Hi John,
In a message dated 11/19/98 5:39:48, you wrote:
>Hi Werner
>
>Thanks for replying.  My question relates specifically to a heatsink bonded
>to the rear surface of the silicon die on top of a Motorola Power PC (which
>uses IBM C4 technology) on a ceramic BGA substrate.  The silicon surface
>area available for heatsink attach is approximately 10mm square and the
>chip power dissipation is 7W.
>
>For previous generation (non BGA) products we used adhesive thermally
>conductive tape bonded to the top of the ceramic pasckage, which worked
>fine as the surface area was about 30mm square. With the BGA package
>structure the bondable area is down to 10mm square but the heatsink has to
>stay big (20mm square) to get the required heat dissipation.
>
>Motorola sort of recommend mechanically clipped heatsinks, but these are
>not practical in our application due to constraints of size and
>availability, leaving us for at least the short term with the need to find
>an adhesive that can cope with the mismatch in CTE.  Tape does not give
>enough bond strength.
>
>We are doing trials at present with Berqquist LA1000, a thermaly conductive
>low modulus adhesive, which gives good bond strength.  The manufacturer
>offers versions with glass beads (4.9mil or 7mil) for control of bond line
>thickness, which may or may not be useful.
>
>We would like to know more about the long term reliability aspects of this
>adhesive. Does anyone have any data in similar applications?
>
>Regards
>
>John

I do not think, that long-term reliability is a problem given your details.
Whether you need a 4.9 or 7 mil bond line thickness with a low modulus
adhesive needs to be determined with testing. If a 7 mil thickness does not
increase the thermal resistance too much I would say go with it. Run a test to
maximize loading by heating the AL-heatsink simultaneously with the die to
exaggerate the thermal expansion mismatch; if the assembly survives I think
you have a viable design. Fatigue is really not an issue with these materials
(silicon, low modulus adhesive, Al).

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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