The laminating is done before the holes are drilled and plated. The   
effect of the curing cycle on reliability depends on the board thickness,   
plating thickness, copper ductility and hole quality. The thicker the   
board, the more stress will be put on the plated hole barrels. My guess   
would be that your single curing cycle would not cause a problem unless   
the board is extremely thick and has thin plating and/or rough hole   
walls. And in that case it would probably have been destroyed by the   
soldering process. The FR4 will probably just cure some more. To be on   
the safe side, why not use 180 deg. Tg multifunctional FR4 or other   
higher temperature material to be decrease Z-axis expansion?

Nick Wander
Unisys, Roseville, MN

 ----------
From:   
 [log in to unmask][SMTP:[log in to unmask]]
Sent:  Monday, February 03, 1997 11:03 PM
To:  [log in to unmask]
Subject:  PCBs at 150oC for more than 2 hours

Hi TechNetters!

We use a multilayer pcb made from basically FR4 with a Tg of 135oC.
There are pths, blind microvias, buried vias and some fine line wiring
(only on top and bottom). One of our processing steps in building up the
assembly (after smd-soldering both sides and cleaning) is die attach
with an adhesive. The recommended curing cycle of the adhesive is 1 h
(actual it may be 2h ore more) at 150oC and is done in a convection oven
under normal atmosphere.

I am interested in the cons of above curing temperature and time. Are
there any reliability issues concerning the pcb? To my knowledge,
laminating the pcb's by the manufacturer is done at even higher
temperatures (although under vacuum).

Any comments are welcome.

Ulrich Korndorfer



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