Patent Application
20070194089
The subject invention generally relates to the field of electronic interconnects, and more particularly to high performance connections between circuit boards.
Electronic instruments and other devices are often assemblies of electronic systems and subsystems. For manufacturing efficiency, different systems are carried on different substrates, which may then be attached to a larger substrate or “mother board.” These connections have several limitations.
One limitation of board-to-board connections is that conventional connections are made at the edge of one board, which has conductive strips that are received in a connector on the mother board. Thus, when a greater number of connections are needed, the edge dimensions may limit the number below a needed threshold. In addition, conventional edge connectors require a perpendicular arrangement between the mother board and the attached board, increasing the volume needed to contain the assembly.
Ball grid arrays have been employed to provide a connection between a board parallel to and overlaying a larger board. While effective for some applications, they suffer from performance deficiencies. A solder ball connection is capacitive, and can create excessive electrical parasitics that impair high-speed applications. In addition, this type of connection is susceptible to generating and receiving electrical crosstalk from other connections, because conventional solder ball connections lack the shielding that is found in other parts of the signal path, including on the boards, in cabling, and in other connections within and outside of the instrument.
Accordingly, there is a need for a method and apparatus for providing a compact, high performance connection between circuit boards. The preferred embodiment provides this in the following:
A circuit assembly has two circuit boards parallel to each other and having major faces facing each other. Each major face has a signal contact connected to a signal trace. At least one of the major faces has a ground contact surrounding the signal contact. The signal and ground contacts are connected by way of a column element defining an axis perpendicular to the board faces. The column element has an inner conductor electrically connecting the signal contact, and a shield electrically isolated from and surrounding the inner conductor. A number of the column elements, as well as solder elements, may be supported by a carrier, which is placed between the boards for an electrical connection process, and which then may be dissolved.
The main board 12 has a major upper face 22 that has a grid array 24 of conductive pad patterns 26, each of which is connected to other circuitry on the board by traces (not shown) that extend on the surface of the board, or in layers within the board. The main board may be a mother board of a computer, or a board for a system or subsystem of any electronic instrument.
Above the board 12, the connector matrix element 14 is a solid body that contains a grid of conductive elements 30, with the grid arrayed the same as the array 24 of the board 12, so that each conductive element is registered with a pad 26.
The secondary board 16 has the chip mounted on the upper surface, and has a pattern of contacts on its lower surface in the same spacing and configuration as those of the matrix element 14 and main board array 24.
The conductive elements 30 are of two different types. For the concentric contact 32, a concentric column element 42 is employed. The column is a cylinder defining an axis 44 that is perpendicular to the surfaces of the circuit boards. The column has a lower end face 46 and upper end face 50 that are perpendicular to the axis. The column has a central conductive signal conductor 52, a surrounding dielectric layer 54, which is surrounded by a conductive shield sleeve 56. The shield is coated by a layer of solder 60, which is in place for subsequent processing to provide connections. In the preferred embodiment, the signal conductor is a copper wire with a diameter of 0.20 to 0.25 mm, the dielectric can be teflon, glass, ceramic, or adhesive with a thickness of 0.15 to 0.20 mm, and the shield is formed of Ni—Au plated Cu, with an outer diameter of 0.6 to 0.75 mm. The solder layer has a thickness of 0.10 to 0.25 mm, giving the column a diameter of 0.70 to 1.00 mm. The column has a length of 0.50 to 3.00 mm.
The second type of conductive element that may occupy one of the cylindrical holes of the matrix 14 is a solder ball 62 that has a diameter to be closely received in the matrix holes. In the preferred embodiment, the matrix is formed of heat resistant, solder phobic material, has a thickness of 0.4 to 2.5 mm, holes on a 0.80 to 1.50 mm center-to-center spacing, and with a diameter of 0.75 to 1.05 mm. The hole diameter and matrix spacing are about the same, so that during heating, the solder will wick to contact both boards above and below. In an alternative embodiment, the matrix may be formed of a water-soluble flux resin carrier material such as poly-oxide resin and dissolvable paper, so that it may be washed away following the soldering process. The matrix may be assembled by either (i) inserting the elements into the an alignment plate/reflow carrier with a grid of holes (with larger diameter than the coaxial columns to enable self-alignment due to solder wetting forces during reflow) corresponding to the pad patterns on the boards, or (ii) a dissolvable matrix may be cast or molded about the array of conductive elements. In the first approach the substrate would be placed, aligned, and reflowed to the grid of elements. The elements would be co-planar in the carrier and protrude above the carrier to ensure solder wetting and attachment.
The upper board 16 has contacts on the lower face that are essentially the same as those shown on the upper surface of the main board. For connecting to a column element, an annular contact 64 connects to the shield 56, and a central signal contact 66 connects to the signal conductor 52. The upper surface of board 16 defines contacts that enable the connection of the chip, via solder bumps or any other means. The board has several internal layers, such as a ground plane 70 that connects to the pad 64, and which has openings where signal paths must pass through. A simple pad 72 on the lower surface provides a contact to the solder ball, and is connected to the ground plane.
The internal layers and conductors of the boards are shown in greater detail in
The board 120 is similar to that in the preferred embodiment, with a grid array of contacts for columns of solder ball connections. However, the pattern for a column connection is as shown, with a circular ground pad 122 defining a radial gap 124 extending from a center point 126 of the circular periphery. A signal trace 130 extends from the center, with a gap 132 on all sides between the ground pad and the signal trace.
