For reliability improvement of power semiconductor modules, silver-sintering and diffusion-soldering technologies have been introduced. Both technologies can be used for die-to-substrate, substrate-to-base-plate and optionally terminal-to-die or substrate joints. Both technologies require pressure during the formation of the joint. In parallel with a pressure profile, a heating profile i.e. a temperature profile is required to set the necessary joining temperature. Die bonding tools are used for diffusion soldering which apply vertical force during die placement on heated substrates. For silvering sintering, the piece parts to be joined are placed on a heated plate and pressed by a tool with a silicone rubber interface. The silicone interface is enclosed in a tool with side walls and substrates to be joined. Under high pressure the silicone behaves like a liquid and hydrostatic pressure is achieved. Between the silicone rubber and the parts to be joined a protecting foil is typically used. The foil is deformed under high pressure and adjusts to the topology of the devices and therefore does not affect the hydrostatic pressure. Another technique involves the use of multi-die tools, where each die receives the same pressure. Again a flexible foil at the interface with the devices protects the surfaces of the devices.
For silver sintering the best joints are achieved when the silver paste, which is mixed with solvents and liquid binders, is dried and heated before die placement. Consequently the die do not adhere to the surface of the paste and can shift after placement and prior to the application of pressure. The sinter layer can be placed on the die during wafer processing. In each case the joining layer is dry and not well suited for adherence. For diffusion soldering, the substrates or die are plated with solder prior to assembly. If the soldering process occurs in a furnace or chamber the die must be placed on the substrate when the solder is below the solder melting point. Again the interface die to substrate is dry and the die do not adhere to the substrates until after placement and a temperature above the solder melting point is applied. Another possibility is to place the die into a wet paste and accepting the disadvantage in the joint. The result of such a sinter process where the die is placed in a wet paste is the formation of channels within the joint, which remain after the joining process.
USPTO Backgrounds
USPTO Backgrounds