Effects of soft solder materials and die attach process parameters on large power semiconductor dies joint reliability

Die attach utilizing soft solders, primarily high lead (Pb) solders for power semiconductor devices are still widely used across the industry. These solders offer excellent mechanical, thermal and electrical properties. Soft solders provide good heat dissipation of typically 35 W/mK is ductile and exhibits desirable thermal expansion properties which tolerate that of silicon (4.2 ppm/K) and copper (16.5 ppm/K), so as to avoid joint failure due to thermal cyclic stresses. Typically, for large dies, bond-line-thickness (BLT), die tilt and post-reflow void rates are important. These factors affect the reliability and performance of the power device. Large, localized voids create air pockets which are detrimental to the device's reliability, both electrically and thermally. The experimental work conducted here is on a large power semiconductor die having an area of 19mm² and Ag coated die back, using a Pb-Sn2-Ag2.5 Type 3 soft solder on bare copper (Cu) substrates. For large dies having unequal width and length, the flow of solder when bond pressure is exerted and released becomes crucial as well as challenging. It directly affects the BLT. When reflowed, most solders undergo flux evaporation which leads to hot slump of the die attach solder, which in turn affects the die tilt and final outcome of void rates between the die and substrate. Here, surface energy concepts and how well the solder material flows and wets both the die back and substrate surfaces need to be understood. Die back and substrate surfaces exert varying stresses before and after reflow. In this literature work, we examine the fundamental concept of impact bonding utilized for soft solders on large power devices, factors which govern the formation of an acceptable bond line thickness, as well as influence of surface conditions for solder paste wetting and spreading during reflow in order to gain a reliable die-to-substrate interconnection.