The mechanical properties of lead-containing and lead-free solders—meeting the environmental challenge

摘要

Reliability is a principal objective in electronics equipment. This imposes a significant challenge, particularly in the face of continued miniaturization, which results in more severe conditions for the interconnection. At present, no sound methodologies exist for reliable life prediction of solder joints and, for lead-free alloys, this problem is accentuated by a shortage of data on mechanical behaviour appropriate to service. To meet the demands of forthcoming environmental legislation, a new generation of lead-free solder alloys is being developed. The paper presents a comparison of the mechanical behaviour of the conventional Sn-37Pb solder with that of Sn-3.5Ag and Sn-0.5Cu alloys (where the compositions are in weight per cent) which are potential replacements. It is demonstrated that the mechanical properties of the lead-free alloys may be better or worse than those of the eutectic Sn-Pb alloys. The ranking of the alloys may change according to the test conditions. Strengths fall to quite low levels with decreasing strain rate and increasing temperature. The Sn-3.5Ag solder has substantially superior creep resistance although, for creep lives less than 1000 h at 75°C, prior ageing impairs this performance, as is the case for Sn-37Pb. During high-strain fatigue, the lead-free alloys are marginally inferior, and the presence of a dwell in the strain cycle generally causes a significant reduction in life. The implications of these findings in relation to the implementation of lead-free technology are considered.