The improvements of high temperature Zn-based lead free solder

To comply with the European regulations regarding the use of high-Pb solders in electronics, researchers have been working for many years to develop a high melting temperature Pb-free die attach substitute. As of now, there is still no known substitute for high-Pb solders that can withstand board-level reflow temperature of 260°C and demonstrate the same reliability that high-Pb solders afford.Our previous study demonstrated a Zn-based solder with a melting temperature above 330°C and excellent thermal and electrical properties, which may serve as a near drop-in alternative to high-Pb solder wires in die attach applications. However, the solder faced several issues, including embrittlement due to CuZn IMC formation when applied to bare Cu lead substrates and insufficient wetting on Ni-plated lead frames. This paper focuses on wetting improvements of the Zn-based solder using a combination of a novel lead frame design and a solder writing technique. In this experiment, the microstructure and mechanical behavior of the Zn-based solder were first characterized. The material was then extruded into wire form and was used to bond Si dies with Ti/Ni/Ag die backside metallization to the newly designed lead frames. The Zn-based solder´s wire properties, wetting and behavior during die bonding process were evaluated.