Development of gold based solder candidates for flip chip assembly

Flip chip technology is now rapidly replacing the traditional wire bonding interconnection technology in the first level packaging applications due to the miniaturization drive in the microelectronics industry. Flip chip assembly currently involves the use of high lead containing solders for interconnecting the chip to a carrier in certain applications due to the unique properties of lead. Despite of all the beneficial attributes of lead, its potential environmental impact when the products are discarded to land fills has resulted in various legislatives to eliminate lead from the electronic products based on its notorious legacy as a major health hazard across the spectrum of human generations and cultures. Flip chip assembly is also now increasingly being used for the high-performance (H-P) systems. These H-P systems perform mission-critical operations and are expected to experience virtually no downtime due to system failures. Thus, reliability of the solder joint is a major critical issue. This reliability is directly influenced by both the phases in the bulk solder and also the intermetallic compounds formed between the solder and the solder wettable layer of the under-bump metallization during both the wetting reaction or/and the solid state ageing. In the present work, an attempt has been made to develop new solder alloys for flip chip assembly using the CALPHAD approach based on gold, the safest element among all the elements being considered for this application. Specifically, efforts have been made to predict the phases in the bulk solder of the promising solder candidates and also the intermetallic compounds formation, using the CALPHAD approach.