(Invited) Advances in Ag Sinter Joining for WBG Power Device Assembly and High Temperature Reliability
Authors: Suganuma Katsuaki , Chen Chuantong , Zhang Zheng , Suetake Aishi , Hsieh Ming Chun , Iwaki Aya 
Published: December 2020

SiC and GaN, wide bandgap semiconductors (WBGs), providing great benefits such as excellent energy saving, high power density, size reduction of power module, and long-life time, have been attracting many researchers for decades. Actually, there are several new products in the world market. At this moment, one of the technology demands for expanding the market of SiC/GaN is on power packaging technology, which can open these great functions. The increasing power density leads to high operation temperature beyond 200 °C at semiconductor junctions. Not only SiC/GaN devices themselves, insulating ceramic substrates such DBC and DBA must possess good heat-conductivity, heat-resistance and heat-shock resistance. Temperature swing range in operation will be from -50 °C to 250 °C, which is quite severe for the conventional packaging materials including not only insulating substrates, but also solders, wiring, molding compounds, and others. In our group, new assembly materials have been demonstrated and new design rules of die-attach and insulating substrates have been explored to realize reliable SiC/GaN power devices.
Sinter joining technology is one of the most promising bonding methods for realizing both high performance and high reliability for WBG devices. The author has developed Ag micron-particles sinter joining with a hybrid of Ag micron/submicron size particles providing excellent heat-resistance beyond 200 °C without any assisting pressure. Ag reacts with oxygen in air resulting in low temperature sintering around 200 °C. The microporous joining structure provides high strength as well as excellent heat/electric conductivity. The die-attached devices are stable even up to 250 °C in thermal shock and in power cycling. For joining Ag sinter materials to various metallization faces such as Au, Ni, Cu or Al, has another critical factor to influence device performance. Ag sinter paste to Ag metallization face is the most recommended one. However, from the various requirements in the market, Ag sinter joining must be also applicable to these dissimilar surfaces. The latest sinter paste enables joining Ag sinter paste even to Al as shown in Fig. 1, which has been recognized as one of the most difficult combination. In this example, the bonding strength reached 35 MPa when bonded at 200 °C.
Metal bonded ceramic substrates are the other key materials for WBG power modules. DBAs of AIN, Si3N4 and Al2O3 can survive even in severe thermal shock test between -50 °C to 250 °C. However, significant deformation of metal plates bonded on rigid ceramic insulators results in serious degradation of the WBG assembly. It was found that the properties of surface plating have a key to suppress this surface deformation.
Operation monitoring in real service environment will be one of the useful technology for power devices. A nondestructive inspection method by using AE monitoring has been proposed to understand the degradation of module during thermal cycling. During power cycling, abrupt stepwise increases in AE occur showing crack extension in a die-attach layer.

Figure 1