Corrosion of Pb-50In flip-chip interconnections exposed to harsh environment

Lead-indium alloys are an alternative to lead-tin solders commonly used. Lead-indium alloys with 25 to 50 wt.% In provide a substantially lower process temperature advantage and overall exhibit a much lower tendency to scavenge gold compared to high Pb-Sn solders. However, a corrosion concern has been identified in relation to Pb-50In solder, an alloy of interest owing to its thermal-fatigue resistance. The results and observations of a study conducted to gain a fundamental understanding of the nature and path of corrosion in Pb-50In flip-chip connections exposed to an accelerated hostile-environment test simulating harsh industrial ambients are described. Among the five major industrial pollutants comprising the test environment, only chlorine corroded the Pb-50In solder-joint metallurgy under the test conditions, and chlorine was the only pollutant species found in the corrosion products. Attack was initially local, forming a product with modular morphology, consisting mostly of In and Cl (probably InCl₃). Lateral growth impingement resulted in the formation of a continuous, quasi-protective surface crust which significantly reduced the bulk corrosion rate. Joint electrical and mechanical integrity depended on the galvanic driving force and hindrances due to space and geometric considerations, etc. which govern the rate of inward corrosion along the ball-limiting metallurgy/solder interface. Successive autocatalytic conversion along the corrosion path likely proceeds via Lewis acid hydrolysis of indium chlorides to corresponding hydroxides and hydrogen chloride. The top surface metallurgy was not attacked. Interface corrosion of C-4 joints was favored over bulk corrosion when chloride was present as part of the corrosion-causing environment