John, Another solution to your problem is to use a heat-spreader with a better CTE match to silicon and the substrate. Since your substrate is alumina, and power dissipation is high, the CTE of the heat-spreader should be somewhat higher than that of Alumina, but not as high as the CTE of Aluminum. There are ways of determining the optimum CTE for a given substrate material. WCu and AlSiC heat-spreaders are CTE-tailored (with a cost premium) for that very purpose. Bottom line is that your adhesive failure problems will go away if the entire structure (substrate/die/ heat-spreader) is "CTE-balanced". With best regards, Jean-Paul _____________________________________________________ Jean-Paul Clech EPSI Inc., P. O. Box 1522, Montclair, NJ 07042, USA SMT / Packaging Reliability Consultants tel: +1 (973)746-3796, fax: +1 (973)655-0815 home page: http://members.aol.com/Epsiinc1/index.html In a message dated 98-11-19 05:31:10 EST, you write: << 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? >> ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################