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Cu Wire Bond Reliability Improvement Through Focused Heat Treatment after Bonding
Keywords: copper bonding, localized heating, bonded ball shear strength
Currently, there is a great interest in applying copper material to wire bonding of fine pitch assembly packaging particularly to improve reliability through minimizing the deterioration of bonded ball shear strength after thermal aging as well as cost saving purposes. However, since copper is much harder than gold, coupled with the work hardening effect of the USG power and force during wire bonding have caused bond pad crack and pad cratering, especially on sensitive pad devices. Such problem can not be resolved by common conventional solutions including the use of heated forming gas to soften the FAB (Free Air Ball) and heating the die during the bonding process by varying the work holder (pedestal) temperature. This study introduces localized heat at bonded ball region with at least 250 degree C after wirebonding to enhance the intermetallic adhesion strength between the copper to the Al bond pad. At such high temperature, localized heat is needed instead of oven baking so as to prevent organic substrate decomposition. This solution facilitates for lower USG power during wire bonding, thus, a lower impact (softer bonding) of the Cu FAB to the sensitive pad structure that has been proven to prevent bond pad crack and pad cratering. The preferred embodiment utilizes a local heat source targeted at the bonded ball region after completing the wire bonding with minimum USG power. The focused heat source will accelerate the IMC growth of Cu-Al to enhance the strength and integrity of the bond. This solution has been proven successful through actual experimental results on a sensitive pad device with focused heat source at 250 degree C for 15 mins, 30 mins and 45 mins. Result showed significant increment in ball shear strength as heating duration increases, while no crack was found on the pads after confirmation with pad etching.
Yow Kai Yun, R&D Engineer IV
Freescale Semiconductors Malaysia Sdn Bhd
Petaling Jaya, Selangor 47300,
Malaysia


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