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October 1997

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From:
Richard MacCutcheon <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Thu, 16 Oct 1997 14:08:00 -0600
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======== Original Message ========
We are attempting to attach a nickel-plated copper heatsink to a 10
layer PWB per a set of manufacturing drawings that only states that "the
heatsink shall be attached with prepreg".  It does not call out a
pressure, temperature, cycle time, or a type of prepreg.  We have
assumed that we would need a "low-flow" type of prepreg.  We have tried
some sample cards with a low-flow prepreg (PC-GF 108V by Westinghouse
Fortin) that we have on hand;  we have had good registration of the
heatsink to card, but the adhesion has not been as good as we would have
liked, and the heatsink seems to have been pressed "into" the board (you
can feel the outline of the heatsink on the backside of the card).  We
ran the press at the lowest possible pressure we can for our press (15
lbs).

We are looking for suggestions from anyone who has experience with
laminating heatsinks to PWBs.

Thanks,

Marty W. Sheppard / LYPME
Manufacturing Engineering Support
(912)-926-9632 DSN 468-9632 Fax: (912)926-7974
email:  [log in to unmask]

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======== Fwd by: Richard MacCu ========
For a bright nickle plated heatsink a low flow epoxy prepreg can be used but
there may be some things you need to try.
If the nickle is thick enough, lightly abrade the bond side with 600 Grit
sandpaper to roughen the surface.  Clean with MEK or toluene. No alcohol or
other degreaser these can leave a film on the surface which can act as a
release agent.  Make sure the package reaches the cure time and temp for
adequate bonding but don't over stay.  The material you mention can stand a
hot press start of 320F, load and keep in the press for 30 mins.
The prepreg needs to see about 15 to 20 mins above 300F to get acceptable
cure/bond strength.

As for the telegraphing of the heatsink into the board. First, accurately
calculate the surface area to be bonded and ensure you are at 15lbs per sq
in.  You may want to reduce to 10 lbs per sq in. If you are over that, place
dummy material of the same thickness in the stack to reduce the force
pressure on the board.  If the press is manually controlled monitor closely,
at the lower pressure when heat causes the stack to expand it increases the
force pressure quickly.  If the heatsink is .020"-.030" thick you should get
good conformity at 10lbs with little effect on the board.

Use a thermocouple for temp monitoring if possible.

Are you using a press pad?  Silicone, expanded paper?  Use on the heatsink
but not under the board since it can cause more telegraphing.

Good Luck

Richard MacCutcheon

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