Peter,
Our greatest concern with the glass transition temperature (Tg) of
dielectrics is that their temperature coefficient of expansion sharply
increases at about that temperature:
delta-length/length
! TCE of dielectric
! /
! /
! /
! /
! /
! /
!-------------------/
!---------------------------- TCE of metal
+-------------------+------------ temperature
Tg
The TCE of copper and other metals is lower and roughly constant at
these temperatures. So exceeding Tg greatly increases the stress on
vias and plated-through holes, and thus the chances of cracking the
copper. The result is a dead product, or worse, one that gets
intermittent at high temperatures...
Many authors recommend keeping the maximum temperature at the printed
circuit board (PCB) 5-10C below Tg for reliability. Another approach is
to plate more than the usual 0.001 inch (25.4um) of copper in the plated
through holes, to greatly strengthen them against Z-axis stress.
IPC-TM-650 Test Methods Manual test method 2.4.25 tells how to measure
glass transition by differential scanning calorimetry (DSC). Test
method 2.4.24 tells how to measure it by thermomechanical analysis
(TMA). From what I've read, these usually give results within a few
degrees C of one another.
I am writing a book for Kluwer, Robust Electronic Design Reference.
Table D-2 in my manuscript lists the range of glass transition
temperatures and the recommended operating ranges for some 40
dielectrics used in PCB's, gleaned from the engineering literature. And
I am still looking...
John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE
dBi Corporation
http://www.dbicorporation.com/
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