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Effect of Bond Pad Thickness on Reliability of Copper Wire Bonds under Extreme Thermal Conditions
Keywords: Wirebonding, Reliability, Thermal Cycling
It has been found in this study that the shear strength of copper wire bonds in commercial microelectronic packages subjected to thermal cycling shows an initial increase, with the growth of first IMC phase, CuAl2, during long dwell thermal cycling and then drops with the onset of growth of the second phase, Cu9Al4, irrespective of the part. In addition, a correlation was found between aluminum pad thickness and the growth of Cu9Al4. It was observed that the second phase of intermetallic became significant only after the first phase (CuAl2) consumed all of the aluminum in the bond pad. This observation led to the hypothesis that thicker bond pads could prolong the onset of Cu9Al4, which is the detrimental phase, thereby increasing time to failure of copper wire bonds. To verify this observation, QFN test packages with seven different pad thicknesses, and copper wire bonds made in the lab, were aged at different temperature conditions to obtain the activation energy for the Cu9Al4 phase formation. Some of the devices were removed at specific intervals to perform cross-sectional analysis and shear strength measurements to verify the effect of bond pad thickness. In addition, interfacial fracture toughness between Cu-Cu9Al4, Cu9Al4-CuAl2 and CuAl2-Al is measured using a nano-indentation technique to prove that the fracture location is at Cu9Al4 and Cu, making it the detrimental IMC phase in the Cu-Al wire bond system.
Patrick McCluskey, Professor
University of Maryland, College Park
College Park, MD
United States

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