PATENT ABSTRACT
A method is provided for testing a built-in component including multiple terminals in a multi-layered circuit board. At least one signal pad is provided on a top surface of the multi-layered circuit board for signal transmission. Each of the signal pads are electrically connected to one of the multiple terminals. At least one test pad is provided on the top surface of the multi-layered circuit board and each of the test pads is electrically connected to one of the multiple terminals. Then, detection occurs regarding one of the signal pads and one of the test pads that are electrically connected to a same one of the multiple terminals in order to determine a connection status of an electric path extending from the one signal pad through the same one terminal to the one test pad.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 11/131,741 filed May 18, 2005, now U.S. Pat. No. 7,345,366, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to high-frequency test technology. More particularly, the present invention relates to an apparatus and method for testing components built in circuit boards. 
       FIG. 1A  is a schematic diagram of a conventional multi-layered circuit board  1  provided with a built-in capacitor  10  in a perspective view.  FIG. 1B  is a cross-sectional view of multi-layered circuit board  1  shown in  FIG. 1A  taken along a line II-II. 
     Referring to  FIGS. 1A and 1B , multi-layered circuit board  1  includes a first dielectric layer  100 , a second dielectric layer  102 , and capacitor  10  built in circuit board  1 . First dielectric layer  100  is formed over second dielectric layer  102 . Built-in capacitor  10  includes a first electrode plate  104  and a second electrode plate  106 .—First electrode plate  104  in the example serves as a signal plate, and second electrode plate  106  serves as a ground plate. First electrode plate  104  is disposed between first and second dielectric layers  100  and  102 , and second electrode plate  106  is disposed on a bottom surface (not numbered) of second dielectric layer  102 . In other words, first electrode plate  104  and second electrode plate  106  are spaced apart by second dielectric layer  102 . A signal pad  108  is formed at a top surface (not numbered) of circuit board  1 , and more specifically, on the top of first dielectric layer  100  on which traces, active components, passive components or integrated circuits may be formed. Signal pad  108  is therefore a circuit node of a functional circuit (not shown) included in circuit board  1 . Since capacitor  10  is built in circuit board  1 , a via  110  is formed through first dielectric layer  100  to electrically connect signal pad  108  and first electrode plate  104 . Via  110  is generally formed by forming an opening through first dielectric layer  100  by a mechanical drill or laser, and then filling in the opening with conductive material. First electrode plate  104  may include a lead  112  and a conductive pad  114  extending therefrom to electrically connect first electrode plate  104  and signal pad  108  through via  110 . 
     During the formation of via  110 , the opening may not be well formed such that an open-circuit issue may occur. For a multi-layered circuit board having built-in components, either passive or active, however, it may be difficult to test if there&#39;s an open-circuiting or short-circuiting in the circuit board. It is desirable to have an apparatus and method for testing a multi-layered circuit board provided with built-in components. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a circuit and a method that obviate one or more problems resulting from the limitations and disadvantages of the prior art. 
     In accordance with an embodiment of the present invention, there is provided a multi-layered circuit board that includes a built-in component including multiple terminals, at least one signal pad formed on a top surface of the multi-layered circuit board for signal transmission, each of the at least one signal pad corresponding to one of the multiple terminals, and at least one test pad formed on the top surface of the multi-layered circuit board, each of the at least one test pad corresponding to one of the at least one signal pad for testing an electric path extending from the one signal pad through the one terminal to the each of the at least one test pad. 
     Also in accordance with the present invention, there is provided a multi-layered circuit board that includes a built-in capacitor including a first electrode and a second electrode, a signal pad formed on a top surface of the multi-layered circuit board for signal transmission in electrical connection with one of the first electrode or second electrode, and a test pad formed on the top surface of the multi-layered circuit board in electrical connection with the one of the first electrode or second electrode for testing an electric path extending from the signal pad through the one of the first electrode or second electrode to the test pad. 
     Further in accordance with the present invention, there is provided a multi-layered circuit board that includes a built-in inductor including a first end and a second end, a signal pad formed on a top surface of the multi-layered circuit board for signal transmission in electrical connection with one of the first end or second end, and a test pad formed on the top surface of the multi-layered circuit board in electrical connection with the one of the first end or second end for testing an electric path extending from the signal pad through the one of the first end or second end to the test pad. 
     Still in accordance with the present invention, there is provided a method for testing a built-in component including multiple terminals in a multi-layered circuit board that includes providing at least one signal pad on a top surface of the multi-layered circuit board for signal transmission, electrically connecting each of the at least one signal pad to one of the multiple terminals, providing at least one test pad on the top surface of the multi-layered circuit board, electrically connecting each of the at least one test pad to one of the multiple terminals, and detecting one of the at least one signal pad and one of the at least one test pad that are electrically connected to a same one of the multiple terminals to determine a connection status of an electric path extending from the one signal pad through the same one terminal to the one test pad. 
