Enhanced stitch bonding concept for QFN package´s Cu wirebonding process

For a products such as QFN base package using a leadframe with Palladium (Pd) pre-plated finish (PPF) is a challenge for the Cu wire bonding. Bondability of Cu wire to PPF leadframe have some constraint with the conventional bonding parameters widely used in Au wire bonding process such as Power, Force and Time. It thus result to a frequent alarm of shortail or even NSOL (non-stick on lead) affecting the Yield and its overall manufacturability performance. Typical bonding concept defined in Au wirebonding process is a “thermosonic” bonding or a Power dominant approach that doesn´t work well in Cu wire bonding. It´s been a nature of Copper wire material which is harder than Au at Vickers hardness of 0.369 versus the Au Vickers hardness of only 0.216. Despite for these challenges, it does not limit to further explore on what is the best combination and suitable process parameter can be use in a PPF base leadframe. A careful understanding and thorough characterization of material and process parameters has been studied to avoid the overbond issue that may impact the process or eventually affect the package reliability. On this paper, it discuss mainly on how to achieve a better stitch bondability through a different approach and parameter concept on QFN (quad flat no-leads) package as the lead vehicle. With the use of Cu wire, basic application of Power, Force & Time resulted to an inconsistent process response due to frequent shortail alarms and NSOL. Second bond stitch visual characteristics observed a “fish tail” like formation with weak or low wirepeel strength. A different approach on the parameter combination has been studied intended with a very low Power or even no Power application combined with a non typical parameter such as the Scrub. The scrub parameter serves as replacement for the Power that delivers weld contact between the Cu wires to leadframe bond finger surface. Pre-installed machine software has a feature tha- able to create a segmented scrub application was utilized as well. Validations were made with a significant improvement in terms of the stitch bondability response and formation. It also addresses to minimize the capillary tip build up that helps to prolong the capillary life which is favorable also for the cost.