Long-term aging study on the solid-state reaction between 58Bi42Sn solder and Ni substrate

The reaction between Ni and eutectic BiSn solder at 85°C, 100°C, 120°C, and 135°C was studied. Reaction times ranging from 25 h to 3600 h were used. Only Ni₃Sn₄ was detected as a result of the reaction. None of the other Ni-Sn intermetallic compounds and none of the Ni-Bi intermetallic compounds were observed. The growth of Ni₃Sn₄ followed diffusion-controlled kinetics and was very slow, with the layer thickness reaching only 16 μm after 3600 h of aging at 135°C. The eutectic BiSn microstructure coarsened very quickly. Substantial coarsening can be observed at 135°C for only 200 h of aging. In addition, fine Bi-rich particles within the Sn-rich phase of the solder were found. The amount of these fine Bi-rich particles increased with the aging temperature. It is believed that the formation of these fine Bi-rich particles is due to the fact that the Sn-rich phase can dissolve substantial amounts of Bi. It was also found that, as aging time increased, the region immediately adjacent to the Ni₃Sn₄ layer was preferentially occupied by the Bi-rich phase. This is because Sn in that region had reacted with Ni to form Ni₃Sn₄, leaving a nearly continuous Bi-rich phase above the Ni₃Sn₄. Since Bi-rich alloys tend to be brittle, a nearly continuous Bi-rich phase might weaken the strength of a solder joint. The Ni₃Sn₄ grain size increased gradually from the Ni/Ni₃Sn₄ interface to the Ni₃Sn₄/BiSn interface, which is probably an Ostwald ripening phenomenon.