     Yet still in accordance with the present invention, there is provided a method for testing a built-in capacitor including a first electrode and a second electrode in a multi-layered circuit board that includes providing a signal pad for signal transmission on a top surface of the multi-layered circuit board, electrically connecting the signal pad to one of the first electrode or second electrode of the built-in capacitor, providing a test pad on the top surface of the multi-layered circuit board, electrically connecting the test pad to the one of the first electrode or second electrode of the built-in capacitor, and detecting the signal pad and the test pad to determine whether there is an open-circuiting in an electric path extending from the signal pad through the one of the first electrode or second electrode to the test pad. 
     Further still with the present invention, there is provided a method for testing a built-in inductor including a first end and a second end in a multi-layered circuit board that includes providing a signal pad for signal transmission on a top surface of the multi-layered circuit board, electrically connecting the signal pad to one of the first end or second end of the built-in capacitor, providing a test pad on the top surface of the multi-layered circuit board, electrically connecting the test pad to the one of the first end or second end of the built-in capacitor, and detecting the signal pad and the test pad to determine whether there is an open-circuiting in an electric path extending from the signal pad through the one of the first end or second end to the test pad. 
     Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the present invention and together with the description, serves to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1A  is a schematic diagram of a conventional multi-layered circuit board provided with a built-in capacitor in a perspective view; 
         FIG. 1B  is a cross-sectional diagram of the multi-layered circuit board shown in  FIG. 1A  taken along a line II-II; 
         FIG. 2A  is a schematic diagram of a multi-layered circuit board provided with a built-in component in accordance with one embodiment of the present invention in a perspective view; 
         FIG. 2B  is a cross-sectional diagram of the multi-layered circuit board shown in  FIG. 2A  taken along a line IV-IV; 
         FIG. 3  is a schematic cross-sectional diagram of a multi-layered circuit board provided with a built-in component in accordance with another embodiment of the present invention in a cross-sectional view; 
         FIGS. 4A to 4C  are schematic, cross-sectional diagrams of multi-layered circuit boards provided with built-in components in accordance with still another embodiments of the present invention; 
         FIG. 5A  is a plot illustrating simulation results in impedance-frequency relationship between a multi-layered circuit board having test pads according to the present invention and a conventional multi-layered circuit board without any test pads; 
         FIG. 5B  is a plot illustrating simulation results in impedance-frequency relationship between multi-layered circuit boards having test pads disposed in different distances from respective signal pads; 
         FIG. 6  is a schematic cross-sectional diagram of a multi-layered circuit board provided with a built-in component in accordance with yet another embodiment of the present invention in a cross-sectional view; 
         FIG. 7A  is a diagram of a built-in inductor in accordance with one embodiment of the present invention in a perspective view; 
         FIG. 7B  is a diagram of a built-in inductor in accordance with another embodiment of the present invention in a perspective view; and 
         FIG. 8  is a cross-sectional diagram of a multi-layered circuit board including a built-in multi-port element in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2A  is a schematic diagram of a multi-layered circuit board provided  2  with a built-in component  20  in accordance with one embodiment of the present invention in a perspective view. In the present example, built-in component  20  includes a capacitor.  FIG. 2B  is a cross-sectional diagram of multi-layered circuit board  2  shown in  FIG. 2A  taken along a line IV-IV. 
     Referring to  FIGS. 2A and 2B , multi-layered circuit board  2  includes a first dielectric layer  200 , a second dielectric layer  202 , and capacitor  20  built in circuit board  2 . First dielectric layer  200  is formed over second dielectric layer  202 . Built-in capacitor  20  includes a first electrode plate  204  and a second electrode plate  206 . In this example, first electrode plate  204  serves as a signal plate for signal transmission, and second electrode plate  206  serves as a ground plate connected to a reference voltage level (not shown). First electrode plate  204  is disposed approximately between first and second dielectric layers  200  and  202 , and second electrode plate  206  is disposed at a bottom surface (not numbered) of second dielectric layer  202 . First electrode plate  204  and second electrode plate  206  are therefore spaced apart by second dielectric layer  202 . 
