Free-air ball formation and deformability with Pd coated Cu wire

With 20μm diameter PCC wire, spherical (well-shaped) free-air balls (FABs) were obtained in both forming gas and N₂ at flow rates between 0.25-0.75 l/min and 0.4-0.5 l/min, respectively. With Cu wire, in comparison, sperical FAB are obtained in forming gas with flow rates between 0.25 and 0.5 l/min while in N₂ gas, FABs are always pointed or golf-clubbed (mis-shaped). Using a shielding gas with Au wire makes the FABs mis-shaped. The FAB deformability is measured as the amount of deformation under a given load. Au had the highest deform-ability. The FABs made with PCC wire in N₂ had generally the same deformability as Cu FABs in forming gas. The force required to achieve a pre-defined deformed ball height (target) is named “force for target deformation”, F_TD, and set to half the FAB diameter. Compared to "deformability", the F_TD method is more similar to the standard tensile or compression tests that determine yield strength. Deforming a PCC FAB obtained with 0.4 1/min N₂ to the target ball height requires 615 ± 32 mN, a value not significantly different from that for a Cu FAB obtained in 5 l/min forming gas. For comparison, the target deformation requires 394 ± 16 mN for a Au FAB.