Abstract:
A method of manufacturing a circuit board is described herein. The method may include adding a resin, forming first and second fiberglass fibers, and forming first and second signal line traces capable of transmitting electrical signals. In some examples, a ratio between fiberglass and resin material near the first signal line trace is similar to a ratio between fiberglass and resin material near the second signal line trace. In some examples, the first and second fiberglass fibers diagonally cross near the first and second signal line traces. In some examples, the first and second fiberglass fibers cross near the first and second signal line traces in a zig-zag pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of co-pending U.S. Pat. No. 8,415,002 filed on Mar. 15, 2010, which is a divisional of U.S. Pat. No. 7,676,917 filed on Mar. 26, 2007, which is a divisional of U.S. Pat. No. 7,459,200 filed on Aug. 15, 2003, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to circuit board design. More particularly, the present invention relates to improving circuit boards to improve electrical performance thereof. 
     Circuit boards, which include printed circuit boards or PCBs, are common in the industry for electrically connecting components in a single unit. The most common type of printed circuit board is made of a material referred to in the art as FR4. This type of circuit board is relatively inexpensive to manufacture. It is made up of a rectangular grid or cloth of fiberglass fibers that are typically bonded to a copper substrate using an epoxy resin. Also, the fiberglass cloth can be impregnated with the epoxy resin. A fire retardant (FR) is added to the board, so that the board may be subjected to high-heat environments (e.g., when soldering components to the board). Electrical conductors (e.g., made of copper) are deposited in or on either side of the board and transmit signals between components (e.g., integrated circuit chips) on the boards, sockets for the insertion of other circuit boards (e.g. a daughter card inserted into a motherboard), etc. In the art, the material used for FR4 circuit boards has been effective at reducing crosstalk between signal lines. 
     FR4 circuit boards are also used to transmit signals in differential point to point interfaces. In such an interface, two signal line traces are provided to transmit one data signal. A first one of the traces transmits a part of the data signal that is 180° out-of-phase with the other part of the data signal transmitted on the second one of the traces. At the receiving end of the two signal line traces, the two out-of-phase signals are differentiated so as to recreate the original data transmitted in the signals. Signal trace pairs are often used in bus architectures. 
     FR4 has been successfully used for the transmission of differential signals at current operating frequencies. As components, including processors, increase in operating frequency, problems may occur in the use of FR4 for the circuit board material. For example, as bus frequencies increase over 1 GHz (gigahertz or 1 billion cycles per second), differential to common mode conversion may become a problem for a signal trace pair. In this conversion, the phase difference between the first and second traces changes from the optimal 180° described above. If the phase difference drops below a certain threshold, the receiving device will be unable to differentiate the two signals to retrieve the original data signal. Though such conversion occurs in FR4 circuit boards, the level of conversion has not been a factor at lower frequencies. At higher frequencies, this conversion may become severe, negating the use of known FR4 circuit boards. 
     One solution to this problem is to replace the FR4 board with a different, and perhaps homogenous, material. Such boards are known in the art, but are generally more expensive than the common FR4 material. 
     In view of the above, there is a need for an improved circuit board material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of the use of FR4 circuit board with a rectangular lattice structure and a pair of signal traces. 
         FIG. 2  is a schematic representation of the use of FR4 circuit board with a zig-zag or herringbone lattice structure and a pair of signal traces. 
         FIG. 3  is a schematic representation of the use of modified FR4 circuit board and a pair of signal traces according to an embodiment of the present invention. 
         FIGS. 4   a - c  are schematic representations of an embodiment of the present invention for manufacturing a circuit board using portions of an FR4 circuit board. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a representation of a portion of an FR4 board with a pair of signal traces is shown. The circuit board  10  includes vertical and horizontal fiberglass fibers  11   a ,  11   b , respectively. Though one rectangular lattice structure is shown, multiple such lattice structures may be provided in the circuit board  10 . The circuit board  10  also includes a resin  12 , such as an epoxy resin, which is disposed throughout the rectangular lattice structure of the fiberglass fibers  11   a - b . In the board of  FIG. 1 , a pair of signal line traces  13   a ,  13   b  are provided. In this example, the signal lines are generally parallel to the horizontal fiberglass fibers  11   b . These signal traces may be embedded into the resin, disposed on top of the resin of the circuit board, etc. As seen in  FIG. 1 , the first signal line  13   a  is generally disposed between adjacent horizontal fibers  11   b . In this example each signal line trace has a width of approximately 5 mils (i.e., 5 milli-inches or 0.005 inches). The second signal line  13   b , however, is generally disposed over one of the horizontal fibers  11   b . The spacing between the traces is 5 mils in this example. 
     When using the first and second signal line traces for differential signaling, it is desirable to have these traces be equivalent in terms of characteristic impedance and propagation constant. With low signal frequencies, the effect of the non-homogeneous material of the FR4 circuit board is negligible on these features of the signal traces. As signal frequency increases, however, differences in the material composition near the signal traces has a considerable effect on these features. At high signal frequencies, these features will have an effect on the magnitude and phase of the signals transmitted by the trace. If the circuit board material affects the characteristic impedance and/or propagation constant in these traces differently, then determining the difference between the signals in these traces becomes more difficult and could result in loss of the data desired to be transmitted. 