     Circuit board  2  includes a signal pad  208  formed thereon. Specifically, signal pad  208  is disposed on a top surface (not numbered) of first dielectric layer  200  where traces, active components, passive components or integrated circuits may be provided. Signal pad  208  is a circuit node of a functional circuit included in circuit board  2 . Since capacitor  20  is built in circuit board  2 , a via  210  is formed through first dielectric layer  200  to electrically connect signal pad  208  and first electrode plate  204 . Via  210  may be formed by forming an opening through first dielectric layer  200  by a mechanical drill or laser, and then filling in the opening with conductive material. First electrode plate  204  includes a first lead  212  and a first conductive pad  214  for electrical connection with via  210 . 
     Circuit board  2  further includes a test pad  218  formed thereon. Specifically, test pad  218  is disposed on the top surface of first dielectric layer  200 . A test pad according to the present invention is used to test whether there is an open-circuiting in an electrical path from a pad of interest to the test pad, or whether there&#39;s a short-circuiting between a pad of interest to the test pad, where an electrical connection should not have been provided. In the present embodiment, test pad  218 , corresponding to signal pad  208 , facilitates an open-circuiting test on an electrical path extending from signal pad  208 , through first electrode plate  204  of built-in capacitor  20 , to test pad  218 . A via  220  is formed through first dielectric layer  200  to electrically connect test pad  218  and first electrode plate  204 . First electrode plate  204  includes a second lead  222  and a second conductive pad  224  for electrical connection with via  220 . 
     During normal operation, test pad  218  is kept at a floating state. During a testing operation, a first probe (not shown) and a second probe (not shown) are applied to signal pad  208  and test pad  218 , respectively for conducting an open-circuiting or short-circuiting test. 
       FIG. 3  is a schematic diagram of a multi-layered circuit board  3  provided with a built-in component  30  in accordance with another embodiment of the present invention in a cross-sectional view. In the present example, built-in component  30  includes a capacitor. Referring to  FIG. 3 , multi-layered circuit board  3  of the present invention includes a first dielectric layer  300 , a second dielectric layer  302 , and capacitor  30  built in circuit board  3 . Built-in capacitor  30  includes a first electrode plate  304  and a second electrode plate  306 . At least one of first electrode plate  304  or second electrode plate  306  serves a signal plate in multi-layered circuit board  3 . First electrode plate  304  is disposed between a first dielectric layer  300  and a second dielectric layer  302 . Second electrode plate  306  is disposed at a bottom surface (not numbered) of second dielectric layer  302 . A first signal pad  308  and a second signal pad  328  are spaced apart from each other at a top surface of circuit board  3 . First signal pad  308  is electrically connected to first electrode plate  304  through a first via  310 . Likewise, second signal pad  328  is electrically connected to second electrode plate  306  through a second via  330 . 
     A first test pad  318  corresponding to first signal pad  308  and a second test pad  338  corresponding to second signal pad  328  are disposed at the top surface of circuit board  3 . First test pad  318  is electrically connected to first electrode plate  304  through a via  320 , while second test pad  338  is electrically connected to second electrode plate  306  through a via  340 . 
     During normal operation, first test pad  318  and second test pad  338  are kept at a floating state. During a testing operation, a first electrical path from first signal pad  308 , through first electrode plate  304 , to first test pad  318  is tested by means of, for example, a pair of probes, to determine whether there is an open-circuiting in the first electrical path. Likewise, a second electrical path from second signal pad  328 , through second electrode plate  306 , to second test pad  338  is tested to determine whether there is an open-circuiting in the second electrical path. Furthermore, during a testing operation, first signal pad  308  and second test pad  338 , which are not electrically connected, are tested to determine whether there is a short-circuiting therebetween. Likewise, second signal pad  328  and first test pad  318 , which are not electrically connected, are tested to determine whether there is a short-circuiting therebetween. 
       FIGS. 4A to 4C  are schematic, cross-sectional diagrams of multi-layered circuit boards provided with built-in components in accordance with still another embodiments of the present invention. In the present examples, the built-in components include capacitors. Referring to  FIG. 4A , a multi-layered circuit board  4  includes a built-in capacitor  40 , a signal pad  408  and a test pad  418 . Built-in capacitor  40  includes a first electrode plate  404  and a second electrode plate  406 . Signal pad  408  is electrically connected to second electrode plate  406  through a via  410 . Test pad  418  is electrically connected to second electrode plate  406  through a via  420 . In the present example, built-in capacitor  40  is a single-port capacitor, in which one of electrodes, i.e., second electrode  406 , serves as a signal plate for signal transmission, while first electrode  404  serves as a ground plate. 
