Mechanically compliant lead-free solder metallurgy: The key element in enabling extreme low-k large-die flip chip devices

Abstract

Significant challenges are encountered to enable large-die fine-pitch lead-free-bump flip chip devices, especially with extreme low-k (ELK) silicon technology for 40 nm and beyond. Such failures are often observed as die ELK stack-up fractures, lead-free bump cracking, underfill delamination, high warpage-induced BGA reliability issues, etc. The underlying root cause can be attributed to elevated thermomechanical stress within the packaging when lead-free bump material is used for the die-to-packaging interconnect. This paper presents an innovative approach to addressing these technical problems by applying mechanically compliant leadfree bump metallurgy. In conjunction with several other aspects of assembly and packaging material optimization, this solution is capable of delivering 100% lead-free flipchip packaging with a die size of up to 20x20 mm and a package size of up to 52.5x52.5 mm per side.