Nano-Cu Sintering Paste for High Power Devices Die Attach Applications

A nano-Cu pressureless sinterable paste was developed for joint formation applications. The material was further engineered in both chemistry and process for enhanced oxidation resistance. This oxidation resistance capability allowed the paste exhibited a shelf life of 6 months when stored at -10°C. The paste was insensitive to either sintering atmosphere or surface finish type, and consistently exhibited a joint shear strength around 13 MPa. Nano-Ag pastes were observed to be sensitive to surface finishes, and performed particularly poorly for OSP-OSP system. The shear strength of nano-Cu was sensitive to sintering temperature and time, with shear strength increased from 220°C/15 min, 220°C/30 min, to 240°C/15 min. The joint shear strength deteriorated with increasing TCT (-40°C/150°C) cycle number. After 3000 cycles, the ranking of shear strength can be shown below: Nano-Cu > Ag-N > Ag-H2 > Ag-H1 > Ag-K > Ag-D. The high Pb joint performed the best. The microstructure of Nano-Cu joint formed at 240°C/15 min sintering condition indicated a continuous sintering progress with increasing TCT cycles, with porosity diminishing and smooth area increasing. This strongly suggests that either a higher temperature or a longer time than 240°C/15 min is desired for improved sintering extent.