Effect of preformed Cu-Sn IMC layer on electromigration reliability of solder capped Cu pillar bump interconnection on an organic substrate

The electromigration behavior of 80μm pitch solder capped Cu pillar bump interconnection on an organic carrier is studied and discussed. In 2011, the EM tests were performed on 80μm pitch solder capped Cu pillar bump interconnections and the effects of Ni barrier layers on the Cu pillars and the pre-formed intermetallic compound (IMC) layers on the EM tests were studied. The EM test conditions of the test vehicles were 7-10 kA/cm² at 125-170°C. The Cu pillar height was 45μm and the solder height was 25μm. The solder composition was Sn-2.5Ag. Aged condition for pre-formed IMCs was 2,000 hours at 150°C. It was shown that the formation of the pre-formed IMC layers and the insertion of Ni barrier layers are effective in reducing the Cu atom dissolution. In this report, it is studied that which of the IMC layers, Cu₃Sn or Cu₆Sn₅, is more effective in preventing the Cu atom dissolution. The cross-sectional analyses of the joints after the 2,000 hours of the test with 7kA/cm² at 170°C were performed for this purpose. The relationship between the thickness of Cu₃Sn IMC layer and the Cu migration is also studied by performing the current stress tests on the joints with controlled Cu₃Sn IMC thicknesses. The samples were thermally aged prior to the tests at a higher temperature (200°C) and in a shorter time (10-50 hours) than the previous experiments. The cross-sectional analyses of the Sn-2.5Ag joints without pre-aging consisting mostly of Cu₆Sn₅, showed a significant Cu dissolution while the Cu dissolution was not detected for the pre-aged joints with thick Cu₃Sn layers. A large number of Kirkendall voids were also observed in the joints without pre-aging. The current stress tests on the controlled Cu₃Sn joints showed that Cu₃Sn layer thickness of more than 1.5μm is effecti- e in reducing Cu dissolution in the joints.