Process Development for Heat Sink Attachment Using Thermally Conductive Liquid Adhesives

Premature field failure of heat sink attachments is one of the most important concerns in the microelectronic packaging industry. In order to minimize failure problems, this study focused on the optimization of the heat sink attachment process and development of an adhesion inspection method. Acrylic-based liquid adhesives were selected for this study to deliver mechanical adhesion strength and high thermal performance of heat sink attachment. Optimal process conditions for volume, mounting pressure, and pressing time were obtained from volume consistency testing, torsion testing, and thermal resistance measurements. Failure analysis was conducted on fracture surfaces after torsion testing. Two failure modes were discovered: interface failure and bulk materials failure. Bond line thickness greater than 254 microns (10 mils), produced more materials failure, which caused reduction in torsion strength. Maximum strength and thermal performance were obtained using contact area of 50% or greater and bond line thickness of 203-254 microns (8-10 mils). Temperature cycling results will also be presented. Torsion test screening was selected to verify heat sink attachment integrity during real-time quality assurance. The torsion strength of a whole soldered package joint was calculated from the nominal shear strength of individual solder joints using the total area approximation. This calculated torsion strength for the package sets an upper limit to torsion testing values.