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Temperature Cycling Reliability Assessment of Die-Attachment on Bare Copper by Pressureless Nanosilver Sintering

Fig. 1. Measured temperature cycling profile in comparison with JEDEC standard
Power electronics engineers use semiconductor switching devices to shape the forms of elec-trical energy to enable efficient and reliable operation of electrical and electronic devices. Per-formance of power electronics devices has been improved significantly with the development of wide-bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN). Com-pared to silicon based devices, wide-bandgap devices are capable of fast turn-on and turn-off, have lower conduction and switching losses, and can be operated at high temperatures (e.g., 250 °C). At the first level of packaging, die-attach material and processing are crucial for the overall performance and reliability of the final product, and therefore innovations in die-attach technologies are urgently needed.

Nanosilver paste, an emerging die-attach material, was formulated by mixing nanosilver particles with several types of organic additives. Extensive research has been done on the ap-plication of nanosilver paste for die-attachment. It has been demonstrated that nanosilver paste can be applied for attaching power devices of different sizes onto silver and gold surfaces by pressureless or low-pressure-assisted (< 5 MPa) sintering, with sintering temperature lower than 280 °C.

The goal of this study was to evaluate the temperature cycling reliability of the pressureless, forming-gas-enhanced sintering process for bonding chips on copper using nanosilver paste. Thick silicon chips were attached to the DBC with a bare copper surface for die-shear testing, while thin silicon chips were attached to copper lead frames for microstructure analysis. Sam-ples were subjected to temperature cycles in the range of -40 °C to 125 °C with a cycling period of 60 minutes. Die-shear tests were performed on cycled samples. X-ray tomography was ap-plied to detect the defects in the die-attach joints for the silicon samples. Microstructures of cross sections of the joints were characterized by SEM.

Fig. 2. Die-shear strength of joints versus number of temperature cycles

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