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

Abstract

Since several years ago, many semiconductors companies have invested a lot of efforts and resources in the wire bonding conversion from gold (Au) wire to copper (Cu) wire in order to achieve the most competitive cost. Meanwhile, the fast growing mobile application and industry continues to leap forwards with miniaturization especially in the most popular QFN (Quad Flat No-lead) packages. From multiple analysis, it looks like breakthrough competitive edge will depend on the success of running wire bonding process of both wire types (Au & Cu) with the ultimate ultra-fine pitch (UUFP) process on the flimsy QFN leadframes. This new process can aim for targeted pitch size of 20um which is the smallest possible in the industry, and most likely it will become the ultimate pitch size for now and future. There are many challenges in the development of such process with major considerations and evaluations focusing on various designs, hardware, machine capability, process constraints, process control and amplification of respective Au and Cu wire process issues. The UUFP process is firstly started with design considerations on both direct and indirect material. From the discussions with major bonding wire suppliers, current feasible wire size of Au wire is 0.4 mils and Cu wire size is 0.5 mils. With both wires selected, next design consideration is the capillary design. The capillary tip needs to be the smallest possible together with the tightest tolerance of every major dimension. These requirements resulted in extremely tedious fabrication process which requires new molding tool and process enhancement. Furthermore, such small capillary can also encounter robustness issue due to the thin capillary wall and dimension tolerances. Upon receiving the wire and capillary, the subsequent challenge is the machine capability. There are two important considerations, resolution of process parameters (ultrasonic power and force) and bonding placement accuracy. In the previous development of 30/35 um pitch, whereby it was found that bond force requirement already hitting only several grams. It is expecting that the UUFP process will require even more stringent process window. It is expected few 0.1 gram force may result in significant change in the wire bonding responses. Despite that, the conventional process issues will get worse, which include resonance effect of QFN wire bonding, Au wire challenges (IMC growth, reliability risk) and Cu wire challenges (lifted ball, crater, short-tail). Therefore, the process window development can be extremely difficult. In order to obtain better success rate, comprehensive six sigma method is highly recommended. Upon completion of process window development, a series of robustness tests should be performed. There are some unique robustness tests which can disclose any potential issue or risk. Upon completion of bonding success, the bonded units will be loaded into reliability test for standard tests and extended tests. Sometimes, the entire development process may need to be repeated from the beginning design phase if there is any stubborn quality issue. With the utilization of comprehensive methodology, the new UUFP process is applicable to full production mode in near future. This process is believed to be the ultimate process for wire bonding pitch, for both Au & Cu wire types.