Thermosonic gold wirebonding to electrolessly-metallized copper bondpads over benzocyclobutene

Multilevel high-density substrates require wirebonding to bondpads which are located over thick polymer dielectric layers. We have studied the ability to thermosonically wirebond gold onto Cu/Ni/Au surface metallurgy consisting of 2 μm Cu representing the bondpad followed by 1 to 4 μm electroless Ni and 1 μm electroless Au over 5 - 20 μm of BCB (Cyclotene®) dielectric, Results suggest that backcooling during sputter roughening of the BCB is i to promoting adhesion between the sputtered Ti sublayer and BCB. At BCB thicknesses of 0 to 10 μm and any Ni thicknesses in the range of 1 to 4 μm, the bonds passed pull testing exhibiting a strength of 9 grams and a clean tensile failure of the bond wire. Bonding over 15 to 20 μm of underlying BCB required higher bonding power and dwell times but reliable bonds could be made with reasonable bonding parameters if 3 or 4 μm ofNi was present to stiffen the pad., Without sufficient Ni thickness the bondpads ripped from the substrate during bond pull testing revealing cohesive fracture damage to the BCB under the pad.