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Power QFN down bond area delamination mechanism study
Keywords: Power QFN, down bond, Contamination
In one customerized PQFN device, the down bond wire (used to provide power to control die) heel crack was eported in both module test and field application. SAM revealed severe delamination between mold compound and leadframe down bond tab. To further analyze the down bond area surface condition, XPS survey scan was conducted on the down bond area after mechanical decap. Narrow scanning of Si reveals there is a significant chemical shift of binding energy for Si2p this leads to the hypothesis that the contaminant on leadframe down bond surface is poly-siloxane since leadframe tape is an Silicone based PSA. The leadframe surface contaminants were extracted followed by FTIR spectrums analysis which showed the contaminant found on down bond area matches well (90%+) with the IR spectrum of PQFN leadframe tape which is the representative of PDMS (Poly Dimethyl Siloxane). The key ingredients of power QFN tape are silicone oil, MQ resin besides organic solvent, Silicon oil ensures good wetting and MQ resin determines the strength of adhesion. Due to its relative lower molecular weight, silicon oil can be outgassed out of the tape during wire bonding, leaving a thin layer of silicone contaminant on down bond surface. The distance of PDMS molecular is relatively larger as 3 hydrogen atoms on methyl occupy larger space under the rotation of methyl. Hence the molecular reaction force is weaker, under the thermo-mechanical load, the silicone contaminant on down bond surface breaks inside bulk layer (cohesive break), leading to delamination between compound and leadframe. All the data drives the conclusion that key contributor to T0 PQFN package delamination is the poly-siloxane outgassed from leadframe tape. Replacing the tape to Acrylic removes the source of contaminant, the post electrical test down bond delamination shows significant reduction, with the other improvement of assembly process, the power QFN device successfully passed TC500 reliability test without down bond failure.
ZhiJie Wang , Assembly packaging and material engineering manager
Freescale Semiconductor (China) limited
Tianjin , Tianjin

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