Abstract:
A circuit board with mounting pads is described for improving the frequency response of routing traces. The present invention is used to etch an etching hole on ground layer corresponding to the surface-mounted devices (SMD) on a routing layer and therefore the parasitic effect from the stray capacitor is reduced, resulting in eliminating the parasitic effect in high-frequency and raising the quality of the PCB as well.

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 093103818 filed in Taiwan, Republic of China on Feb. 17, 2004, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is used to eliminate the parasitic effect of the circuit board with mounting pads by etching the area on the ground plane corresponding to the surface-mounted devices on a routing layer for increasing the response of routing traces and raising the quality of the circuit board. 
   2. Description of Related Art 
   Since the advancement of IC technique, the required wiring pins are getting more complicated and the more routing traces are needed oppositely. Therefore, the design of a PCB is getting more complicated and integrated due to the area spared and multi-layer design of the PCB. 
   The method for manufacturing the conventional PCB is to press a plurality of layers with different functions, including at least a power plane, an element layer, a signal layer, a ground plane and some interposed insulator layers. Since the distance of separation ground plane and the routing layer is narrow, the parasitic effect of mounting pads is brought out and exceeds the original design of the PCB. 
     FIG. 1  shows a schematic drawing of a multi-layer PCB with a routing layer  11  and ground plane  13  in the prior art, wherein the conducting line  12  and the mounting pad  14  connected with are formed on the routing layer  11 . The mounting pad  14  is used to solder the surface-mounted device (SMD)  16  above the routing layer  11 . The mounting pad  14  can be the place of a lead terminal or connector of probe needle, which has a larger region than the conductive line  12 . In the drawing, there is a ground plane  13  next to the routing layer  11  as an electrical reference. 
   In high-frequency circuit, the printed circuit board with the scale of mil should be designed to prevent the response degradation of multi-layer board. The degradation occurring is like the metal mounting pads due to a tiny distance between ground plane  13  and routing layer  11 . The parasitic effect appearing between the layers will degrade the performance and frequency response of the PCB, especially in the region of surface-mounted device  16  as shown in  FIG. 1 . 
     FIG. 2  is the diagram of curves of the parasitic capacitor from the mounting pads in high-frequency circuit. The curve  21  and the curve  23  respectively show the capacitor effect while the distance between the routing layer  11  and the ground plane  13  is 4 mils and 12 mils. Thus, the capacitance shown on curve  21  is larger than the value shown on curve  23 ; therefore, the parasitic effect should be considered as the devices on PCB are thinner and smaller. 
   For improving the demerit of the prior art, in which a severe parasitic effect occurs between the layers in current PCBs because the width of a PCB is too narrow, the present invention provides a ground plane with at least one etching hole etched therein and corresponding to SMD on the routing layer. 
   SUMMARY OF THE DISCLOSURE 
   The present invention is to provide a circuit board with mounting pads for reducing parasitic effect. The present invention comprises at least one mounting pad located on a first conductive layer of the circuit board and used to carry a terminal of a surface-mounted device, and at least one etching hole located in a second conductive layer of the circuit board and corresponding to the mounting pad on the first conductive layer. Therefore, the parasitic effect from the stray capacitor is reduced, resulting in eliminating the parasitic effect in high frequency and raising the quality of the PCB as well. 
   Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and in which: 
       FIG. 1  is a schematic drawing of a mounting pad on a multi-layer PCB with a routing layer and a ground layer of the prior art; 
       FIG. 2  is a schematic diagram of curves of the capacitance of the mounting pad of the prior art; 
       FIG. 3A  is a schematic drawing of the first embodiment of the present invention; 
       FIG. 3B  is a schematic drawing of a top view of the first embodiment of the present invention; 
       FIG. 4  is a schematic drawing of the second embodiment of the present invention; 
       FIG. 5  is a schematic drawing of the third embodiment of the mounting pad of the present invention; 
       FIG. 6A  is a schematic drawing of the fourth embodiment of the present invention; 
       FIG. 6B  is a schematic drawing of a top view of the fourth embodiment of the present invention; 
       FIG. 7  is a schematic drawing of a top view of the fifth embodiment of the present invention; 
       FIG. 8A  is a schematic drawing of the sixth embodiment of the present invention; 
       FIG. 8B  is a schematic drawing of a top view of the sixth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The main purpose of the present invention is to provide a circuit board with mounting pads for minimizing parasitic effect and raising the frequency response. 
