Pb-Free Assembly With Low-Ag Solders For High Yield And Solder Joint Performance
Authors: Pericles A. Kondos, Michael Meilunas, and Peter Borgesen Company: AREA Consortium, Universal Instruments, and Binghamton University Date Published: 5/4/2011
ICSR (Soldering and Reliability)
Abstract: The properties of solder joints formed by a variety of low-Ag alloys on boards with Cu/OSP, ENIG or electrolytic Ni/Au pad finishes were studied as a function of peak reflow temperature, in an effort to determine the "coolest" reflow profile that can be used for such alloys and still produce good, reliable solder joints. The profiles varied from one with a "definitely too cold" peak temperature of 225°C to one "safely hot", with a peak temperature of 256°C. A requirement was to use SAC305 paste to attach the alloys to be tested to the PCBs. Several properties were considered during this project, but the present discussion will be limited to ball shear strength, solder microstructure, and performance of assembled parts in accelerated thermal cycling. Each alloy/pad finish combination was treated as a different system, since the interaction between the pad metallizations and the solder alloys changed the final composition and microstructure of the joints, therefore potentially the joint properties. Generally it was seen that when there were clear differences between joints made with the coldest profile and the others, the second coldest resulted in behavior that often was in between while the rest gave results practically indistinguishable from each other. Combining the conclusions of the individual tests, a peak temperature was determined that appeared to be the absolute minimum that every single solder joint should reach to ensure good soldering.