     Referring to  FIG. 4B , a multi-layered circuit board  41  including a built-in capacitor  42  has a similar structure as multi-layered circuit board  4  shown in  FIG. 4A , except that an additional signal pad  428  and an additional test pad  438  corresponding to the additional signal pad  428  are provided. Vias  430  and  440  electrically connect signal pad  428  and test pad  438 , respectively, to first electrode plate  404 . Built-in capacitor  42  is a dual-port capacitor, in which both of electrodes, i.e., first electrode  404  and second electrode  406 , function to serve as signal plates for signal transmission. 
     Referring to  FIG. 4C , a multi-layered circuit board  45  includes a first electrode and a second electrode. The first electrode includes a first layer  43 , a second layer  45  and a third layer  47  electrically connected to each other by vias  450 . The second electrode includes a first layer  44 , a second layer  46  and a third layer  48  electrically connected to each other by vias  460 . A first signal pad  431  and a first test pad  432  corresponding to first signal pad  431  are disposed on first layer  43  of the first electrode and electrically connected to one another by vias  450 . A second signal pad  441  and a second test pad  442  corresponding to second signal pad  441  are disposed on first layer  44  of the second electrode and electrically connected to one another by vias  460 . 
     During normal operation, first and second test pads  432  and  442  are not connected to any power sources, i.e., floating. During a testing operation, first layer  43  and second layer  45 , or first layer  43  and third layer  47  of the first electrode are tested by applying a pair of probes to first signal pad  431  and first test pad  432  to determine whether there is an open-circuiting. Likewise, first layer  44  and second layer  46 , or first layer  44  and third layer  48  of the second electrode are tested by applying a pair of probes to second signal pad  441  and second test pad  442  to determine whether there is an open-circuiting. Furthermore, during a testing operation, by applying a pair of probes to first signal pad  431  and second test pad  442 , or to second signal pad  441  and first test pad  432 , it is able to determine whether there is a short-circuiting between the first and second electrodes. 
       FIG. 5A  is a plot illustrating simulation results in impedance-frequency relationship between a multi-layered circuit board having test pads according to the present invention and a conventional multi-layered circuit board without any test pads. Referring to  FIG. 5A , a curve  51  represents the result of simulation of a multi-layered circuit board provided with test pads, for example, multi-layered circuit board  2  shown in  FIG. 2A  or  2 B according to the present invention. A curve  52  represents the result of simulation of a multi-layered circuit board without any test pads, for example, multi-layered circuit board  1  shown in  FIG. 1A  or  1 B. In either of multi-layered circuit board  1  or  2 , as an example, first electrode plate  104  or  204  has an area of 20×20 mil 2 , via  110  or  210  has a diameter of 5 mil, and signal pad  108  or  208  has a diameter of 10 mil. Curve  51  has a self-resonance point at approximately 15.6 GHz, while curve  52  has a self-resonance point at approximately 16.6 GHz. By comparison, the self-resonance frequency of capacitor  20  of multi-layered circuit board  2  is smaller than that of capacitor  10  of multi-layered circuit board  1  by approximately 1 GHz. Such a 1-GHz decrease, due to an increase in parasitical inductance as test pads and corresponding vias are added. 
       FIG. 5B  is a plot illustrating simulation results in impedance-frequency relationship between multi-layered circuit boards having test pads disposed in different distances from respective signal pads. Referring to  FIG. 5B , a curve  53  represents the result of simulation of a multi-layered circuit board having a longer distance between signal pads and test pads, while a curve  54  represents the result of simulation of a multi-layered circuit board having a shorter distance between signal pads and test pads. Curve  53  has a greater self-resonance frequency than curve  54 . The shorter the distance between signal pads and test pads, the greater the self-resonance frequency. In one embodiment according to the present invention, the center-to-center distance between a signal pad and a test pad ranges from one to one and a half folds of the signal pad or test pad diameter. 
       FIG. 6  is a schematic cross-sectional diagram of a multi-layered circuit board  6  provided with a built-in component  62  in accordance with yet another embodiment of the present invention in a cross-sectional view. In the present example, built-in component  62  includes one of an inductor or resistor. Examples of a built-in inductor will be discussed later by reference to  FIGS. 7A and 7B . As to a built-in resistor, since skilled persons in the art will understand that a conductive line or trace in a layer of a multi-layered circuit board may function to serve as a resistor, illustration of a built-in resistor is not necessary. Referring to  FIG. 6 , multi-layered circuit board  6  includes dielectric layers  63 ,  64  and  65 , and a built-in inductor  62 . A first signal pad  608  and a first test pad  618  corresponding to first signal pad  608  are provided on a top surface (not numbered) of multi-layered circuit board  6 . First signal pad  608  is electrically connected to first test pad  618  through vias  610  and  620 , traces  650  and a first terminal  621  of inductor  62 . A second signal pad  628  and a second test pad  638  corresponding to second signal pad  628  are provided on the top surface of multi-layered circuit board  6 . Second signal pad  628  is electrically connected to second test pad  638  through vias  630  and  640 , traces  660  and a second terminal  622  of inductor  62 . 