     In the example of  FIG. 1 , the board material near the second trace  13   b  has a relatively high ratio of fiberglass-to-resin material, while the board material near the first trace  13   a  is made up of a much lower ratio of fiberglass-to-resin material. At a signal frequency of 604 MHz (i.e., 604 Million Hertz or 604 Million cycles-per-second), a 5 mil wide trace of an FR4 circuit board has a dielectric constant E r  that varies between 3.32 and 3.50. With two signal line traces having a width of 5 mils and a spacing of 5 mils, it is estimated that total mode conversion of differential to common mode (i.e., complete signal loss) will occur at 3.5 GHz for 30-inch trace lengths due to the accumulated phase shift between the pair. 
     According to an embodiment of the present invention, a circuit board is fabricated using the same types of materials contemplated for FR4 circuit boards. A representation of this embodiment is shown in  FIG. 2 . Referring to  FIG. 2 , in the portion of the circuit board  20  shown, first and second fiberglass fiber sets  21   a ,  21   b  overlap each other. These sets form a zig-zag or herringbone design in this embodiment. In other words, each fiberglass set is characterized by straight line segments connected by vertices (e.g., as in a periodic, triangular wave-form). Though two sets of fiberglass fibers are shown, one or more such sets may be provided. As in the FR4 circuit board, a resin is added to form the circuit board structure. As with the FR4 circuit boards known in the art, there will be areas where the ratio of resin material to fiberglass material will be relatively high (e.g., at area  25 ) and areas where this ratio will be relatively low (e.g., at area  27 ). Again, two signal line traces  23   a ,  23   b  are disposed in or on the circuit board  20 . As seen in  FIG. 2 , the circuit board material residing near the signal line traces  23   a ,  23   b  is a mixture of different ratios between the resin and fiberglass materials. The effect, in this embodiment, is that the material near the first signal line trace  23   a  will be similar, overall, in content compared to the material near the second signal line trace  23   b . Since the materials near first and second signal line traces  23   a ,  23   b  are similar, overall, there is less differential to common mode conversion when using these traces for differential mode signaling. 
     In the embodiment of  FIG. 2 , the fiberglass material is formed in a zig-zag or herringbone design. The spacing between adjacent fibers may be similar to that of the fiberglass lattice of FIG.  1 —approximately 5 mils. As signal frequencies increase and signal trace width decreases, the density (i.e., thickness and spacing) of the fiberglass material lattice may be adjusted so as to reduce differential to common mode conversion in signal line pairs. 
     The rectangular lattice for the fiberglass cloth used in the circuit board  10  of  FIG. 1  is made in a conventional manner. The cloth is made by taking fiberglass fibers and weaving them together or interleaving horizontal fibers and vertical fibers. To make the zig-zag or herringbone design of  FIG. 2  may be implemented by modifying systems that currently manufacture fiberglass cloth. For example, in creating the first set  21   a  of fiberglass fibers, the apparatus emitting the fiberglass fibers can be moved side-to-side as the fibers are laid down onto a surface. Alternatively, the surface upon which the fibers are laid can be moved side-to-as the fibers are laid down. Once the zig-zag or herringbone fiberglass fiber is made, it can be adhered to a copper substrate with an epoxy resin as with standard FR4 manufacturing methods. Accordingly, signal line traces  23   a ,  23   b  may be formed by removing unwanted copper from the circuit board in a conventional manner. 
     The embodiment of the present invention shown in  FIG. 2  may be made at a low-cost similar to conventional FR4 circuit boards that are currently available. The manufacture of the board of  FIG. 2  may use much of the same equipment as is used in the manufacture of conventional FR4 circuit boards. 
     Another embodiment of the present invention is shown in  FIG. 3 . In  FIG. 3 , an FR4 circuit board is made in a conventional manner. In other words, a fiberglass cloth of horizontal and vertical fiberglass fibers  31   a ,  31   b  are provided in an epoxy resin  33 . After the fiberglass fibers and resin are formed together the entire apparatus is rotated and then cut to size. For example, in  FIG. 3 , the fiberglass lattice is rotated 45° (i.e., either the horizontal fiberglass fibers or the vertical fibers form a non-right angle of 45° with cut line). When placing two conductive traces  35   a ,  35   b  onto the circuit board  37 , it can be readily seen that both conductors pass by areas of higher fiberglass-to-resin material and lower fiberglass-to-resin material, thus assisting in reducing differential to common mode conversion between signals in the two conductive traces. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Furthermore, certain terminology has been used for the purposes of descriptive clarity, and not to limit the present invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims. 
     For example, one or more conventional FR4 boards may be rotated (e.g., by 45°) and sliced into a number of squares. The individual squares from the board(s) can then be rotated again to desired orientations and reformed together. An example of this is shown in  FIGS. 4   a - c . In  FIG. 4   a , an FR4 board is rotated 30° and sliced into a number of sections or squares  41 . In  FIG. 4   b , a second FR4 board is rotated 45° and sliced into a number of squares  42 . The individual squares can then be rotated if desired, and then reformed together to form a new FR4 board with conductors  43   a ,  43   b.    
     Though the embodiment of  FIGS. 4   a - c  is shown with rectilinear shapes, other shapes may be used. Also, as shown in  FIGS. 4   a - c , the density of the fiberglass fibers need not be the same between the sections used to create the new FR4 board.