Thermosonic gold wire bonding to palladium finishes on laminate substrates

As the laminate substrate industry moves from hot air solder level (HASL) finishes, alternate plating finishes are being proposed, such as immersion gold/electroless nickel, electroless palladium and electroless silver. This paper presents results of an evaluation of the thermosonic gold ball wire bondability of electroless palladium. Two palladium thicknesses, with and without a nickel underlayer, from two vendors were evaluated. In each case, a thin gold passivation layer was deposited by immersion plating over the palladium. The initial evaluation criteria included bondability (number of missed bonds and flame-off errors), wire pull strength, standard deviation, bond failure mode and visual inspection. The bonding window was determined by independently varying force (4 levels), power (4 levels) and time (2 levels). The stage temperature was maintained at 150°C, compatible with BT laminate material. The effects of different preconditioning environments such as a solder reflow cycle, high temperature storage (125°C) and humidity storage (85% RH/85°C) on initial bondability were also considered. Rutherford backscattering and Auger analysis were used to examine the surface finishes. The stability of the bonds was investigated by high temperature storage (125°C) with periodic electrical resistance and pull strength testing