Microstructures of joints between a Ni-8 mass%P UBM (under bump metallization) and three different Pb-free solders, i.e., Sn-Ag, SnAg-Cu and Sn-Ag-Cu-Bi were studied by transmission electron microscopy. The phases formed near the joint interfaces as well as inside the solders during soldering were identified on the basis of electron diffraction and EDX analysis without ambiguity. In the Ni-P under bump metallization near the interface with the solders, P-denuded Ni-20 mass%P layer was observed, where a high density of columnar voids were formed. Also, in Ni(Cu) SnP intermetallic, which is adjacent to the P-denuded Ni-20 mass%P, a high density of spherical voids were observed. These voids degrade the joint strength. Change of the above microstructures during thermal cycling between 253 K and 453 K was also observed.
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 b Sn matrix dispersed with Ag 3 Sn particles. Analysis of the diffraction patterns shows that there are more than one orientation relationships between b Sn and Ag 3 Sn. 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.
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