Low cost Pd coated Ag bonding wire for high quality FAB in air

A 25 μm diameter Ag wire is coated with a nano-layer of Pd to form a Palladium coated Silver bonding wire (PCS). A new method is used in which the coating is directly attached to the bonding wire without subsequent drawing process. Palladium coated Silver wire produced with this direct coating method (DPCS) is tested for use in wire bonding. The target free-air ball (FAB) diameter in this study is 50 μm. Before forming the FAB, the vertical distance from wire end to the fixed electrode is chosen to be 50 μm - which is intentionally short - to artificially introduce a higher than optimal FAB variation. Under these test conditions, the diameters of FABs produced with PCS in air are found to be more consistent than those obtained with Au wire, having a standard deviation of about 1% of the FAB diameter compared to 3% for Au. These results show the PCS wire forms FABs with excellent uniformity in air, ie. without the use of any shielding gas. The bond process results are improved, too. Average values for the stitch pull force and neck pull force of PCS are 55% and 35% higher, respectively, than those of Au. The shear force is ≈8% higher than that obtained with Au. The PCS bonds pass a 24 h thermal bake at 175 °C in Nitrogen with neck pull and ball shear force values of 12.5 gf and 27.9 gf respectively, higher than those obtained with Au. The fusing current for 25 μm diameter 2.92 mm long wires at 25 °C is measured for Au, Cu, and PCS wires. As Silver is the most conductive of all metals, the fusing current for PCS wire is highest, its average is found to be 1.24 A, i.e. 37% higher than that of Au wire and 18% higher than that of Cu wire.