Advanced materials for drop in solution to Pb in high temp solders: The next generation of zinc based solder alloy

The search for an acceptable die attach solution to the European End of Life Vehicle RoHS Directive has proven problematic for the industry with several potential materials being evaluated. The mandate affects a full array of semiconductor device types from Power Modules, Smart Power ASICs to Power MOS-FETs & IGBTs in SMD and Through-Hole packages, which all contain high temperature lead containing solders until the RoHS exemption expires. Several IDMs have looked at four base material technologies: adhesives, Ag sintering, hybrid materials and solders with each one posing hurdles for mass production. The adhesives can use current equipment but are costly and have low thermal conductivity. Ag sintering has shown promising temperature cycle result but is costly and requires development of new equipment and processes. Hybrid materials can use established equipment sets but are also costly and have not been proven in high volume. There has been extensive development of solder technologies as it poses the highest likelihood of delivering a “drop in” solution to lead. Companies have spent years developing a solution that performs similarly to lead in all of the different applications. One of those solutions is Zn based alloys which have shown potential to provide a nearly drop-in solution to lead-based materials without the use of expensive precious metal or capital expenditure. Prior generations of solder development initially looked at Bismuth alloys due to the material similarities with Lead. Although Bismuth alloys are available on the market, they have two flaws that prevent them from mass adoption. The first flaw is the low thermal conductivity, achieving only 12 to 17 W/m-K. The second is the process requirement of surviving three times reflow at 260C. Since Bismuth alloys melt from 255 to 275C, they are unable to meet the reflow process constraint. This paper will focus on the development of Zn alloys for the replacement of lead in high temp- rature solders. Zinc solders overcome the two hurdles Bismuth encounters and offer a solution with thermal conductivity more than 100W/m-K and melting temperature higher than 300C. In addition, the solder material has cost benefits compared to the alternative solutions while providing improved thermal conductivity and resistivity over current lead solders. The process window of the Zn alloys are not as wide as its lead solder counterparts, requiring substantial composition and process development, and depending on device type and use, metallization requirements. Material development has lead to further understanding of intermetallic compound (IMC) formation and impacts to bond lines during reliability evaluation while using bare copper lead frames and nickel plated lead frames. Following the recommended guidelines, Zn based alloys are an acceptable alternative to lead for use within high temperature solders.