| This research reviews and discusses experimental results obtained from a microstructural analysis of Sn-Cu-(Ni) solder joints formed on both immersion silver and Ni/Au PCB finishes.; The shear strength of Sn-Cu-(Ni) solder joints was found to be equivalent to or better than Sn-Pb solder joints, depending on conditioning. Intermetallic layer growth rates at 125°C in Sn-Cu-(Ni) joints suggested interface-controlled kinetics (i.e., linear in time), while those in Sn-Ag-Cu and Sn-Pb joints suggested diffusion-controlled kinetics (i.e., parabolic in time). The microstructure of Sn-Cu-(Ni) solder joints was determined to be (Cu, Ni)6Sn 5 phases in a Sn matrix, as well as Au-Sn and Ag-Sn intermetallic phases from the reaction of the solder with the circuit board finish. Coarsening of intermetallic phases was observed. Unlike Sn-Ag-Cu solder joints, Sn-Cu-(Ni) solder joints had no evidence of Sn dendrite formation. One possible explanation for this is that Ni inclusions in the Cu sublattice of (Cu, Ni)6Sn 5 promoted faster nucleation, reducing constitutional supercooling and suppressing dendrite formation.; Accelerated thermal cycling results showed that Sn-Cu-(Ni) solder joints had a characteristic life of 30%--49% that of Sn-Pb solder joints, depending on board finish. The characteristic life of Sn-Ag-Cu joints was roughly equivalent to Sn-Pb joints on immersion Ag finish, but only about 63% that of Sn-Pb joints on plated Ni/Au finish. Crack propagation in both Sn-Cu-(Ni) and Sn-Ag-Cu joints was intragranular and predominantly through the bulk solder, not in the interfacial intermetallic region. In Sn-Pb solder joints, cracks propagated along grain boundaries in the Pb-rich region near the resistor termination. (Abstract shortened by UMI.)... |
