Low cycle fatigue test for solders using non-contact digital image measurement system

In a strain controlled low cycle fatigue test it is customary to use a contacting extensometer. However the extensometer can cause premature fracture at the contact points by inducing localized plastic deformation, particularly in a soft metal such as a lead–tin solder. In order to avoid this problem, a non-contacting, digital-image measurement system has been developed to measure the displacement of the specimen gage length during a cyclic loading. The capability of this system for strain-controlled fatigue testing was studied in preliminary experiments, and the effect of variables in the digital image measurement system on the accuracy and the reliability were established. The results confirmed that this system was capable of measuring the displacements in strain-controlled fatigue tests. Low cycle fatigue tests on three solder materials at 20°C were then carried out using this system. The solders were: Ag–Sn eutectic solder (3.5Ag/96.5Sn), Sn–Pb eutectic solder (63Sn/37Pb) and Sn–Pb solid solution solder (5Sn/956Pb). It was found that at a given strain range the Sn–Pb eutectic solder (63Sn/37Pb) had the lowest low cycle fatigue resistance. The lead free eutectic alloy (3.5Ag/96.5Sn) has the highest low cycle fatigue resistance in low strain range regime (Δϵp <1%). At higher strain range regime (Δϵp >1%), the 5Sn/95Pb alloy has better low cycle fatigue resistance. For all three types of solder, the initial failure mode at the surface of specimens was intergranular. For the 63Sn/37Pb alloy the subsequent fracture path was also intergranular, but for the 3.5Ag/96.5Sn and the 5Sn/95Pb alloys, the subsequent crack paths were generally transgranular.