Thermal stress analysis of 3D die stacks with low-volume interconnections

Silicon die stacking with low-volume interconnections is an attractive method for 3D integration. It offers such benefits as extension to fine-pitch integration, increased vertical heat transfer and hierarchy for repeated thermal processes without re-melting. The process uses low-volume solder to form joints of few microns high. The low-volume solder mostly forms intermetallic compounds with underlying metals. The formation of intermetallic compounds increases the strength of the solder joints. However the joints formed by intermetallic compounds can be brittle and less resistant to mechanical shocks as compared to the joints mostly formed by pure solder. The joint´s mechanical properties play an important role in the system´s reliability. Therefore in-depth evaluations of joint´s mechanical properties are crucial to further advance this technology. We considered two metallurgies with different mechanical properties for interconnections between silicon dies: Cu/Sn and Cu/Ni/In. Earlier article reported that the Cu/Sn joints has a higher shear strength than the Cu/Ni/In joints. However the Cu/Ni/In joints showed better a result in the impact shock testing. In this report, we conducted the thermal cycle tests on the silicon die stack systems with the two joint metallurgies. The thermal cycle tests showed that the Cu/Ni/In joint systems have less failures than the systems with Cu/Sn joints. The energy dispersive X-ray (EDX) analyses of the solder joints after the 2250 cycles of thermal cycle tests showed that the CuSn intermetallic compounds dominate the Cu/Sn joint whereas the region of mostly pure indium region still remains in the Cu/Ni/In joints even after the tests. We also conducted a finite element analysis of the Si die stack with the Cu/Sn joints on an organic substrate. The analysis showed that increasing the Si interposer thickness can reduce stresses in the intermetallic compound joints.