Toronto
     


______________________________ Reply Separator _________________________________
Subject: DESIGN:  Aspect Ratio versus Material choice
Author:  [log in to unmask] at INET
Date:    2/20/97 7:06 AM


     We are redoing our specification and are wondering if anyone has design 
guidelines that take into account either aspect ratio (or drilled hole size) 
and choice of material into account.  For instance on a 0.125" board with a 
0.0135" hole( ratio 9.4), or a 0.093" board with a 0.0135"  mil hole(ratio 
6.9) do you specify to the designer use of a higher glass transition 
material that you do on the same hole size on a 0.062"        ( ratio4.6)? 
 An example would be polyimide for the 0.125 thickness, multifunctional for 
the 0.093 thickness and standard FR-4 for the 0.062 thickness or a chart 
that shows increasing thickness or aspect ratio versus what material should 
be used.  Anybody have any thoughts on this or comments?  Or is there any 
fabrication industry consensus?
          Thanks in advance for your inputs;
               [log in to unmask]  
     
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