QUICK ASSESSMENT OF THE MINIMUM PEAK REFLOW TEMPERATURE REQUIRED IN LOW-AG SOLDER ASSEMBLY

摘要

There is an ongoing interest in minimizing peak process temperatures for some SMT assemblies, notably when using inexpensive printed circuit board materials or complex multi layer substrate constructions. Each combination of materials and designs may, however, call for a different minimum. The thorough qualification of a proposed process is a major undertaking that usually allows for the inclusion of only a very few, if any, alternative parameters. There is therefore often a need for an initial rapid identification of the specific process parameter values to include in the final test matrix, e.g. for an initial prediction of the peak temperature range to consider. This was accomplished for selected low-Ag alloys through a combination of microstructure analysis and simple tests, the criterion being that structure and properties should not differ significantly from those achieved using more conventional, higher temperatures.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. The goal was a preliminary estimate of the "coolest" reflow profile that can be used for each of these 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 in the overall project, but the present discussion will be limited to ball shear strength, solder microstructure, and performance of assembled parts in accelerated thermal cycling. Combining the conclusions of the individual tests for a given set of solder alloy and pad finishes, peak temperatures were determined that appeared to be the absolute minimum that every single solder joint should reach to ensure good soldering.