The impact of process sequence on the quality of pressurized sintered silver joints

Sintered silver is being increasingly recognized as an ideal method of joining in situations where a relatively low temperature process is required to form a bond that can survive service at elevated temperatures, e.g. in the mounting of power semiconductor dies. In the optimization of this process much consideration is given to the parameters of the sintering stage but the integrity and reliability of the bond achieved during sintering can also be affected by the preheat parameters. Since the goal is minimum organic residues in the finished bond it is essential that excess solvent added to create the rheology required for the application method, e.g. printing, dispensing, be removed before sintering begins. However, the solvent also plays a role in holding the particles together to maximize interparticle contact, imparting tackiness to hold the die in place during handling and protecting the particles from oxidation. To achieve the best result the optimum level of residual solvent has to be achieved during the preheat stage to provide the best preparation for the sintering stage of the bonding process. An important consideration is whether the preheat is conducted before or after the die is placed since that affects the ease with which the solvent can evaporate. In the experiments reported in this paper the effect of temperature and time on the final bond strength and microstructure was determined for preheat conducted before and after die placement. The conclusion was that the best bond strength and a wider process window could be achieved if the silver paste is preheated before die placement.