# Silver Sintering for Silicon Carbide Die Attach: Process Optimization and Structural Modeling
## 1 Introduction  
The current diffusion of high-performance hybrid and electrical vehicles is pushing semiconductor industries to engineer novel chip based innovative materials. Wide band gap semiconductors, particularly silicon carbide (SiC), offer a concrete option for the development of devices with higher efficiency and temperature capability with respect to the classic silicon (Si) based. The introduction of these improved semiconductors is strongly related with the evolution of packaging manufacturing. Dedicated and advanced technologies are required to allow the full exploitation of the capability of new semiconductors. There the excellent electrical and physical properties of SiC power MOSFETs that enable high switching speeds, power losses reduction and junction temperature higher than 200 \({}^{\circ}\)C [1] are already known. Packaging improvement is also needed to withstand the strict requirements in terms of performance, efficiency and durability [2]. Considering the high temperature supported by SiC devices, the main reliability bottleneck is solder degradation which is the current state of the art for die attach in power modules. Due to current standards, solder for new automotive power modules are made of tin-based compounds that are not suitable for supporting temperatures higher than 150 \({}^{\circ}\)C due to their low melting temperature and related creep degradation. A replacement of suchtechnology is required to improve performance and reliability over the current Sn-solder brazing process. The sintering layer is an option to substitute the solder joint from chips to carrier substrates. A reliable and stable bond can be manufactured considering a suitable combination of time, temperature and pressure. As already highlighted by literature [3], pressure acts for reducing the minimum temperature to execute sintering process, resulting in a reduction in induced thermo-mechanical stress in package components. Chips and substrates are coupled together by means of metal powder, typically silver or copper-based, sintered by a thermo-compression process [4; 5]. Sintering joints have excellent thermal conductivity and thanks to their very high melting point (about 962 \({}^{\circ}\)C for silver sintering), reduce the amount of creep with respect to conventional solder compounds. Experiments based on uni-axial tensile tests have shown that, for the same level of stress and at a given temperature, lower strain rates have been found for the sintered Ag compared to SnAgCu solder. At a temperature of 150 \({}^{\circ}\)C and a stress of 10 MPa, creep magnitude is roughly one order of magnitude lower in sintering than in solder [6; 7]. The package reliability improvements triggered by sintering joints shall be evaluated due to the aging generated by a typical mission profile. The durability benefits of sintering with respect to solder solutions, in passive thermal cycling as well as in an active power cycling lifetime, have been demonstrated [8; 9; 10; 11]. Liquid-to-liquid thermal shock (TS) is an option, as considered by some authors [12; 13], to reproduce the very aggressive temperature rate (up to 2500 \({}^{\circ}\)C/s) that can happen during the SiC device switch-on [14]. Thanks to the better convective exchange provided by liquid media, liquid-to-liquid TS can ensure a higher temperature rate and strain rate to the device under test than the air-to-air thermal cycle. The stress-strain constitutive law for silver sintering depends on the strain rate: increasing the strain rate sinter layer behaves stiffer with higher mechanical stress [15; 16]. In last analysis, thermal shocks are a reasonable methodology to exploit fatigue behavior much faster than other reliability techniques such as passive or active cycling [17]. In this framework, thermal shock tests are employed to optimize the pressure applied during the sintering manufacturing process.