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In this paper, the effects of Al pad contamination, epoxy molding compound [biphenyl (BP) and ortho-cresol novolac (OCN)] and wire (Au and alloy) on the propensity for the interfacial degradation of wire bond in a quad flap package under high temperature storage (HTS) tests at 125degC, 150degC, and 170degC are meticulously investigated. The interfacial degradation intends to be explicated in regards to change in surface morphology of Au-Al and alloy-Al intermetallic compound (IMC) and bonding strength as a function of HTS test. The combination of atomic force microscope and Auger electron spectrometry reveals that initial bonding strengths from wire pull and ball shear test decrease with increasing the thickness of contamination layer on Al pad, carbon and oxygen, and subsequent surface roughness. Indeed, the plasma exposure on Al pad prior to wire bonding enhances both mechanical bonding strengths up to 10% and 15%. It is found that the failure behaviors at 125degC are dissimilar to 150degC and 170degC. We first report that Sb diffused from the OCN exists at the intermetallics of Au-Al bonds, leading to rapidly deteriorate mechanical integrity. Furthermore, inductively coupled plasma mass spectrometry affirms that the OCN is a resource of Br. Above 150degC, the interdiffusion of Br and Sb from the OCN significantly impacts the integrity of Au-Al bonds. In turn, such physical degradation mechanism governed by Sb and Br can be linearly accelerated. It is also found that in the case of Au-Al bonds, the life time with the BP is much longer than that with the OCN under the given HTS tests due to less content of halogen ions. In contrast, neither Sb nor Br was found from the intermetallic layers of alloy-Al bond encapsulated with the OCN and BP. Thus, alloy-Al bonding strengths are intact even after longer stressing. With an alloy wire having Pd as an impurity, the growth kinetics of IMC fueled by Br and Sb seems to be sluggish, providing better reliability than Au wire.- - Obviously, lower flame retardants and higher are critical intrinsic material properties that should be taken into account when a new epoxy molding compound is introduced to pursue cost effectiveness without loosing reliability. Finally, upon painstaking work over a wide range of analyses herein, a quality affordable guideline for the selection of wire type associated with epoxy molding compound that can ensure the long-term reliability is presented in order to secure reliable supplyline management as well as package assembler.
Components and Packaging Technologies, IEEE Transactions on (Volume:30 , Issue: 4 )
Date of Publication: Dec. 2007