A reliability evaluation of lead-free ball grid array (BGA) solder joints through mechanical fatigue testing

Abstract

Lead is a hazardous substance which, when ingested can be toxic to humans; therefore it has been banned by a European Union directive in an aim to reduce its harmful effects on health and the environment. The ban, which comes into force on July 1/sup st/ 2006, means that electronic manufacturers must transfer from a tin-lead soldering process to a lead-free process. In this paper a reliability evaluation of a tin-silver-copper (SnAgCu) solder is presented with a baseline of tin-lead (SnPb). An experiment was carried out to optimize the surface mount reflow process and the reliability of the resulting solder joints was investigated using a torsional mechanical fatigue test method. The test vehicle comprised of an 8-layer FR4 printed circuit board (PCB) mounted with four ball grid array (BGA) components - each package comprising four daisy-chains. The basic principle of the torsion test was to stress the BGA solder joints repetitively in order to determine the number of cycles to failure. Graphs of the cycle number versus resistance were created and the numbers of cycles to failure were determined. The failure mechanisms were examined using cross-section and scanning electron microscope (SEM) techniques which showed cracking that initiated at the upper corner of the solder joint and propagated in the solder along the upper copper pad. This failure mechanism was observed for both SnAgCu and Snb solder joints. From a comparison of number of cycles to failure, the reliability of SnAgCu BGA solder joints was found to be superior to that of SnPb joints in torsion tests.