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Date: | 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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