Mapping Mechanical Properties of Lead-Free Solder Joints

Abstract

ABSTRACT Mechanical properties of intermetallic compounds and tin-rich area were investigated using nano-indentation. Measurement of hardness, Young’s modulus and strain rate sensitivity were acquired at room temperature for a SAC305 solder alloy with addition of Au (5% of weight percent). Additionally, a fast nano-indentation measurement technique was employed to produce surface maps of hardness. Every indentation cycle takes less than three seconds, which includes surface approach, contact detection, application of the force, withdrawal, and movement to the next indentation. Traditional nano-indentation analyses are applied to the force-displacement measurements from each indentation. Scanning-probe technology with advanced information storage and presentation allows mapping of the mechanical properties on small features. Consequently, this nano-indentation technique produces three-dimensional images of mechanical properties which are stored and manipulated just like scanned images. In the present work, a fast technique was used to map the hardness of a SAC 305 solder joint with Au plating. After prolonged isothermal aging, the solder joint is comprised of three constituents: a tin-rich matrix, an interfacial intermetallic (Cu, Ni, Au)6Sn5 and a bulk intermetallic AuSn4. The softest material is the tin-rich matrix, which has a hardness of 0.51 ± 0.07 GPa. The hardness of the bulk AuSn4 intermetallic is 2.12 ± 0.18 GPa. The interfacial intermetallic has extraordinary hardness — greater than 8 GPa. Strain rate sensitivity revealed that the IMC phase has a lower tendency to creep than the tin-rich area. Under uniform plastic strain, the mismatch in hardness between the interfacial intermetallic and surrounding material may increase the local stress intensity factor which drives interfacial fracture.