Wire bonding with Pd-coated copper wire

The last three years have seen an aggressive implementation of fine-pitch copper wire bonding in face of ever-increasing gold prices. Numerous technical hurdles associated with copper wire bonding have mostly been overcome. However, achieving process robustness equivalent to gold wire bonding is still challenging and requires enhanced manufacturing control procedures. Although the 2nd bond in copper wire bonding is generally very reliable, it often requires additional process steps, such as skidding or scrubbing motions. These processes require additional time and cause an overall reduction in wire bonder throughput. Pd-coated Cu wire is emerging as an alternative to bonding with bare Cu wire. While not quite achieving the same cost savings as bare copper wire, it offers the promise of a more robust bonding process. Being a noble metal, the Pd-coating prevents oxidation of the wire surface. This improves 2nd bond performance, increases shelf life and protects the wire against corrosion. Better 2nd bond performance may allow for faster 2nd bond processes in some applications and hence greater throughput relative to bare copper. The effect of the Pd on the first bond is less certain. While residual Pd on the ball surface may enhance adhesion by analogy to the improvement seen at 2nd bond, the distribution of the Pd on the surface and within the ball is generally not uniform. Cross-sectional analysis of free air ball formed under different EFO parameters revealed different Pd distribution within the ball with more non-uniform mixing and swirling structures at high EFO current settings. In addition, formation of Pd-Cu alloys in the ball will increase its hardness at high EFO settings compared to low settings and may in turn exacerbate damage to bond pads, which is already a problem in copper wire bonding. This paper compares the bonding performance of Pd-coated Cu wire to bare copper wire at both 1
^st
and 2
^nd
bond. Bond strengths and process window are characterized. It is shown that Pd-coated wire can be successfully bonded using nitrogen rather than forming gas as the cover gas during ball formation. Bonding with nitrogen would partially recover the higher cost of the wire relative to bare Cu.