Scientific imaging arrays need to have their individual imaging elements arranged in a close-spaced mosaic. The typical single imaging element is a silicon chip mounted on a larger support frame. This excess area of the support frame takes away valuable imaging space from the mosaic. This appears as a grid of black (no data) in the overall mosaic image. Making the support frame smaller makes the amount of lost data smaller, and the imaging elements can be spaced more closely together. Eliminating the support frame altogether brings the imaging elements even closer. This is referred to as four-side buttable.
In the case of a backside-illuminated imager, one way is to wire bond to receiving pads located on the inclined walls of a central support structure. The degree of inclination, in this case, is chosen to permit a line of sight to the detector’s bond pads. Typical wire bonding stations allow observation of the pads through a microscope that has a line of sight fixed at 30° from vertical. While respecting this observational constraint, it is possible to make a wire bond going from the device’s pads up to the receiving pad by rotating the part 60° between the two bondings. The fixture to rotate the part has to do so smoothly without breaking the fine wire at the wire bonder’s capillary, or pulling the wire out of its firstbond position. This limit of motion is about 100 microns.
In general, any degree of inclination could be accommodated, provided there is a means to see the bond pads when they are rotated into position. Also, physical access of the wire-bonding tip to the pad is required. Key to its operation is an x–y adjustment of the holding fixture that moves the rotational axis close to the line of bond pads. This adjustment only needs to be made and set once for a given imager.
The ability to mount and wire bond electronic chips on tilted surfaces will allow designers to create smaller and lighter instruments for space missions. Electrical interconnections for the imaging elements of a focal plane array can be moved out of the imaging plane, and gaps between sensors can be reduced.
Similar procedures can be applied to the general assembly of electronic components. This approach to interconnecting sub-assemblies would allow novel configurations and space-saving solutions.
This work was done by Todd J. Jones of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Technology Transfer at JPL
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Refer to NPO-49716.
This Brief includes a Technical Support Package (TSP).
Wire Bonding to Pads in Tilted Planes
(reference NPO49716) is currently available for download from the TSP library.
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Overview
The document outlines the objectives and results of a research and technology development (R&TD) project led by Tim Goodsall and a team at NASA's Jet Propulsion Laboratory (JPL). The primary goal was to deliver two 2kx2k delta-doped CCD (Charge-Coupled Device) detectors to the Caltech Optical Observatories (COO) in a four-side buttable package. These detectors were designed to maximize broadband response from 320 to 1000 nm, enhancing UV sensitivity significantly, particularly from 350 nm to below 100 nm.
The project is part of the Wafer Scale Prime (WaSP) imager upgrade for the 200-inch telescope at Palomar Observatory, which aims to replace the existing six-CCD mosaic with a single 6kx6k CCD. The upgrade necessitates dedicated guide and focus CCDs. The guide CCD is crucial for providing feedback to ensure accurate tracking of observing targets during long exposures, while the focus CCD maintains image focus as thermal effects cause drift throughout the night.
To achieve accurate tracking, the guide CCD must take high-cadence exposures (up to 1 Hz), necessitating maximized blue response due to low guide star flux in the U-band. The CCDs used are 200-micron thick STA devices, operated in a fully-depleted mode, which enhances red sensitivity by up to six times compared to conventional thinner CCDs.
The document also details the packaging design of the detectors, which consists of an aluminum nitride support plank for precise bonding, a thicker invar support for mechanical stability, and custom flex cables for electrical connections. The design allows for close abutment of the package to surrounding detectors, achieved through beveled edges and a flexure mount design that prevents mechanical interference.
A significant innovation discussed is the development of a technique for wire-bonding between non-parallel planes, which allows for the integration of flex connector pads in different orientations relative to the CCD bond pads. A custom wire-bonding jig was created to facilitate this process, enabling the detector to be rotated during assembly.
Overall, the document highlights advancements in CCD technology and packaging that enhance the capabilities of astronomical imaging systems, showcasing JPL's commitment to innovation in aerospace technology.
