PREPARATION, STRUCTURE, AND FRACTURE MODES OF PB-SN AND PB-IN TERMINATED FLIP-CHIPS ATTACHED TO GOLD CAPPED MICROSOCKETS

Solder-bump flip-chip (SBFC) interconnection technology has proven to be reliable, cost-effective, and extendable [1]–[3]. This type of chip-to-substrate connection, also referred to as a controlled-collapse chip connection or C-4, is used in IBM's multilayer ceramic (MLC), metallized ceramic (MC), and thick film packages. On MLC substrates, flip-chip interconnects are achieved by placing diced chips face down and simultaneously reflowing the chip I / O's directly to corresponding substrate pads, termed microsockets. The substrate's molybdenum thick film conductor material is not directly wet by lead-tin or lead-indium terminated chips. Several preparatory metallization steps are required to render the surface suitable for solderability and chip attach. Fatigue life of thermal cycled solder joints directly depends upon the integrity and metallurgical properties of the joint interfaces and bulk solder characteristics. This paper discusses the effects of selecting nickel, deposited from an electroless phosphorus-based bath, as the base metal to achieve solder wetting. Specifically discussed are the degrading effects on solder wetting due to precipitate formation during process thermal exposure and the use of a gold cap to prevent the problem. Also addressed are the overall metallurgical characteristics of both lead-tin and lead-indium C-4 joints, as well as their relation to performance (e.g., fracture mode and fatigue life). © 1990 IEEE