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.
Pd coated wire is increasely being used as a substitute for bare Cu wire. Being a noble metal, Pd coated wire has high resistance to oxidation enabling longer shelf life. Its chemical properties also exhibit better second bond-ability on micro PPF lead frame enabling simple bond process translating to high throughput and yield. It has higher stiffness which is able to minimize the wire sweep especially for LQ FP, as well as thermo-mechnical robustness.However, there are a few challenges to be overcomed before the bonding process can be released. Basically, Pd will diffuse non-uniformly into F AB after EFO sparking. The formation of Pd-Cu alloys will increase F AB hardness resulting in higher risk of oxide crack issue. Technically, due to its physical properties, Pd coated wire will produce higher bonding impact on the bond pad in order to achieve stable and reliable 1 s t bond process. Conversely, this approach is not feasible for XoAA material. A new 1s t bonding process has to be developed that can produce a stable bond yet able to meet all buy off requirements. This paper will show the study of Pd coated wire interaction with pad metallization of NiP/Pd/ Au on XoAA material. In the 1s t bond process technology development, the effect of capillary on pad structure are examined. The traditional capillary design appears to be detrimental to XoAA material. A special capillary design was introduced that incorporated a different bond mechanism with the consideration of wire properties and pad structure. An extensive Design of Experiment (DOE) is carried out to defme a robust process window.New analysis method by using optical profiling was also introduced for quick and reliable assessment for pad deformation.Bond interface was also validated. Transmission Electron Microscopy (TEM) with EDX line scan analysis showed the presence of Cu-Pd at the bond interface. This study also established two criterias to control oxide crack issue.This process technology is proven and able to meet automotive requirement. In short, the requirements to achieve stable bondability and reliabililty has been developed in this study.
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