Development of advanced wire bonding technology for QFN devices

Quad Flat No-Lead (QFN) is one of the fastest growing semiconductor packages. It offers a variety of advantages including near-chip scale footprint, reduced lead inductance, thin profile and low weight. It also provides good thermal and electrical performance. Wire bonding is an ideal choice to connect integrated circuits (IC) and leadframes in QFN devices, due to its low cost, flexibility and reliability. However, wire bonding on QFNs is more challenging than on BGAs and QFPs, especially for stitch bond (2nd bond). Copper wire binding increases the challenges. K&S has been developing advanced wire bonding solutions for QFN devices. Finite element analysis has been conducted to fundamentally understand the mechanical behavior of the wire bond, including the effect of the process parameters (force and scrub), the capillary design, and the substrate material. In our experiments, standard metrology has been developed to characterize the process window, stitch pull strength, Cu remains, and other important responses. A new 2nd bond process, named ProStitch Plus, has been developed to solve most of the challenging issues with QFN wire bonding. This feature provides significant improvements in both bondability and productivity. It provides significantly wider process window, improved stitch strength, more Cu remains and reduction in peeling and short tail. It also has improved tolerance to poor clamping. Especially, it enables bare Cu wire bonding on most challenging materials - roughened μPPF QFNs. This will dramatically increase the quality of QFN packages and hence reduces packaging cost. In order to understand nano-scale interface between Cu wire and μPPF, high-resolution transmission electron microscopy (HR TEM) has been performed. A nanoprobe beam was used for composition analysis with energy-dispersive X-ray spectroscopy (EDX) in scanning (S)TEM mode.