New developments in high performance solder products for power die assemblies

The use of special purpose solder pastes in power die attach is well-established offering low voiding and reliable bonding in volume manufacturing. However these materials are designed around high lead alloys and applied by dispensing. IGBT circuits are made by printing high tin alloys to multiple die sites, placing die and reflowing in a process more similar to conventional PCB or hybrid thick film assembly. This paper describes how the opportunity was taken to make use of the latest developments in Pb-free SMT flux technology and re-optimize them to the different requirements of IGBT die attach. We rehearse the attributes and requirements of IGBT circuitry and then go on to show how a high performance Pb-Free solder paste has been developed to meet the requirements of large power die attachment (LDA) in IGBT module manufacturing processes. The paste has excellent print and handling characteristics and routinely returns less than 0.5% voiding under large die over a wide range of vacuum reflow conditions. The flux vehicle chemistry offers ease of cleaning to be compatible with the next stage processes of wire bonding & circuit encapsulation.