Interconnections of Low-Temperature Solder and Metallizations

Abstract

The development of solder is from Pb-bearing to Pb-free solder because of toxin. Nowadays, Sn-based solder is usually chosen as the prime materials in electronic packaging. However, the melting point of Sn-based solder is higher than Pb-bearing solder about 40 °C. High process temperature causes the side effects on devices such as warpage, crack and damage. Requiring the melting point of solder is below 200 °C to overcome thermal budget. In is a good candidate to replace Sn owing to its good mechanical properties. Recently, a Cu/In structure is developed for 3D integration applications. Nevertheless, fewer researches investigate low temperature reaction between Cu and In. Previous literature of Cu-In interfacial reactions focused on thick In layer and high temperature reactions. Studying thin In layer and low temperature reactions are important for 3D IC integration. Microstructure evolution of Cu-In compounds at low-temperature are regarded as useful database for interfacial reaction. This study not only concentrates on Cu/In reaction but also discusses deeply the reaction between Cu, In and Ni. Electroplating In layer on Cu metallization and subsequently electroplating Ni on In. The electroplating procedure is one of the major challenges because of the rapid diffusion of Cu and In. Microstructure observation are from 100 °C to 140 °C. The samples were mounted in epoxy resin and ground with SiC paper and Al2O3 powders. The reaction zone was observed using an optical microscope and a scanning electron microscope (SEM). The compositions of Cu-In and Ni-In compounds were determined using an electron microprobe (FE-EPMA). The main results were as follows: (1) (Cu,Ni)In2 formation after electroplating, (2) (Cu,Ni)In2 is stable at 100 °C, (3) (Cu,Ni)11In9 is dominant phase at 140 °C, (4) Microvoids within (Cu,Ni)11In9 and (Ni,Cu)3In7 formation at 140 °C. The result provides detailed Cu-In and Ni-In compounds development and key findings related to the reliability of In-based solder. This study presents the growth of intermetallic compounds in Cu/In/Ni scheme resulting from aging at 100 °C and 140 °C.