Understanding Die breakage during heavy Al wedge bonding for large/thin chip (39mm~2/75μm) in J-alloy- Cu system

摘要

TO big package was developed with large and thin chip on Copper (Cu) die pad frame and Lead-free solder in chip by chip (CbC) die attach process. Die breakage during thick Aluminum (Al) wire bond process was encountered. Failure analysis observed high magnitude of void under the wire bond area and also porosity in between the chip backside metallization (BSM) and solder layer. Several studies and analysis were conducted to prove all the contributing factors. The investigation shows Sn element from solder wire, oxide from the Cu lead frame and solder wire tend to dissolve through the molten solder and contribute to a porous mixed layer in between the chip BSM and Copper Tin (CuSn) intermetallic compound (IMC). The porosity of the mixed layer and depletion of the chip backside metallization can be exaggerated when higher thermal budget is supplied towards the bonded units during the die bonding process either due to longer machine idling time or high bonding temperature as experienced by the chip. Besides, high presence of Tin Oxide (SnO) from the solder wire during solder dispensing and spanking process is causing poor wettability towards chip backside as well as on die pad surface. This resulted in high magnitude of solder voids formation. The chip strength became weak with the present of big voids and porosity and thus not able to withstand high force during thick Al wire bonding process resulting to broken die defect. Controlling the thermal budget in die attach process towards the material and increasing the solder volume became essential to eliminate the problem.