| Abstract |
| The morphology and thermal stability of the interfacial phases in the joints between lead free solder (Sn3.5AgO.7Cu) and electroless Ni-P (12 at%P) and electro Ni under bumper metallizations(UBM) were investigated as a function of thermal aging. Samples were prepared by reflowing at 250℃, N₂ atmosphere and aging in the range of 125 to 170℃ for up to 1000h. After reflow, in the electrolytic Ni/solder joint, the interfacial IMC was mostly Nirich (Ni,Cu)₃Sn4 with an addition of about 8 at% Cu while in electroless Ni-P/solder joint it showed both Cu-rich (Cu,Ni)_(6)Sny and Ni-rich (Ni,Cu)₃Sn4. After relatively short aging time, the interfacial IMC on electrolytic Ni layer has developed into the dual-layed structure of (Ni,Cu)₃Sn₄(bottom) and (Cu,Ni)_(6)Sny(top). But IMC on electroless Ni-P layer were (Cu,Ni)_(6)Sn_(6) and changed into the dual structure only after long aging time of 500h. The growth rate of IMC with aging was higher on electrolytic Ni layer than on electroless Ni-P. The different behavior of electroless Ni-P/SAC joint came from Ni₃P formation between IMC and electroless Ni-P layer which retards diffusion of Ni and Sn to the opposite direction. The thickness of Ni₃P layer was propositional to that of IMC. For longer aging time, NiSnP layer was found at the interface between IMC and Ni₃P layer. Interestingly, in spite of low consumption rate of electroless Ni-P than electrolytic Ni layer by suppressing IMC growth, the interface of Ni-P/IMC was locally hollowed by the accelerated reaction of the Ni-P layer through local thickness reduction of Ni₃P and NiSnP layers. This reduced the effective remaining thickness of Ni-P layer. |
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| Key Words |
| Lead-free solder, IMC, Intermetallic compound, Electroless plating, BGA, Ball grid array, UBM, Under bump metallization, Solder joint |
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