Hi Gerard,
As far as co-efficient of thermal expansion is concerned there is no
difference between BT & FR4 below their respective Tg's, the usual quoted
figure is 12 -16 ppm/degC. But CTe is not the only reason for dimensional
instability. Other things which can affect stability are:- the pre-preg
style used to make a multilayer, tension on the glass fabric during pre-preg
manufacture, incorrect heat-up or cool-down during pressing,resin content of
the finished board.
Hope this helps
Geoff Layhe
www.lamar-uk.co.uk

-----Original Message-----
From: Gerard O'Brien [mailto:[log in to unmask]]
Sent: 13 September 2001 16:34
To: [log in to unmask]
Subject: [TN] dimensional stability of substrate material


Can anyone tell me if there is a significant difference in dimensional
stability below Tg between FR4 and BT materials, in particular with regard
to HDI applications and laser drilling?

Gerard O'Brien.
Photocircuits Corporation

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