Monitoring the Growth of the α Phase in Tin Alloys by Electrical Resistance Measurements

Abstract

The β phase of tin is stable only down to 13°C. Below that temperature the thermodynamically stable phase is α tin, more commonly known as tin pest. This phase has gained notoriety because of the catastrophic consequences of the transforming metal. Interest in this subject increased after the transition from tin-lead to lead-free solder, driven by European restriction of the use of hazardous substances (RoHS) legislation. Lead-free alloys contain from 95% to 99% tin and could potentially be susceptible to the β/α transformation mentioned above. The nucleation of the α phase takes a long time to occur, months or even years, especially in alloys of tin. For this reason this phenomenon is particularly difficult to study. The growth, however, occurs faster, and this paper describes a procedure for observing and measuring growth based on electrical measurements. Furthermore, a sample preparation technique that considerably accelerates the nucleation in alloys is described. Seven binary alloys were tested with this procedure and ranked in terms of their propensity to transform. Experimental results demonstrated that the α crystal growth is not stopped by alloying, but just decelerated, and that lead and bismuth are the most effective at this. The same procedure was then demonstrated successfully on printed circuit boards and hence is shown to be a general method for monitoring α tin growth in critical field applications.