Hi Ken,

This is a very interesting problem. I can't say I've come across this complaint before. I guess it is possible that there is a torque limit before you start to introduce micro-fractures in the glass fibers that may be close to the surface. If that were the case then subsequent heat applications may just " push it over the limit " and make the fractures visible. But that is just a guess. Does the mounting holes have a  surface pad?  If not, perhaps that may just solve your problem. Or as a minimum hide it. You would have to evaluate any potential reliability issues. 

Sorry I couldn't be more helpful. 

Ed Cosper

----------
From:  Chafin, Ken G.[SMTP:[log in to unmask]]
Sent:  Wednesday, June 24, 1998 3:38 PM
To:  [log in to unmask]
Subject:  [TN] Measling/Crazing

We have a FR-4 board (11" by 8" by 0.093") on which is mounted a large
sink beneath two power transistors.  When the screws which secure the
transistors to the heat sink and to the board are torqued to 5
inch-pounds, significant measling beneath the heat sink (visible from
solder side) occurs when the board is flow soldered.  When the torque is
reduced to 3 inch-pounds the measling disappears.

Does the above information suggest there is a problem here beyond a
simple process problem which is addressed by reducing the torque from 5
inch-pounds (which the drawing specifies) to 3 inch-pounds?  (For the
record, I don't know if this should be called measling or crazing--both
the mechanical and thermal stress are required to produce the
phenomenon.

It seems odd that the relatively small amount of torque change could
have such a dramatic effect--if the material is within specification
limits and the flow solder process is close to optimal.

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