3D IC 微銲點與不同基材之介面反應與機械性質研究

Abstract

Current development of solder joints in flip-chip microelectronic packages has range about 100 μm in diameter. However, the diameter of a typical micro-joint is only about 10 μm in three-dimensional integrated circuits (3D IC). The volume of a solder joint shrinks by a factor of 1000 during the transition from flip-chip to 3D IC between generations. Due to such large reduction in volume, there are some unique requirements and challenge for 3D IC. One such requirement is that solder joints are confined to a much smaller space compared to solder joints in outer levels of packages. As a result, some new issues may be arisen due to such a large miniaturization of the solder volume. This research is divided into two parts. The first part studies the mechanical properties and interfacial reactions of 3D IC micro-joints. In micro-joints, two kinds of microstructures can be present in Cu/Sn/Cu sandwiches during assembly bonding process or under a long-term aging process: (1) joints with unreacted remaining solder and (2) joints fully occupied by intermetallic compounds (IMCs). According to the shear test results, the shear strength decreases with aging time in both cases. This study points out that the strength of the joints fully occupied by IMCs is much higher than the joints with remaining solder. Finally, this study generalizes a series of failure mechanisms for different microstructures. The strength of the micro-joint is very sensitive to the microstructure of IMCs. For the joints with remaining solder, the main contributing factor was the planarization of the Cu6Sn5 morphology through the aging process. For the entire joint occupied by IMCs, characterization of microstructure indicates that the void formation at Cu3Sn/Cu interface is the main factor to deteriorate the mechanical properties of the joints. The second part of this study is to investigate the interfacial reactions and mechanical properties of 3D IC micro-joints with different surface finishes under space confinement, including ENIG (Ni/Sn-Au/Ni), ENIP (Ni/Sn-Pd/Ni), and ENEPIG (Ni/Sn-Au-Pd/Ni). In the 3D IC packaging, the size of solder joints becomes relatively smaller, which can result in Au or/and Pd embrittlement as the Au or/and Pd content in the solder is too high. The phenomenon of Au or/and Pd embrittlement caused by such space-confined region in 3D IC can severely influence the microstructure of solder joints. The formation of intermetallic compound of (Au,Ni)Sn4, (Pd,Ni)Sn4 or (Au,Pd,Ni)Sn4 will appear to be a continuous layer across entire solder joints, even if the Au or Pd layer is very thin. This brittle layer presented in the joints could significantly deteriorate the reliability of the solder joints. Furthermore, the effect of Au and Pd on interfacial reaction is different. The Pd embrittlement phenomenon is much severe than the Au embrittlement when the effective Pd/Au concentration is the same (Sn0.8Pd/Sn0.8Au). The key reason is that more Ni can diffuse into the (Pd,Ni)Sn4 IMC, so the (Pd,Ni)Sn4 layer is much thicker than (Au,Ni)Sn4. To prevent such serious Au/Pd brittle effect under space-confined 3D IC construction, it is suggested that with addition of Cu into the reaction by applying Cu under bump metallization instead of using Cu added solders can successfully inhibit the formation of (Au,Ni)Sn4, (Pd,Ni)Sn4 or (Au,Pd,Ni)Sn4, and eliminates the risk of Au or/and Pd embrittlement.