Dual-phase solid-liquid interdiffusion bonding, a solution for the die attachment of WBG

Wide band gap (WBG) semiconductors such as SiC and GaN devices are expected to replace Si power devices in the next generation power modules for renewable energy and smart grid to enhance their energy conversion efficiency through a characteristic of high frequency switching. However, the temperature of the WBG power module may reach 250°C as operating, which is much higher than the melting temperatures of the conventional solder materials like Sn37Pb (187°C), Sn3.0Ag0.5Cu (217°C) and Sn0.7Cu (227°C). Therefore, a new die attaching method is an urgent research subject for the assembly of WBG power modules. Sandia National Laboratories won the R&D 100 award in 2009 by the world´s first full SiC module, in which the die attaching of SiC devices was accomplished by a transition liquid phase (TLP) method, 2 intermetallic phases including Ag₃Sn and Ni₃Sn₄ were formed to bond SiC devices on a direct bond aluminum (DBA) and mount the DBA on a metal matrix composite baseplate, respectively, and the power module was capable to operate under a condition in excess of 400°C. Subsequently, Infineon announced a power chip embedded technology named BLADE for renewable energy applications in 2011, a same process so-called diffusion soldering was used to attach Si MOSFET device on an organic carrier by producing an intermetallic layer like Cu₆Sn₅ between them. However, some disadvantages have been found in the previous cases. First, the bonding time is too long to achieve a high throughput production when the process temperature is lower than 300°C, whatever, the power ICs are easily damaged when the process temperature is as high as 350°C. Second, the intermetallic joint might degrade due to the voids induced by a volume contraction as heated. Third, there are some defects such as Kirkendall voids are formed at the interface between different intermetallics, which s- gnificantly impacts the reliability performance of the joint. A dual-phase solid-liquid interdiffusion bonding process was developed to solve the above-mentioned issues in this study. By the elemental design of the electrode compositions on both chip and substrate, a dual-phase intermetallic joint was formed to attach the chip on the substrate, and the bonding temperature was decreased to just 260°C. Furthermore, almost no void was found within the joint because they were rapidly stuffed by the formation of secondary intermetallic. The shear strength of the intermetallic joint was measured being higher than 20 MPa, even though experienced a temperature cycling test (Condition B, JESD22-A104), meaning that the new bonding technology was reliable and capable of manufacturing WBG power modules.