Low temperature die-bonding with Ag flakes

Operating temperature of power semiconductors is expected to rise beyond 150°C especially such for GaN or SiC. Die-bonding must maintain good reliability in such high temperatures. Currently, the conventional Ag-epoxy conductive adhesives and Pb-free solders have been used for current Si die attachment. They are, however, difficult to satisfy the requirements for such ultra-heat resistant uses. In order to develop of ultra-heat resistant die-attach technology at low temperature, new die-bonding material was proposed, i.e., nano-meter thin Ag flakes. The Ag flakes have large surface area and thickness less than 100 nm. They are mixed with alcohol as a solvent to be formulated into a paste. The other Ag pastes were also made by using Ag micron particles and Ag particles with submicron. These Ag pastes were stencil-printed on a Cu substrate with or without electrolytic plated Au or Ag. Finally, die-bonding was carried out. These samples were bonded in a temperature range from 160°C to 200°C for 60 minutes. As the result, the die-bonding with the new Ag nano-meter thin flakes exhibits an about twice times higher bonding strength than the conventional Ag paste die-bonding. By observing cross-sections of the samples after sintering revealed that the nano-meter thin Ag flake die-bonding has denser microstructure than the conventional die-bonding. Shear strength of die-bonding was improved by using the nano-meter thin Ag flakes. It was revealed that the nano-meter thin Ag flakes can form a reliable interconnect below 200°C.