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A method and apparatus for improved impedance control on printed wiring board-based products

IP.com Disclosure Number: IPCOM000004729D
Publication Date: 2001-Apr-23
Document File: 7 page(s) / 120K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method and apparatus for improved impedance control on printed wiring board (PWB) based products. Benefits include an improved reference ground without changes to the commonly available laminate thickness and higher RF design frequencies.

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A method and apparatus for improved impedance control on printed wiring board-based products

Disclosed is a method and apparatus for improved impedance control on printed wiring board (PWB) based products. Benefits include an improved reference ground without changes to the commonly available laminate thickness and higher RF design frequencies.

Improved impedance control for PWB products enables design improvements like increased bandwidth and higher density I/Os. These changes result in faster performance and smaller form factors. As routing densities increase, the reference ground moves closer to the signal trace to maintain standard interface impedance values, such as 50 ohms. The disclosed method provides a closer reference ground for line impedance control without making changes to commonly available laminate thickness.

Most PCBs have an embedded glass weave that improves the material's structural integrity (see Figure 1). In the disclosed design, a portion the weave's strands are made conductive to provide the reference ground for the impedance controlled circuit (see Figure 2). For the conductive filaments to be effective as trace shielding, they must connect to a ground at some point. When the weave epoxy assembly is cut into boards, the ends of the filaments are exposed at the board edge. These exposed ends are electrically connected to a ground placed at the periphery of the laminate or PCB (see Figure 3 and Figure 4).

One possible method for manufacturing this feature is to replace a portion of the glass strands with copper strands. In another method, conductive strands are grown through a controlled conductive filament formation. Conductive filament formation is a well-known defect in PCB manufacturing. When the glass strands are pulled, air bubbles can form in the filaments making them hollow. After the board is manufactured, metal can migrate into the hollow filaments from a board via or other metal structure. Under uncontrolled circumstances, this condition may result in unwanted shorts. However under controlled circumstances, this phenomenon can provide needed trace shielding.

Conventional high performance and high frequency designs use a microstrip or stripline design based on a continuous reference ground plane spaced a given distance from the controlled impedance circuit. Line widths for the impedance circuit can be reduced to provide the same circuit impedance if the ground plane can be moved closer to the circuit trace. By moving the reference ground plane within the laminate, the circuit trace can be reduced in accordance with the dielectric constant for the substrate material. The advantage is finer circuit lines are achieved with less circuit real estate consumed.

The use of an internal laminate ground opens the design parameters for two impedance-controlled circuits on opposing sides of the reference ground plane. The circuits could be etched onto a copper-clad internal ground laminate using standard subtraction chemistries. Th...