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Operating ICs Hotter to Dissipate Power
Keywords: Operating at 175C Tj-max, High temperature reliability, Over passivation metal
Operating life shortens as the IC temperature rises; therefore, today’s ICs are designed to safely operate below 125C and to spike only briefly hotter. However, with attention to wire bonds, to mold compound plus thermal management, few silicon ICs already operate reliably up to 175C. Reliability of packaged ICs degrades because of accelerated failure at higher temperatures. Limiting IC life-span are: 1) brittle intermetallic Au/Al failure, purple plague and Kirkendall voiding 2) higher aluminum corrosion rate at higher temperatures 3) temperature above the mold compound’s glass transition results in higher package mechanical stress 4) electromigration of thin, interconnect aluminum 5) testing packages to operating temperatures + burn-in 6) few weak semiconductor devices and substrate leakages. Options are available to cool packages and to limit Tj-max of ICs to 125C. To be presented is an alternate option: electroplated, gold top metal layer over the chips’ final passivation to protect ICs. This Power Gold layer improves the reliability of ICs 6x to 100x so that engineers can reliably operate ICs up to 175C Tj-max. Gold is the most noble metal and gold pads are corrosion resistant. A diffusion barrier separates bond pad Power Gold from aluminum. Gold diffusion into aluminum is slowed by this high temperature barrier metal to avoid Kirkendall voids and to extend high temperature operating life of wire bonds. Gold wires bonded onto Power Gold pads are stronger plus the mono-metallic gold/gold bond interfaces are ductile. ICs protected with Power Gold are still functional even with package mold compound delamination following extreme MSL1 preconditioning, AATC and then autoclave. Nine microns thick Power Gold lowers device resistance, Rdson, to reduce on chip heat generation. Power Gold can carry high current to prevent electromigration of thinner interconnect metal layers plus allow both hot and cold wafer probings. Data show greatly enhanced reliability by adding Power Gold onto existing IC designs before packaging. Operating packaged ICs at 175C Tj-max versus 140C Tj-max, greater heat dissipation will deliver more power through every package/PCB. With cost of Power Gold comparable to adding one layer of interconnect metal, one can minimize exposed flag package, costly thru-vias, and/or bulky heat-sink in order to significantly lower total system cost.
James J. Wang, Founder
Power Gold
Phoenix, AZ

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