Patent Application
20060012452
This invention is generally related to electrical conductors, and more particularly to electrical conductors on printed circuit boards.
It is generally desirable to provide electronic products in the smallest practical size. Consequently, there is a long term development trend of reducing the size of various electronic components such as integrated circuits, passive components, power supplies, and printed circuit boards. In the case of printed circuit boards this trend has prompted development of circuit boards with ever-increasing complexity and density. Increases in density are typically associated with reduction of both the width of transmission lines and the separation between transmission lines to the extent practical with existing manufacturing technology. Current manufacturing techniques enable reliable production of printed circuit board with very small transmission lines. While this is beneficial to increasing the density of the circuitry the lines are becoming so small that their electrical properties are an impediment for certain applications. For example, cross-talk between closely spaced transmission lines is an impediment to high data rate transmission applications. Similarly, electromagnetic interference (“EMI”) can be a problem for high data rate transmission. Resistive and dielectric losses associated with “skin effect” and transmission line length in relatively narrow conductive traces are also problematic.
In accordance with the present invention a printed circuit board transmission line has an outer conductive wall surrounding an inner dielectric core. The outer conductive wall may be formed by joining multiple transmission line traces into a single transmission line of rectangular cross-section. The resulting transmission line has a greater conductive surface area for a given area of substrate than a standard conductive trace, and hence mitigates loss due to skin effect. The interior dielectric portion can reduce dielectric loss associated with induced magnetic fields. Dielectric-core transmission lines may be disposed both within inner layers of printed circuit boards and on outer layers of printed circuit boards.
In an alternative embodiment, the dielectric-core transmission line is disposed inside a grounded shielding to provide a form of coaxial conductor that mitigates cross talk from adjacent transmission lines and EMI. The shielding may include a dielectric portion disposed within an outer metallic wall, both of which are disposed around the dielectric-core transmission line.
In one embodiment of the invention edge coupled differential pairs of dielectric-core transmission lines are disposed inside grounded shielding. The edge coupled differential pairs are disposed in parallel with each other on a plane defined by a layer of the printed circuit board, i.e., side-by-side. The paired transmission lines are separated by a dielectric material which may also occupy substantially all of the volume outside of the individual transmission lines and inside the shielding.
In another embodiment of the invention broadside-coupled differential pairs of dielectric-core transmission lines are disposed inside grounded shielding. The broadside-coupled differential pairs are disposed in parallel with each other in a stack which is orthogonal with a plane defined by a layer of the printed circuit board, i.e., one on top of the other. The paired dielectric-core transmission lines are separated by a dielectric material which may also occupy substantially all of the volume outside of the individual transmission lines and inside the shielding.
The edge-coupled and broadside coupled configurations provide common noise rejection. However, because of limitations associated with printed circuit board geometry including layer thickness and trace thickness it may not be practical to form the walls of the transmission lines which are orthogonal with a plane defined by a layer of the printed circuit board with as great a width as the walls of the transmission lines which are parallel with that plane. Consequently, broadside-coupled differential pairs may have a greater coupled surface area relative to edge-coupled differential pairs, e.g., by a ratio of 8:1. The relatively greater coupled surface area results in a relatively cleaner signal transmission. Further, the broadside-coupled configuration occupies less area on a substrate than an edge-coupled configuration with similarly sized transmission lines. However, the relatively greater coupled surface area and thicker substrate may also result in relatively greater signal attenuation per unit length. Those skilled in the art will recognize how to use these different advantages to achieve a desired result for a specific implementation.
In another embodiment of the invention a plurality of dielectric-core transmission lines such as combinations of single-ended transmission lines, differential pairs in either edge-coupled or differential-coupled or mixed architecture, are disposed within a single grounded shielding structure. The resulting group of dielectric-core transmission lines are separated by a dielectric material which may also occupy substantially all of the volume outside of the individual transmission lines and inside the shielding. This embodiment may be employed, for example, for a bus or other group of related transmission lines.
One aspect of the invention is a method for manufacturing the dielectric-core transmission lines and shielding. To form a single-ended dielectric-core transmission line an intermediate layer having an intermediate conductive trace on a substrate is disposed above a lower, ground layer. An upper signal layer having an upper conductive trace of lesser width than the intermediate conductive trace is disposed over the intermediate layer such that the upper conductive trace is substantially centered over the intermediate conductive trace. Note that the upper and intermediate conductive traces are now separated by a dielectric. A controlled-depth laser cut or mechanical mill is then made along both edges of the upper conductive trace. In particular, trenches reaching to the intermediate conductive trace are formed on either side of the upper conductive trace. Sidewalls connecting the upper and intermediate conductive traces are then formed in the trenches by filling the trenches with conductive paste or other conductive material, or alternatively by electro-less or electroplate deposition of a conductive material such as copper. It should be noted that the relatively lesser width of the upper conductive trace facilitates formation of the trenches and sidewalls by enabling trench formation in a plane orthogonal with the plane defined by a layer of the printed circuit board. Hence, the laser or mechanical milling bit may be situated perpendicular to the top and bottom of the printed circuit board and the relative difference in ablation properties between the substrate and the intermediate conductive trace may be used to advantage.
