Drop Shock Reliability of Lead-Free Alloys - Effect of Micro-Additives

The drop shock reliability of solder joints has become a major issue for the electronic industry partly because of the ever increasing popularity of portable electronics and partly due the transition to lead free solders. Most of the commonly recommended lead-free are high Sn alloys which have relatively higher strength and modulus. This plays a critical role in the reliability of Pb-free solder joints. Further, even though metallugically it is the Sn in the solder alloys that principally participates in the solder joint formation, details of the IMC layers formed with SnPb and Pb-free alloys are different. The markedly different process conditions for SnPb and Pb-free alloys also bear on solder joint quality. Brittle failure of solder joints in drop shock occurs at or in the interfacial IMC layer(s). This is due to the inherent brittle nature of the IMC, defects within or at IMC interfaces or transfer of stress to the interfaces as a result of the low ductility of the bulk solder. In developing improved performance alloys, Cookson Electronics has addressed both issues -improved ductility and modification and control of the intermetallic layer. A broad range of base alloy compositions together with selected micro-alloying additions to SnAgCu alloys have been evaluated with the objective of controlling bulk alloy mechanical properties and the diffusion processes operating in the formation and growth of the intermetallic interfacial layer(s). In the present article a detailed study of a range of micro-alloy additives is presented. The alloy additives generally act as diffusion modifiers slowing interdiffusion between substrates and solder thereby reducing IMC thickness or the propensity for void formation. Alternatively additions can be made that act as diffusion compensators^copy. It should be noted that the level of the micro-additions does not measurably modify the bulk mechanical properties of the base alloys. Our results show that dramatic improvements - in the solder joint reliability, as demonstrated by high-speed ball pull and drop shock tests, can be achieved.