Breakthrough development of ultimate ultra-fine pitch process with gold wire & copper wire in QFN packages

Abstract

In order to obtain breakthrough competitive edge in semiconductors industry that requires cost effective miniaturization, the wire bonding process needs to be renewed to highest possible level. The combination of extremely small pitch size with both gold (Au) and copper (Cu) wire in QFN packages provided the best opportunity of success. However, development of such process is also extremely challenging, it requires significant improvement in all aspects of wire bonding process. Base on the detailed analysis, the wire bonding pitch size of 20um should become the target of ultimate ultra-fine pitch (UUFP) process. The bonding with Au & Cu wire provides both options of quality and cost, and QFN were selected as vehicle package which represents the most popular package platform. The major challenges of UUFP process include development of smallest wire size, capillary design, machine capability and process window optimization. From multiple engineering studies, it was determined that the smallest wire size is 0.4 mil (10 um) for Au and 0.5 mil (12.5 um) for Cu. These wires were developed through research of dopant contents, process flow modification enhancement (additional die sets) and parameters optimization. In parallel, capillary dimensions and tolerances were revised several times to accommodate these extremely small wire sizes. Most of the major capillary dimensions were designed up to the physical material limitations. In the early design stage, there were several incidents of capillary tip breakage due to its thin wall was not able to withstand too much bonding stress. The final optimum capillary design could achieve reasonable good results with new tighten tolerances and further enhancement could even achieve stable and robust production performance. With the readiness of direct and indirect material, subsequent study was conducted on machine capability. Most of the parameters resolution needs to be improved in order to enable most precise setting within the tight operating range. The required task was working out optimum process window through comprehensive study, involving multiple DOE (Design of Experiment) and RSM (Response Surface Method). In the process optimization, the critical challenges are not only limited to making smaller bonded ball. All the previous discovered issues with Au wire, Cu wire and QFN bonding would be amplified. Some of those issues include Cu wire displacement/damage, Cu wire lifted bond, Cu wire 2nd bond challenges, QFN leadframe resonance issue [1], Au wire inter-metallic issue, etc. All these challenges have main effect and interaction effect, making the screening and optimization efforts becoming very complicated. Only with comprehensive optimization, a robust and good process window could be obtained. The optimum UUFP process promised to offer the most miniature package together with the most effective cost, as the ultimately competitive solution to obtain largest market share.