Lead Forming and Outer Lead Bond Pattern Design for Tape-Bonded Hybrids

Tape automated bonding (TAB) has progressed to such an extent that hybrid devices are now being designed and built with this technology. In contrast with monolithic device configurations, where inner and outer lead bonds are coplanar, the hybrid device requires the leads to descend from the chip top surface level to the substrate level. It is desirable that the leads follow a carefully designed trajectory to match a substrate bonding pattern of a definite repeatable design. Funded by contracts from the U.S. Army Electronics Command (ECOM) and by internal sources, Honeywell Avionics Division developed a number of different designs for lead forming and outer lead bonding. These designs are now matured to the point that they are being standardized and incorporated into manual and automatic bonding equipment built by the Jade Corporation. They are also being submitted to American Society for Testing and Materials (ASTM) for consideration. Early experimentation with lead forming revealed a two-piece forming tool unable to shape the leads to the designed trajectory. This trajectory, where the leads simultaneously descend and spread out, was termed "fan-out on the slope." The lead frame was redesigned to present identical cross-sectional areas for all leads in the bending areas. Persisting alignment problems after forming resulted in the design and trial of a more complex "triple-action" forming tool. Finally because of continued questionable results, the fan-out on the slope approach was abandoned, and a "fan-out on the level" approach was selected, where the lead fan-out is accomplished on the chip top level before the lead descends to the substrate surface. Next, the development effort was directed to standardize tooling for manual and automatic bonding of multiple chip sizes. Resulting were a set of three outer lead bonding patterns: small, medium, and large, corresponding to carder tape window sizes of 5, 7, and 10 mm, respectively. The basic premise was to make use of all the available beam length by excising as close as possible to the Kapton window in order to allow maximum chip sizes in each of the patterns. The small size measures 170 rail square and can accept chips up to 80 mil square and up to 24 leads. The medium-size measures 250 mil square and can accept chips up to 160 mil in largest dimension and up to 40 leads. The large size measures 350 mil and can accept 250-mil chips and up to 48 leads. The small pattern is designed so that, with 10-mil spacing between adjacent pads and 50-mil spacing from the edges, exactly five patterns fit on a 1inch substrate length. With the same 10-mil spacing, exactly two small patterns fit in the space of one large pattern. The lead trajectory of all three patterns is identical up to a point 40 rail from the excise line, inside of which it is matched to the chip layout. A provision is made for vias and corresponding clearance underneath the lead trajectory inside the outer lead bonds