Wide band gap semiconductors such as SiC and GaN have been expected to be established as the next generation power semiconductors, which are capable to reduce huge energy-loss in electric power conversion. The application of them will increase power density resulting in operation temperature rise beyond 200 °C. In addition, both increase of current and of frequency will also increase stress on devices. In general, power devices must possess robustness and high reliability for safe control of power conversion. Heat dissipation from the hot die in operation is one of the essential key factors for establishing high performance WBG devices. Here we report the current status of high temperature packaging materials including die-attach, wiring, substrates and molding compounds. In addition, reliability issues including thermal shock, power cycling and transient thermal properties will be summarized.

A series of DBC and DBA substrates, composed of Al and Cu as metal layers and of Al₂O₃, AlN and Si₃N₄, were examined by thermal shock test between -50 °C and 250 °C. DBC substrates with Al₂O₃ and AlN fractured only after 10 cycles due to severe thermal stress while all DBA and DBC with Si₃N₄ substrates survived. Al and Cu layer of all DBC and DBA substrates severely deformed by thermal shock. By microstructural observation, grain boundary sliding during thermal shock has a key to cause deformation of metal layers on ceramics. Ni and Ni-P plating on metal layer also deformed followed by Al and Cu layer deformation. Ni-P plating showed severe cracking which can be attributed to brittleness of Ni-P plating while Ni plating did not.

Ag sinter joining can be successfully applied for die attach by low temperature and low pressure bonding in air. For better bonding quality, Ag coating is the best metallization due to the specific reaction of Ag in air. By modifying solvent, Ag particles paste was modified to form a good bonding with Au surface finish. It can provide not only high temperature structural stability but also excellent thermal dissipation capability. High die-shear strength beyond 30 MPa was maintained by thermal shock between -50 °C and 250 °C up to 1000 cycles for a DBC die-attach structure. The thermal resistance of Ag sinter joined die attach is much lower than that of solder.　Figure shows the example of a GaN die-attached on DBC substrate with a new Ag sinter materials at 200 °C in air without any pressure. Even on the Au metallization, the GaN die was bonded with high strength. The interface shows a good bonding between Au and Ag.

This work was supported both by JST ALCA project on GaN packaging and by METI standardization project for evaluation methods of thermal properties of ceramics substrates.