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”, FTD, and set to half the FAB diameter. Compared to "deformability", the FTD 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.