Influence of IMCs volume ratio in micro-scale solder joints on their mechanical integrity

Abstract

Since miniaturization of electronic devices makes the joint height of solder joints shrink to tens of microns. IMCs take noticeably high volume ratio in solder joints, which seldom happens in BGA or bulk solder joints. Growth of IMCs volume ratio will definitely affect mechanical integrities of micro-scale solder joints, which is rarely covered in pervious publications. In this paper, copper/SAC305/copper solder joints with a joint height of 20μm were utilized as specimens. They were aged at 150°C for different durations to obtain different thickness of IMCs. Microstructure of specimens was studied before tensile tests. The growth of IMCs and aging time follows parabolic law, similar to reported results. Tensile tests were carried out to investigate the effect of IMCs volume ratio on tensile strength and fracture mode. Fracture surface of some selected specimens after tests was observed to determine their dominant fracture modes. When specimens were aged for 1 hour, a significant decrease in tensile strength was observed which is due to the Ag 3 Sn particles locating along grain boundaries and results in intergranular fracture. The tensile strength reaches a peak, 86.1MPa, at 9 hours with the IMCs volume ratio of 21.95%. When the aging process lasted for 16 hours and IMCs volume ratio increased to 23.35%, this is a transition point of fracture mode from ductile to brittle fracture with a difference of tensile strength exceeding 20MPa. As the aging duration continued increasing, the tensile strength falls continuously due to the coarsening of grains with the brittle fracture area being extended.