Voids formation and Cu3Sn growth mechanisms in Cu/Cu3Sn/Cu6Sn5 system under air in Cu/SnAg joints for microelectronic packaging

The technological advances reached today in semi-conductor devices and their applications increased the challenges in terms of power density. The introduction of new and high-temperature mission profiles made the integrated circuits packaging reach their limit and emerged new reliability issues that did not exist for previous generation devices. As a response to the increasing power density issues, the transient liquid phase bonding (TLPB) technique, which consists of transforming pure metals or alloys into intermetallic compounds (IMCs) with high-thermal stability, is one of the most studied techniques nowadays. The same principle is used also to increase the thermal stability of a system by the solid-state reaction (SSR) technique. The present work focus on the solid-state reactions occurring during transformation, under air atmosphere, of the initial Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu system to the final Cu/Cu3Sn/Cu system with a much higher thermal stability. The experiments were performed on interconnections using Cu pillar and SnAg solder alloy technologies. Cu and Sn-2 wt%Ag bumps with a size of 90 x 90 mu m(2) were deposited using electro-chemical deposition process with a thickness of 20 mu m and 15 mu m, respectively. In a first step, the initial system was obtained using the TLPB process in the Cu/liquid Sn/Cu system performed under air at 250 degrees C. Afterwards, the isothermal SSR process in the Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu system was studied at 250 degrees C under air for holding times from 1 to 124 h. After the experiments, cross-sections of the samples were characterized by optical and scanning electron microscopy. The growth kinetics of Cu3Sn compound was studied for samples with two initial thicknesses of Sn-Ag alloy: 15 and 30 mu m. A critical thickness of Cu3Sn layer beyond which a drastic change in its growth kinetics was observed. This phenomenon was accompanied by a significant change in the microstructure of the joint. An island-shape microstructured Cu3Sn within Cu was observed accompanied by formation of Cu oxide layers. The growth mechanisms of Cu3Sn phase responsible for different observed microstructures are presented and discussed.