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. After 1000 thermal cycles, 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. During thermal cycling, the Al pad and Ti layer between the Si chip and Cu column bumps were displaced due to thermo-mechanical stress. Based on the low cycle fatigue model, the accumulation of equivalent plastic strain resulted in thermal fatigue deformation of the Cu column bumps, and ultimately reduced the thermal cycling lifetime. In addition, the normal plastic strain of the y-direction, 822, 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. Next, the effect of Cu column height was investigated for the enhancement T/C reliability. As results of T/C test for 60 um and 85 um Cu column heights, flip chip assemblies with thicker Cu column height showed better T/C reliability. In the real time moire interferomerry, shear strain and normal strain of the x-direction was almost same regardless of Cu column height. On the other hand, the normal strain of y-direction (perpendicular direction to the Si chip) at Si chip/Cu column interface for 85 um-thick Cu samples shows significantly reduced value compared with 60 um-thick Cu samples. This relaxation of the normal plastic strain of the y-direction is the origin that thicker Cu column height guarantees better T/C reliability.