Processing and characterization of a 100% low-temperature Ag-sintered three-dimensional structure

Due to RoHS restrictions, researches on lead-free packaging have increased over the past decade. Low Temperature Joining Techniques (such as silver sintering or Ni-Au Transient Liquid Phase Bonding) are particularly studied because they are processed below 300°C and the attaches obtained are reliable at high temperature. Silver paste sintering technique for die backside attach is now a well-known technology used in both industry and academic research. The objective of this work is to adapt the procedure related to conventional silver paste sintered assembly to produce a three-dimensional module. For such purpose, this paper presents some theoretical considerations on silver sintering in order to briefly introduce the technique. Then, sintering procedures for realizing both die backside attach and three-dimensional structure will be described. Experimental results (electrical and mechanical tests, cross-sections, thermal measurements) will be given and performances of the double-side structure will be compared to die backside attaches and with lead-free solder. This work demonstrates for the first time the feasibility of a 100% silver sintered three-dimensional module using silicon diodes and Ni-Au substrate. Mechanical properties of the two attach-packages are better than classical solders and the performances of each attach are similar to silver sintered die backside attach. These promising results open the way to the use of silicon devices for high power density assemblies because the dice will be cooled down on both sides. This technology just requires silver metallization on the two sides of the device and can be extended to new materials such SiC or GaN for high temperature applications.