Modelling and experimental measurement of multiple joint lead free solder interconnects subjected to low cycle mechanical fatigue

Ball Grid Arrays (BGAs) are one of the dominant technologies for high-end packaging solutions of electronic circuits, especially where a high interconnect density is required. This paper aims at studying the solder isothermal fatigue failure characteristics in test structures that resembles BGAs. Test structures comprising between 1 and 8 small solder joints have been made using 96.5Sn3.0Ag0.5Cu lead-free solder and subjected to cyclic mechanical loading that produced a shear force across the joints. The same structures were modelled using finite element analysis. The loading response distribution profile through the joints is analysed and the regions of likely failure identified to be along the shear band and at the stress concentration areas in the corners of the joints. Volume weighted average accumulated creep strain per cycle is used to analyse the failure of the individual joints where it is shown that greater damage occurs in the outer joints. Solder joint models of three different shapes are investigated: rectangular, convex and concave shapes. Results have shown that less damage occurs in the concave joints..