Tg being the transition temperature where CTE (commonly designated as alpha
1) shifts to a higher value, FR4 Tg is no longer 14 or 15 ppm past its Tg.
The CTE past the Tg (alpha 2) should be listed on the material datasheet.
Higher CTE will cause more reliability problems than a lower CTE depending
on the operating temperature at a site or for the system. To determine the
effect of Tg, operating temperature needs to be measured or simulated.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bryan Montney
Sent: Tuesday, November 11, 2008 1:49 PM
To: [log in to unmask]
Subject: [TN] Tg specification

I'm looking for a guideline or resource (IPC spec?) on how to select and
specify the proper Tg for a given design? I'm not talking about process wise
(Tg140 is acceptable in our lead-free processes), but design wise.
For example, is Tg140 material sufficient for a board running 80amps through
it and sitting in an enclosure on a hot summer day in Arizona?
Also, I'm looking for how best to test the Tg robustness during production.
Any help would be greatly appreciated. 

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