Studies on the Thermal Cycling Reliability of Fine Pitch Cu/SnAg Double-Bump Flip Chip Assemblies on Organic Substrates: Experimental Results and Numerical Analysis

A thick Cu column based double-bump flip-chip structure is one of the promising alternatives for fine pitch flip-chip applications. In this study, the thermal cycling (T/C) reliability of Cu/SnAg double-bump flip-chip assemblies was firstly investigated and the failure mechanism was analyzed through correlation of T/C test and the finite element analysis (FEA) results. In addition, the effect of Cu column height was investigated for the enhancement T/C reliability. The T/C failure site was the Cu column/Si chip interface, where was identified via a FEA as the location of the maximum stress concentration during thermal cycling. In the T/C test, the Al pad and Ti layer between the Si chip and Cu column bumps were displaced due to the accumulation of equivalent plastic strain. The normal plastic strain of the y-direction, ¿₂₂, was determined to be compressive and was a dominant component in relation to the plastic deformation of Cu/SnAg double-bumps. As the number of thermal cycles increased, normal plastic strains in the perpendicular direction to the Si chip were accumulated on the Cu column bumps at the chip edge in the low temperature region. Thus it was found that displacement failure of the Al pad and Ti layer, the main T/C failure mode of the Cu/SnAg flip-chip assembly, occurred at the Si chip/Cu column interface by compressive normal deformation during thermal cycling. Furthermore, flip chip assemblies with thicker Cu column height showed better T/C reliability.