The role of friction coefficient on the stitch bondability in Pd coated Cu and bare Cu wire bonding

Due to the cost advantage, copper and palladium coated copper (Pd/Cu) wires are replacing gold wires in wire bonding. The Pd coating enhances the shelf storage time and stitch bond quality of Cu wire as it protects Cu from oxidation. For future developments, it is important to understand the effects coating has on the bonding process. One major process mechanism is the plastic deformation of the bonding wire. The mechanical properties of the wire, geometry of bonding tool (capillary), and the friction coefficient between contacting parts all affect the deformation. A mixed level three factor design of experiments (DOE) is used to investigate the stitch bond process with Au, Cu, and Pd/Cu wires. The stitch bond width and stitch bond length are determined as measures for deformation. It is found that the Pd/Cu wire deforms substantially more than the bare Cu wire, under identical process conditions. The average stitch bond width with Pd/Cu wire is 42.74 μm, ≈10% larger than that with Au and Cu wires. The average stitch bond length with Au wire is 19.86 μm, ≈15% smaller than that with Cu and Pd/Cu wires. A finite element (FE) analysis is carried out to understand the strong effect the Pd coating has on the wire deformation. To support the FE analysis, the friction coefficients between Pd/Cu wire and Au substrates or alumina substrate (capillary) are determined to be 0.687 ± 0.064 and 0.725 ± 0.041, respectively. The frictions coefficients between bare Cu wire exposed to air for three days, and Au substrates or alumina substrate (capillary) are determined to be 0.584 ± 0.049 and 0.619 ± 0.030, respectively. Consequently, the different degrees of deformation in the bonding are contributed to by the different friction coefficients.