Shielding may be formed around the resulting dielectric-core transmission line in a similar manner. In particular, an uppermost layer having an uppermost conductive trace of greater width than the intermediate conductive trace is disposed on the uppermost layer such that the uppermost conductive trace is substantially centered over the intermediate conductive trace, after which trenches reaching to the ground layer are formed on either side of the uppermost conductive trace and sidewalls connecting the ground layer and uppermost conductive trace are formed in the trenches. It will now be recognized by those skilled in the art that this technique can be employed to manufacture the various configurations of the invention.
In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only.
Referring to
The single-ended transmission line 10 provides improved performance because of increased outer surface area of the conductor relative to a typical conductive trace. For example, the transmission line has a greater outer surface area than a single conductive trace of width and thickness similar to one of the sidewalls. In practice, this relative increase in surface area yields improved electrical properties relative to the conductive trace having lesser surface area because of the “skin effect.” The skin effect is a well known and studied phenomenon. In brief, electrical currents such as signals in conductive traces generate magnetic fields. As a signal is transmitted there is a delay in the response of the magnetic field to changes in current associated with signal transmission, which in turn tends to push current flow towards the outside of the conductive trace. The effect increases with frequency and at very high frequencies, e.g., 10 GHz or 10 Gbps, current only flows through a thin skin on the outer surface of the conductive trace. Hence, the useful cross-sectional area of the conductor is effectively reduced such that undesirable impedance is increased as frequency is increased. The transmission line of the present invention thus mitigates the increase in impedance by increasing the outer surface area relative to a conductive trace occupying a similar amount of substrate surface area.
The dielectric material in the interior portion 14 of the transmission line further enhances the electrical properties of the transmission line. For example, the interior dielectric portion reduces “dielectric loss” because the induced magnetic fields described above are partially dependent on the dielectric constant of the conductor core. By employing a dielectric material such as thermoplastic polymer modified epoxy resin and glass cloth (e.g., FR4, FR5), polytetrafluoroethylene (“Teflon®”), helium gas, nitrogen gas, ceramics, mica, polyethylene, glass, and metal oxides or other materials having similar dielectric constant, the magnitude of the induced magnetic field is reduced, thereby reducing loss.
Referring to
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The edge-coupled and broadside coupled configurations of
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Following the bonding process, a controlled-depth laser cut or mechanically milled trench is made along both edges 68a, 68b of the second conductive trace 64. In particular, trenches 69a, 69b are formed which reach through the substrate 66 of the second layer S1 to the first conductive trace 60 substantially along the edges 68a, 68b of the second conductive trace 64. Sidewalls 70a, 70b connecting the first and second conductive traces 60, 64 are then formed in the trenches by filling the trenches with conductive paste or other electrically conductive material, or alternatively by electro-less or electroplate deposition of a conductive material such as copper. It should be noted that if unequal conductor widths are employed the relatively lesser width of the second conductive trace 64 facilitates formation of the trenches and sidewalls by enabling the trench formation in a plane orthogonal with the plane defined by a layer of the printed circuit board. Hence, the laser or mechanical milling bit may be situated perpendicular to the plane defined by the printed circuit board and the relative difference in ablating properties between the substrate and the first conductive trace may be used to advantage. However, it should be noted that conductive traces 60 and 64 may have equal widths and the trench cutting/ablating process may partially cut through the second conductive trace 64 or be disposed at an angle which is not perpendicular with the traces. Indeed, various cross-sectional shapes may be employed to achieve the desired result.
Shielding around the resulting dielectric-core transmission line may be formed in a similar manner. In particular, a third layer S0 having a third conductive trace 72 of greater width than the first conductive trace 60 on a dielectric substrate 74 is disposed on layer S0 such that the third conductive trace is substantially centered over the first conductive trace, after which trenches 76a, 76b reaching to the ground layer S3 are formed on either side of the third conductive trace, and sidewalls 78a, 78b connecting the ground layer and third conductive traces are formed in the trenches. It will now be recognized by those skilled in the art that this technique can be employed to manufacture the various configurations of the invention.
While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.
