Sn-Ag系鉛フリーはんだ接合部の信頼性に及ぼす銅表面めっきめ影響

摘要

The microstructure and strength for the EGA joint of Pb free Sn-Ag and Sn-Ag-Cu solder balls with Au/Ni-P plating were investigated by high resolution TEM observation. For the EGA joint using Sn-Ag solder, Ni_3Sn_4 reaction layer formed at the solder/pad interface and also P-rich layer formed in Ni-P plating. The P-rich layer was confirmed to be composed of the Ni-P-Sn ternary compound layer and crystallized Ni_3P layer. The spherical voids formed by Kirkendall effect were observed within the Ni-P-Sn ternary compound layer. Defects were also observed in the crystallized Ni_3P layer, but the morphology was spindly. Since the area surrounding these voids was hardly crystallized, these defects are assumed to be formed by the volume changes from amorphous to crystallized structure. Formation of these P rich layers lowered the joint strength. On the contrary, the alloying of Cu into solder changed the reaction layer to (Au, Cu, Ni)_6Sn_5 and such P rich layers were not formed. Between (Cu, Ni)_6Sn_5 layers and Ni-P plating, thin layer composed of Sn, Cu, Ni and P was observed by TEM observation, but it still retained the amorphous structure free from voids.