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July 2002

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Fri, 12 Jul 2002 07:46:08 +0800
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HI, Joyce,

Can't think of another answer to the one you've come up with yourself.
Sounds very like you have some very high internal stresses in the heatsink
that have been relieved at soldering temperature. Clamping the heatsink and
annealing it after its manufacture should be enough to keep it flat - just
make sure it cools slowly or you'll still trap stresses.

For those heatsinks already chipped, heating them up, clamping them and
continuing to anneal them might get them back into shape, but the
tolerences may be a bit out afterwards unless you have a very good
clamp/jig and a good annealing process.

What happened to the board the heatsink was presumably mounted to for
reflow? Has it potato chipped as well?

Peter



joyce <[log in to unmask]>    11/07/2002 08:16 PM
Sent by: TechNet <[log in to unmask]>

Please respond to joyce.koo

              To:  [log in to unmask]
              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: [TN] warpage heat sink








Guys,
I got a heat sink look like potato chips after reflow using conduction hot
plate (230-240 C).  The heat sink is 1 cm thick Cu with few position screw
holes on 2 sides.  The size is 13.0x10.0x1.0 cm with 0.45 mm diagonal
height
variation at the moment (one corner point to the sky and one down to the
earth).  Any remedy for the "cure"at this stage? ?  (it is Au/Ni plated).
Anything I can do to prevent from happening besides specify the annealed Cu
must be used?  (can't reflow on the other side, it has to bond to fan by
thermal compound).  Never see anything like that.  The internal residual
stress must be extremely high!
Thanks in advance for all your help...;-(
                                                                       jk

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