Transmission electron microscopy of joints between the Sn-Ag solder and the electroless Ni-P of an electronic device

Microstructure of Pb-free Sn-Ag solder and joints between the Pb-free solders and electroless Ni-P was examined by means of transmission electron microscopy. Microstructures of the solders consist of beta-Sn matrix dispersed with Ag3Sn particles. Analysis of the diffraction patterns shows that there are more than one orientation relationships between,beta-Sn and Ag3Sn. The solder joint of the Pb-free solders is essentially similar to the joint of Pb-Sn eutectic solders. Analysis of distribution of the elements by EDX analysis shows that Ag does not diffuse into Ni-P. Both of the as-deposited Ni-P and P-enriched Ni, the latter being formed as a result of reaction with Sn-Ag solder, have FCC structures based on pure Ni. In other words, P is dissolved in Ni to form a solid solution.