Comparative study of lead-free alloys submitted to thermomechanical ageing

Following the European RoHS directive’s restriction of certain dangerous substances, including lead in a range of electronic and electrical equipment, the SnPb eutectic, which was the most commonly used solder alloy in electronic assemblies, is replaced by SnAgCu (SAC) alloys and improved solder alloys (SAC+), by introducing small quantities of dopants such as Bi or Sb. In this study, based on accelerated experimental tests and finite element analysis, the fatigue behaviour of two different sizes chip resistors with two different types of solders joints (SAC305 and SAC+: Innolot) subjected to cyclic temperature variations, is studied. From experimental tests, the most vulnerable region in different solder joints where the crack may originate and propagate have been identified and their path length quantified as a percentage. The creep properties of both studied solder joints have been described using the Anand model. Creep strain, Von Mises stress and stress-strain hysteresis loop at the critical region were extracted from simulations for the solder alloys. Numerical results showed a good concordance with cross-sectional observation by optical microscopy. A significant difference was observed between the two different solder alloys: doped SAC has better performance against thermal cycling than the SAC305. A comparison of the size of the chip resistor showed that the smaller component seems to be more performant than the larger one (R1210).