Wire Looping Optimization in Fine Pitch Dual Row In-line Pad Devices

摘要

Fine pitch wire bond technology for IC devices continues to evolve. More challenging designs have been developed in order to provide opportunities for tighter pitch, smaller die size, more die per wafer, as well as provide more functionality to customers. The traditional in-line bond pad design would utilize smaller bond pads and smaller wire diameter and capillary, which creates a myriad of assembly processing and reliability difficulties. The industry is still very reluctant to embark on volume production at 35um pitch using 15um wire diameter. Staggered bond pad designs provide a tighter effective pitch while employing less aggressive assembly technology. However, this design layout tends to have large die corner keep-out, thus increasing die size. Dual row in-line bond pad layouts have now emerged, where two rows of bond pads are placed around the die perimeter, in line in both the X and Y directions. In a 47pm dual in-line pad layout (44um pad width and minimum 3um between pads), the effective pitch of 23.5um can be achieved with a 20um wire diameter. However, there are numerous challenges in wire bonding a dual row in-line bond pad design. These challenges include: the ability to create multiple wire tiers and special wire looping to ensure no wire shortage between the rows, clearance between the top of the encapsulation and the tallest loop, and long wire length to accommodate high I/O count. This paper will begin with a brief description of the 47um pad pitch dual in-line pad design. The paper then focuses on the assembly challenges associated with dual in-line design. Systematic experiments were conducted to evaluate the wire bonder's stability in forming different wire loop heights, kinks, and lengths consistently. Evaluations on die thickness on different package platforms were carried to define the die thickness necessary to allow sufficient clearance above the Silicon and the top of the encapsulation. Wire bond parameter optimization was critical to achieve both ball size reduction and bond strength stability. Wire loop height and scheme were optimized to minimize wire sweep during encapsulation. Package reliability testing was also performed. The 47um dual in-line design can indeed deliver a robust fine pitch solution without stretching assembly into ultra fine pitch production.