抄録
An investigation has been made of the structure and tensile properties of the Pb-Sn solders which are important micro-soldering and bonding materials used in the fabrication of semiconductors ICs (Integrated Circuit) and LSIs (Large Scale Intergrated circuit).
During the soldering operation the Pb-Sn solders are undergoing a melting and solidifying process, and are in the so-called casting state; they exhibit a structure with significant segregation in the composition. This structure is thought to have a large influence on the reliability of the soldered joint.
To evaluate the mechanical properties of the micro-soldered joint, cast solder samples (Pb/Sn(in mass%)=100, 99/1, 97/3, 95/5, 93/7, 90/10, 85/15 and 37/63 (eutectic composition)) were solidified and formed at the conventional cooling rate (about 1 K/s). The micro-structure and tensile properties of these samples were investigated. The results are summarized below.
(1) The cast solders exhibit a dendrite structure with the segregation of composition. They show very little elongation as compared with rolled materials.
(2) For Pb-rich solder alloys, tensile deformation occurs primarily within the grains, and virtually no grain boundary sliding of the type seen in rolled materials is observed.
(3) Eutectic phase crystals, caused by the concentration of Sn into the grain boundary and between the dendrites (i.e., within the grains), are observed in the cast structure as a result of segregation in composition. In Pb-rich solders, this eutectic phase suppresses the rotation of the crystalline grains and the grain boundary sliding, and thereby causes the reduction of tensile elongation.
