Challenges of 43µm Cu bonding on very thin & softest Al bond pad structure

As the popularity of Cu wire bonding continues to grow, it inspires more packages to introduce Cu bonding to enhance product electrical, thermal & reliability performances at the same time enjoying unit package cost down. Risk of damaging the bond pad structure underneath in Cu bonding is always a challenge especially bonding on very thin bond pad structure with large wire size. 43 μm Cu wire was selected to replace 50 μm Cu wire size with the consideration of its equivalent electrical resistance & thermal performance to 50 μm Au wire. 43 μm Cu wire enjoying the benefit of reducing bond pad stresses during bonding in addition to the line complexity reduction and standardization by replacing 38 μm & 50 μm Au wire with only 43 μm Cu wire. Besides, it is also able to increase bonder capacity and throughput by bonding less wire in a package without sacrificing performance when replacing 38 μm wire size. Fundamental Cu free air ball (FAB) quality assessment in achieving spherical shape and oxidation free during sparking is always a key factor prior to start of actual Cu bonding process. Orthogonal response surface methodology (RSM) experiment design was adopted in defining the critical optimum process window for robust Cu bonding both process-ability and reliability. 43 μm Cu bonding qualification on very thin bond pad of 1.4 μm Al thickness without pad damage and obtain minimum 0.2 μm Al thickness remained underneath is a major challenge in this paper. With the thorough process optimization, 43 μm Cu wire was finally ready for qualification with 1.4 μm Al thin pad thickness & beyond for consumer products whereas 3.2 μm and beyond Al bond pad thickness products for automotive qualification.