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A reliability comparison of electroplated and stencil printed flip-chip solder bumps based on UBM related intermetallic compound growth properties

Authors Gong, JF HKUST affiliated (currently or previously)
Chan, PCH HKUST affiliated (currently or previously)
Xiao, GW HKUST affiliated (currently or previously)
Lee, RSW View this author's profile
Yuen, MMF View this author's profile
Issue Date 2006
Source IEEE transactions on components and packaging technologies , v. 29, (1), 2006, MAR, p. 164-172
Summary The effects of under bump metallurgy (UBM) microstructures on the intermetallic compound (IMC) growth of electroplated and stencil printed eutectic Sn-Pb solder bumps were investigated. The process parameters and their effects on UBM surface morphology and UBM shear strength were studied. For the electroplating process, the plating current density was the dominant factor to control the Cu UBM microstructure. For the stencil printing process, the zincation process has the most significant effect on the Ni UBM surface roughness and Ni grain sizes. In both processes, the good adhesion of UBM to aluminum can be obtained under suitable UBM processing conditions. Samples with different UBM microstructures were prepared using the two processes. The resulting samples were thermal aged at 85 degrees C, 120 degrees C, and 150 degrees C. It was observed that the Cu UBM surface roughness had larger effect on the IMC growth and solder ball shear strength than the Ni UBM surface roughness. The thickness of Cu3Sn and Cu6Sn5 IMC depended strongly on the UBM microstructure. However, for Ni/Au UBM, no significant dependence was observed. More likely, the thickness of Au-Ni-Sn IMC near the IMC/solder interface was controlled by the amount of gold and the gold diffusion rate in the solder. Shear tests were performed after thermal aging tests and thermal/humidity tests. Different failure modes of different sample groups were analyzed. Electroless Ni UBM has been developed because it is a mask-less, low-cost process compared to electroplated Cu UBM. This study demonstrated that the process control was much easier for Ni UBM due to its lower reactivity with Sn material. These properties made Ni UBM a promising candidate for the lead-free solder applications. © 2006 IEEE.
ISSN 1521-3331
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Language English
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