This paper describes the tests performed to evaluate the solder capillary action which occurs within a gap between two solderable surfaces during soldering. The goal was to determine the optimal gap distance for maximum capillary flow in the attainment of hermetic solder joints capable of withstanding extreme temperature cycles and various mechanical shocks. One of the test conditions was arranged so that the gap thicknesses would vary while the width of the gaps remained constant. In a second condition, the gap thicknesses remained constant while the gap widths varied. Three plating designs were evaluated. They were nickel plating; nickel overplated with gold; and nickel, copper intermediate, with tin overplate. The capillary action of all three plating combinations deposited onto aluminium specimens, with the gap configurations previously described, was evaluated. The capillary results were measured with X‐ray and microstructural data. End use solder joint designs were determined from the capillary results. These designs are shown and they include the best plating design for the application. In addition, an unexpected result was obtained that is useful for testing the solderability of all finishes—the Configured Capillary Solderability Test.