Low Cycle Isothermal Fatigue Properties of Lead-free Solders

A major concern is the reliability of the solder joints, which has traditionally been predicted using modeling techniques with established SnPb solder materials data. Although the mechanical behavior of lead-free solders is known to be different from that of their traditional counterparts, there is a dearth of credible measurement methods, in order to evaluate the likely lead-free solder performance. However, currently available test instruments do not readily meet these constraints. This paper describes a new approach developed at NPL to overcome these constraints, resulting in the design of an instrument IPTM (Interconnect Properties Test Machine), with a sample geometry that permits small solder volumes to be studied in pure shear [Ref 1 pp.468]. Example data are presented and illustrate how the low cycle fatigue resistance of solders can be characterized. Advantages of the new approach include: (i) it can accommodate various solder alloys and surface coatings, (ii) the solder joint volumes mirror those in modern assemblies, (iii) the solder is under shear, (iv) the samples are relatively easy to manufacture, (v) the construction of the specimen allows direct microscopic examination during the test at room temperature. A 4-point measurement system for resistance monitoring has also been evaluated and found to correlate well with load decreases recorded during fatigue testing of solders. The results of this study reveal that lead-free solders at high temperatures have increased lifetimes when creep constitutes major part of mechanical cycle. A route to predict a fundamental reliability variable of solder has been identified.