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 degrees C, 100 degrees C, 120 degrees C, and 135 degrees 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 mu m after 3600 h of aging at 135 degrees C. The eutectic BiSn microstructure coarsened very quickly. Substantial coarsening can be observed at 135 degrees C for only 200 h of aging. Ln 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.