Issue link: https://iconnect007.uberflip.com/i/270052
68 SMT Magazine • March 2014 hesion to the die or substrate. Another factor could be the roughness of the finish itself, e.g., copper on DCB. For that challenge a concept with micro silver has been developed. This al- lows a process without pressure, or with low pressure, at 220°C. Diffusion Soldering/hotPowcon (hPc) Different investigations described 11, 12 an increase of thermal stability by using a dissolu- tion process for creating intermetallic connec- tions. Therefore, an increase in reliability after temperature cycling 3 means a combination of thermal solidification and isothermal solidifica- tion, which could be the basis for a new type of interconnection, as seen in Figures 11 and 12. From this basic investigation, the idea for this project was developed: to create structure designs by developing high melting-point in- termetallic solder paste. For this purpose, a soft solder alloy was used based on Sn with a high metal concentration and additives, and the pos- sibility of isothermal solidification at low/stan- dard solder temperatures, as well as operating temperature above liquidus temperature after soldering. Before the project began, an investi- gation had been carried out on realistic DCBs with IGBTs. The process was printed with a 20 µm stencil, pick-and-place of IGBTs and solder- ing in vapor phase with vacuum. The solder paste was based on type 6 eutectic SnCu powder with and without Cu powder in different sizes. The result after soldering without Cu powder is shown in Figure 13. By using this process, intermetallic in a se- lective connection between substrate and die was created. By using additional Cu powder, as in Figure 14, most of the solder joint was creat- ed with the standard interconnection based on the soft solder alloy and selective intermetallic of the Cu powder. Figure 12: Creating of intermetallic with Cu in a SnCu system (Source: Fraunhofer iZM). figure 11: Phase diagram of a two element system. feaTure aSSEmBLY maTErIaLS FOr hIGh-TEmPEraTUrE aPPLIcaTIONS continues