Pad bending improvement on copper wire bonding on NiP/Pd/Au bond pad

Abstract

Due to the high gold price, conversion from gold wire to copper wire has been a widely accepted method in semiconductor assembly for cost reduction. However, conversion from gold wire to copper wire is not a straight forward conversion. Copper wire is about 30% harder than gold wire and the commonly used bond pad metallization is Aluminium base (which is softer compare with Copper). Potential challenges include pad cratering, Al splash, lifted ball and reliability concern. Therefore, harder bond pad (plated on top of Aluminium) with Nickel as the base material was introduced. Ni which is harder than Cu, offer protection to the underlying structure, especially for probe and bond over active area products (XoAA). Technically, NiP will only bend down when the bonding impact is applied. The underneath Aluminium layer will be squashed out exhibited bond pad bending. Bond pads bend down at the centre coincide with the ball bond position and bend upward at both edges of bond pad. More severe pad bending will be observed at the direction parallel to the direction of ultra-sonic vibration and potentially cause oxide crack underneath the bond pad. Thus, the understanding of interaction between wire bond parameters and bond pad bending is very crucial to prevent oxide crack. This paper presents pad bending improvement study focusing on NiP thickness, bonding parameters and capillary design. In the stage of screening to identify the key input parameters, the results show very significant reduction on pad bending by lowering ultrasonic power. However, there is a limit for ultrasonic power in order not to compromise on the wire bond performance on non stick on pad occurrence. Thus, there is a need to consider capillary with build in design that can function with lower ultrasonic level. From experiment run, it is proven that capillary design can greatly reduce the ultrasonic power required (more than 50%) without occurrence of non stick on pad. Besides this, Ni plating thickness also shows significant impact on pad bending, a DOE approach was used to characterize and define suitable bond parameters window. A three factors (Ni plating thickness, ultrasonic power and bond force), two level factorial design was used to examine interaction and main effect. The experiment shows significant main effect by Ni plating thickness and ultrasonic power. Pad bending is less severe for lower ultrasonic power and thicker Ni plating thickness. With the defined window, XoAA device passed XoAA assessment and subsequently meet reliability requirement. Thus, it proven that this methodology is workable. In summary, Copper wire bonding on NiP based bond pad (plated on top of Al) is feasible and inline with published papers [1,2]. However, pad bending need to be considered to avoid cratering on XOAA device. Pad bending can be improved with lower ultrasonic power, capillary design and thicker Ni plating thickness. NiP plating thickness should also be included in the DOE to define suitable parameter window.