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Break Down Failure Process of the Flip Chip Bumps Caused by Current Stressing
Keywords: electromigration, Pb-free-solder, bump reliability
Controlled collapse chip connection (C4) has been used to connect Si chip and organic substrate in flip chip packaging structures. As flip chip solder bumps move to scale down, electromigration is one of the most important reliability issues in the bumps. In this work, we investigate the failure process of the C4 bumps under current stressing. C4 bumps used for the electromigration experiments in this work consisted of Cu post and SnAg3.0Cu0.5 solder. Under bump metallurgy was electroless Ni(6um)/Au(0.48) on Cu pad. Electromigration tests were curried out on the condition of electron flow that direction was from organic substrate side (Cu pad) to chip side (Cu post) with current density of 4.0E+4A/cm2 at 165 deg.C. Current stressed samples at various periods of time were cross-sectioned and examined by scanning electron microscopy (SEM) and electron probe microanalyser (EPMA). As a result, the failure process of the C4 bumps under current stressing was found to be composed of several stages. As the first stage, electromigration of Au was significantly occurred in a few hours. In the second stage, Cu6Sn5 intermetallic compound (IMC) formation was enhanced on the surface of the Cu post and the IMC was reached to the surface of the under bump metallurgy. In the third stage, this stage was frequently occurred at the same time of the second stage, Ni diffusion into Cu6Sn5 IMC and the formation of (Cu,Ni)6Sn5 IMC were occurred. In the last stage, narrow gaps and voids were formed at the intra- and interfaces of (Cu,Ni)6Sn5 IMC. Consequently, C4 bumps resistance increased abruptly. It will be discussed that the failure process of C4 bumps under current stressing is affected by high anisotropic behavior of tetragonal crystal of Sn and Au thickness of under bump metallurgy.
Shigeaki Suganuma,
Shinko Electric Industries Co., Ltd.
Nagano-shi 381-0014,
Japan


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