The influence of low level doping on the thermal evolution of SAC alloy solder joints with Cu pad structures

The behaviour or BGA solder joints with Cu pad structures was studied as a function of reflow conditions, thermal aging and Pb-free alloy composition. All Pb-free solder compositions studied were slight compositional modifications of the standard Sn-3.0Ag-0.5Cu (SAC 305) alloy. Emphasis was placed on studying the effect of low level doping with Co, Fe, In, Ni, Zn and Cu with respect to two anomalous behaviours, found in this system, and which depend on the source of plated Cu: the variability within the rate of elemental Cu consumed under the same reflow and thermal aging conditions, depending on the source of plated Cu; and the variability, depending on Cu plating source, in Kirkendall void formation and growth, during thermal aging, in the Cu-Cu₃Sn interfacial solder joint region. To assess and characterize the effects of these dopants on these behaviours and on the solder, itself, parts were sampled and tested after various reflow and thermal aging conditions, using ball pull testing, metallurgical cross sectioning, polarized light microscopy, SEM examination and hardness testing of the solder. Low level Zn additions to the standard SAC 305 solder have a significant effect on solder joint interfacial intermetallic compound structures and on the basic interfacial, Kirkendall voiding phenomenon. Low level Zn additions drastically reduce the rate of formation in the interfacial, Cu₃Sn phase, significantly reduce the Cu pad consumption rate, during reflow and thermal aging, and dramatically lower the Kirkendall void formation at the interface, during thermal aging. The effects of the other alloying elements, defined above, on the interfacial intermetallic compound formation, solidification behaviour, solder properties and interfacial void formation are less dramatic; but, are, also, discussed in detail