Lead Free Solder for Flip Chip

The electronics industry is moving to replace lead in electronic assemblies. The driving factors are potential legislation, primarily in Europe, and global market pressures for more environmentally friendly products. The search for a ‘drop-in’ solder alloy replacement has been ongoing for more than 10 years and none have been found. The current industry trend is to use the eutectic (or near eutectic) Sn-Ag-Cu alloy. The melting point of this alloy (~217C) is significantly higher (34C) than eutectic Sn-Pb (183C). This will impact the assembly process. In addition, the lead-free alloys have a higher modulus than eutectic Sn-Pb. This may change the stress distribution during thermal cycle testing, affecting reliability and failure modes. This paper examines the assembly process for flip chip die with Sn-Ag-Cu solder bumps and initial reliability