     During normal operation, first and second test pads  618  and  638  are kept at a floating state. During a testing operation, first signal pad  608  and first test pad  618  are probed to determine whether a path denoted as A is open-circuited. Second signal pad  628  and second test pad  638  may be probed to determine whether a path B is open-circuited. Furthermore, first test pad  618  and second test pad  638  are probed to determine whether a path C extending through inductor  62  is open-circuited. In other embodiments of the present invention, first signal pad  608  and second signal pad  628  are probed to determine whether a path (not numbered) extending through inductor  62  is open-circuited. First signal pad  608  and second test pad  638 , or second signal pad  628  and first test pad  618  are probed to determine whether there is an open-circuiting in respective paths (not numbered). 
       FIG. 7A  is a diagram of a built-in inductor  71  in accordance with one embodiment of the present invention in a perspective view. Referring to  FIG. 7A , built-in inductor  71  includes a first terminal  72 , a second terminal  73 , and a plurality of conductive lines or traces  74  and  75  extending from first terminal  72  to second terminal  73  through vias  76 . Traces  74  are disposed in a layer  702  of a multi-layered circuit board (not numbered), and traces  75  are disposed in another layer (not shown) of the multi-layered circuit board. A first signal pad  708  in electrical connection with first terminal  72  and a first test pad  718  corresponding to first signal pad  708  are disposed in still another layer  700  of the multi-layered circuit board. Furthermore, a second signal pad  728  in electrical connection with second terminal  73  and a second test pad  738  corresponding to second signal pad  728  are disposed in layer  700 . The testing operation for inductor  71  has been previously discussed by reference to  FIG. 6 . 
       FIG. 7B  is a diagram of a built-in inductor  81  in accordance with another embodiment of the present invention in a perspective view. Referring to  FIG. 7B , built-in inductor  81 , which is a solenoid-type inductor, includes a first terminal  82 , a second terminal  83 , and a winding conductive line or trace  84  extending from first terminal  82  to second terminal  83 . First terminal  82 , second terminal  83  and trace  84  are disposed in a layer  802  of a multi-layered circuit board (not numbered). A first signal pad  808  in electrical connection with first terminal  82  and a first test pad  818  corresponding to first signal pad  808  are disposed in another layer  800  of the multi-layered circuit board. Furthermore, a second signal pad  828  in electrical connection with second terminal  83  and a second test pad  838  corresponding to second signal pad  828  are disposed in layer  800 . The testing operation for inductor  81  has been previously discussed by reference to  FIG. 6 . 
     Embodiments of a passive component such as a capacitor, an inductor or a resistor built in a multi-layered circuit board have been illustrated. Skilled persons in the art, however, will understand that the present invention may be applied to an active component or a multi-terminal component in addition to the two-terminal components previously discussed. In one embodiment according to the present invention, the multi-terminal component includes one of a multi-port microwave passive element or a transistor.  FIG. 8  is a cross-sectional diagram of a multi-layered circuit board  9  including a built-in multi-port element  92  in accordance with one embodiment of the present invention. Referring to  FIG. 8 , built-in multi-port element  92 , for example, a filter or a balun, includes a first port  921 , a second port  922  and a third port  923 . First, second and third ports  921 ,  922  and  923  are respectively electrically connected through vias (not numbered) to a first signal pad  908 , a second signal pad  928  and a third signal pad  948  formed on a top surface of multi-layered circuit board  9 . A first test pad  918  corresponding to first signal pad  908  is formed on the top surface for testing whether a first electrical path extending from first signal pad  908  through first port  921  to first test pad  918  is open-circuited. Likewise, a second test pad  938  corresponding to second signal pad  928  is formed on the top surface for testing whether a second electrical path extending from second signal pad  928  through second port  922  to second test pad  938  is open-circuited. Furthermore, a third test pad  958  corresponding to third signal pad  948  is formed on the top surface for testing whether a third electrical path extending from third signal pad  948  through third port  923  to third test pad  958  is open-circuited. 
     As an example of a transistor, which generally includes a gate terminal, a source terminal and a drain terminal, at least a test pad corresponding to one of the gate, source or drain terminal may be formed on a top surface of a multi-layered circuit board for testing an electric path extending from a signal pad formed on the top surface through the corresponding one terminal to the test pad. 
     The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.