Copper wire bonding challenges and solutions of small outline packages

Moving towards next level of major cost saving in small outline packages, conversion from gold wire to copper wire became necessary action. With normal production expectations of fast bonding and good quality performance on flimsy leadframe, copper wire challenges of small outline packages were incredible. Furthermore, situation became worse when new copper wire process was found requiring additional bonding time and mechanism due to harder material properties. In order to enjoy full saving of material cost, there was no loss expected on yield, quality, throughput, and machine efficiency. Under these stringent requirements, development of copper wire bonding process requires full considerations and optimization of all possible root causes, with targeted performance matching gold wire bonding process. By utilizing Six Sigma DMAIC methodology, appropriate characterization and optimization were performed on major elements, material, machine and bonding method. For wafer technology that come with thin top metal, bonding optimization was important in ensuring additional stress from Cu wire process would not cause any metal peeling. In order to achieve that, new bonding method was introduced by flattening bonded ball and then applying gentle ultrasonic vibration. This approach was later discovered to be one of the most important factors in ensuring successful reliability tests. When established process ran on more wire bonders, copper wire bonding was found very susceptible to leadframe clamping condition. Any slight unit floating caused positional type non-sticking, and most of time it happened regardless to any parameter settings. In order to resolve this issue on machines with multiple bonding sites, a new machine characterization methodology was established. This method focused and emphasized on variations of machine parts which are in contact with the leadframe. The most important tool created was the new quantifiable measuring sub-systems on machine parts, absolutely different from conventional type of product buyoff. On the machine itself, bonding mechanism was analyzed through new established external instruments, especially for some older generation machines that never provide such utilities in the operating software. Other than that, comprehensive study was also performed on the material factors. From the study of capillary, certain design and surface roughness provided significant advantage in the wire bond responses. As for the wire properties, study showed that wire coupling with optimum electrical firing parameters and air cushion could provide robust Free Air Ball formation, free from oxidation risk. In the overall combination of optimum settings and conditions, copper wire bonding showed significantly improved performance, whereby cycle time and reliability was not different from previous gold wire process. The project success was more precious when older machines were beneficial from the study, and therefore copper wire conversion did not require any capital investment. This project has enabled substantial amount of cost reduction possible for small outline packages, with zero impact to capacity or customer quality risk.