     FIG. 3A  is the schematic drawing of the first embodiment of the present invention, which shows a first mounting pad  31   a  and a second mounting pad  31   b,  but these are not limiting the implementation of the present invention. The mounting pads  31   a  and  31   b  are located on first layer of the PCB, and the second ground plane  30  is next to one side of the first plane. For preventing a stray capacitor from occurring between the mounting pads  31   a,    31   b  and second layer  30  and affecting the response of SMD, an etching hole  33  is etched in the second plane  30  corresponding to the area of the first mounting pad  31   a  and second mounting pad  31   b.  The etching hole  33  overlaps the above mounting pads  31   a,    31   b  completely, and can be used to eliminate the stray effect between the mounting pad itself and its edge and the second plane  30 . 
     FIG. 3B  shows the top view of the first embodiment, and discloses that the etching hole  33  formed on the second plane  30  overlaps the first mounting pad  31   a  and mounting pad  31   b  of the first layer and eliminates the stray effect between the mounting pads  31   a,    31   b  and the second plane  30 . 
   The etching hole  33  described in first embodiment is bigger than the area of the mounting pads  31   a,    31   b,  so that the edge stray effect between the mounting pads  31   a,    31   b  and second plane  30  can be eliminated. But in actual implementation, some of the following situation will be shown: 1. the etching hole is the same as the corresponding area of mounting pads; 2. the etching hole completely or partially overlaps the mounting pads; 3. the etching hole completely or partially overlaps the edge of the mounting pads; 4. the etching hole completely or partially overlaps the comers of the mounting pads; 5. the etching hole completely or partially overlaps the area between the mounting pads; and 6. the etching hole can be any shape in the embodiment. Reference is made to the following detailed descriptions regarding other embodiments. 
     FIG. 4  is a schematic drawing of the second embodiment of the present invention and shows that the area of etching hole  43  etched in the second plane  30  overlaps the area of the first mounting pad  31   a  and second mounting pad  31   b  of the first layer and also overlaps the area between the two pads in the first layer properly. Briefly, the etching hole  43  is formed for eliminating the stray effect between the mounting pads  31   a,    31   b  and the ground plane  30 . 
     FIG. 5  shows the third embodiment of the present invention without the edge effect occurring, probably from the mounting pads, in which the etching holes  51 ,  53  is the same as the corresponding area of the mounting pads  31   a,    31   b  respectively. 
     FIG. 6A  is a schematic drawing of the fourth embodiment of the present invention. This fourth embodiment shows that the H-shaped etching holes  61 ,  63  etched on the second plane  30  overlap the area of the first mounting pad  31   a  and the second mounting pad  31   b  and further overlap the corners of the these mounting pads of the first layer. The parasitic effect occurring between the mounting pads  31   a,    31   b  and the ground plane  30  can be eliminated, and the edge stray effect formed at comers of the mounting pads due to the fringing electric field occurring at metal tips can be further reduced. 
   Reference is made to  FIG. 6B , an illustration of a top view of the fourth embodiment. Apparently, for eliminating the parasitic effect between the first layer and the second plane  30  efficiently, the corners of the mounting pads are also overlapped by the etching holes  61 ,  63 . 
     FIG. 7  is the fifth embodiment of the present invention. The etching holes  71 ,  73  entirely cover the area of the first mounting pad  31   a  and the second mounting pad  31   b,  respectively, and even expand the edge area of the etching holes  71 ,  73  for eliminating the edge-fringing effect between the mounting pads  31   a,    31   b  and the second plane  30  further.  FIG. 8A  shows the sixth embodiment of the present invention. The etching hole  83  etched on the second plane  30  overlaps entirely the area of the mounting pads  31   a,    31   b,  and even overlaps the edge corresponding to the two pads and the part between the two pads. 
     FIG. 8B  further shows the top view of the sixth embodiment of the present invention. The etching hole  83  completely overlaps the area of the first mounting pad  31   a  and the second mounting pad  31   b,  and even the area corresponding to the part between the two mounting pads and the edge thereon. 
   The etching hole described in best implementation above of the present invention, which relates to a circuit board with mounting pads for reducing parasitic effect, can be implemented as any shape, and can be arranged as any requirement. 
   The many features and advantages of the present invention are apparent from the written description above and it is intended by the appended claims to cover all. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.