Fluxless bonding of silicon to copper with high-temperature Ag-Sn joint made at low temperature

A fluxless bonding process between silicon and copper with high-temperature Ag-Sn joint is developed by low temperature bonding using Ag-Sn-Au multilayer composite structure. The copper substrate is plated with a thick silver layer as stress buffer. We bond 5mm, x 5mm silicon chips onto Ag/copper substrates using electroplated lead-free Ag-Sn-Au solder. Si chips were produced with two different kinds of Under Bump Metallization (UBM) and both are compared. i.e. Si/Cr/Au and Si/Cr/Ni/Au. Electroplating method becomes an attractive deposition technique in that thicker films can be fabricated. It also has an economical advantage over vacuum deposition technique. To achieve high quality joint with few voids, a fluxless bonding process is developed in vacuum environment (50 militorrs) to suppress tin oxidation. Comparing to bonding in air, the oxygen content is reduced by a factor of 15,200. Nearly void-free solder joints are made. Using Cr/Au UBM structure, initial joints comprising of three distinct layers of Sn-rich layer, Ag3Sn intermetallic compound, and Ag layer have been achieved. Newly proposed UBM structure with Si/Cr/Ni/Au bonding samples show a very high quality joint and becomes to have higher re-melting temperature joint by proper annealing step. Microstructure and composition of the joint are studied using optical microscope and Scanning Electron Microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). This technique becomes an innovative success for overcoming the very large mismatch in thermal expansion between silicon of 3x10(-6)ppM/degrees C and copper of 17x10(-6)ppm/degrees C. To evaluate the reliability of the solder joint and the bonded structure, samples will go through thermal cycling test and failure modes will be evaluated in the future. Microstructural changes of the solder joints during thermal cycling test will be investigated and assessed.