Comparative performance of gold wire bonding on rigid and flexible substrates

This paper reports comparative performance of wire bondability of electrolytically plated Au/Ni/Cu bond pads on rigid FR-4 and bismaleimide trazine (BT) PCBs, as well as flexible polyimide (PI) substrate. The metallization surfaces were treated with plasma to study the effect of bond pad surface cleanliness on wire bondability. Process windows were constructed as a function of bonding temperature and bond power for the individual substrate materials. Significant improvements of wire pull strength and process window were noted after plasma treatment with a substantial reduction in minimum bonding temperature from 120°C to 60°C for both the rigid and flexible substrates. The minimum bond power required to produce successful bonds decreased with increasing bonding temperature. At a bonding temperature of 120°C, the process window for the flexible substrate was wider than the rigid substrates. The wire bondability and wire pull strength of rigid substrates decreased with increasing bonding temperature above 120°C due to softening of the substrate which adversely affected the effective bond force and the transmission of ultrasonic energy. In contrast, the wirebonding performance of the flexible substrate remained stable at 120°C or above because the thermo-mechanical properties of flexible PI substrate were rather insensitive to temperature. The process windows of flexible substrates with and without stiffener showed similar bondability.