Patent Publication Number: US-8981790-B2

Title: Electronic device and noise current measuring method

Description:
TECHNICAL FIELD 
     The present invention relates to an electronic device that is configured to prevent an electronic component disposed on a PCB (Printed Circuit Board) or PWB (Printed Wiring Board) inside an enclosure from breakdown or malfunction due to noise such as static electricity. The invention also relates to a method of measuring a noise current caused by static electricity or the like applied to the electronic device. 
     BACKGROUND ART 
     Electronic devices such as consumer devices, manufacturing devices, and inspecting devices are specified to undergo an immunity tolerance test (IEC61000-4-2). The immunity test aims to confirm that the electronic device will not malfunction even if it is subjected to electrostatic discharge caused by people touching a portion of the electronic device or aerial discharge. Recently, immunity tolerance of semiconductors has deteriorated since operating margin has been reduced for lower source voltage, and non-metalization of enclosures has been promoted for weight saving of electronic devices. Thus, the environment of the electronic device regarding static electricity is becoming severe from year to year. Countermeasure costs needed to pass the above-mentioned electrostatic test tend to increase as well. Further, malfunctions due to static electricity occur even in the devices that have passed the test, which is now a major problem. 
     In such situation, a demand for a technology to improve the immunity tolerance and electrostatic resistance characteristics of an electronic device is increasing. It is therefore urgent to develop a component, a PCB or PWB, and an enclosure structure which are adopted to control noise current flowing into a semiconductor (e.g. LSI) that is susceptible to static electricity and to construct designing technology for them. Further, establishing a technology for identifying current paths during the immunity tests or electrostatic tests and elucidating mechanisms where faults occur are important in order to construct the designing technology of the noise suppression component, the PCB or PWB and the enclosure structure. 
     A current that flows through a screw/spacer portion connecting an enclosure with a PCB or PWB in an electronic device is a key factor for solving immunity issues including static electricity. This is because, particularly in an electrostatic issue, the noise current caused by electrostatic discharge may flow from the enclosure via the screw/spacer to the PCB or PWB, and the noise current that has flown into the PCB or PWB may be released to the enclosure via the screw/spacer. 
     The current flowing through a screw/spacer portion is also an important factor regarding a problem related to EMI (Electromagnetic Interference) from an electronic device. The noise current generated in a semiconductor such as LSI flows from the PCB or PWB via the screw/spacer portion into the enclosure, thereby causing EMI. 
     The following technologies are proposed for these issues and problems. “Patent document 1”, “patent document 2” and “patent document 3” describe a screw-like component having a transmission line structure composed of inductance and capacitance to hold low-pass filter characteristics. 
     In the electronic devices disclosed in “Patent document 4”, “patent document 5”, and “patent document 6”, an enclosure and a PCB or PWB is secured by a metal spacer and the surrounding of the spacer is covered with a magnetic core, thus forming an integrated spacer. “Patent document 4” further describes a technology for inserting a washer-like resistance sheet between a spacer and an enclosure. 
     On the other hand, in order to identify the cause of EMI from the overall electronic device, “patent document 6” discloses a technology for measuring an electric current flowing through a screw portion connecting the enclosure and PCB or PWB of an electronic device. This relates to a current probe having a coil for detecting magnetic flux caused by the current flowing through the screw portion between the enclosure and PCB or PWB and to a measuring method using the current probe. 
     PRIOR-ART DOCUMENTS 
     Patent Documents 
     
         
         Patent document 1: JP-2001-267134 
         Patent document 2: JP-2001-76931 
         Patent document 3: JP-2001-76932 
         Patent document 4: JP-A-H09-23083 
         Patent document 5: JP-A-H10-163665 
         Patent document 6: JP-A-2007-85741 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     “Patent document 1”, “patent document 2” and “patent document 3” describe a low-pass filter structure in which the inductance and parasitic capacitance of a screw connecting an enclosure and a PCB or PWB are arranged with respect to an electric current in a distributed manner. However, the inductance and capacitance of the screw with a length of approximately 5 to 10 mm and an opening portion with a diameter of approximately 2 to 6 mm are very small. Thus, cut-off frequency of the low-pass filter is predicted to approach GHz. The cut-off frequency band width may therefore be insufficient for a noise current having a wide band of several MHz to several GHz such as an electrostatic discharge current. 
     The documents also describe that the cut-off frequency may be several hundred MHz if the parasitic capacitance between the PCB or PWB and the enclosure are taken into account. However, this cut-off frequency is determined by the structure of the PCB or PWB and the enclosure and cannot be controlled purposefully. The cut-off frequency band width may be insufficient as well. Further, since a resistance component serving as a loss is not considered, the current may adversely increase when the inductance and the capacitance resonate. 
     In patent documents 4 and 5, the enclosure and the PCB or PWB are connected to each other by the metal spacer surrounded by the magnetic material, whereby the EMI due to cavity resonance in the enclosure is suppressed. In addition, a resistance sheet is inserted between the enclosure and the metal spacer, whereby a high-frequency current flowing through the screw is suppressed. 
     However, in the metal spacer surrounded by the magnetic material, the inductance component and the resistance component possessed by the magnetic material are not inserted in series with respect to the current flowing from the PCB or PWB to the enclosure. An electric current may partially flow through the metal spacer so that a targeted reduction effect is possibly not produced. 
     The resistance sheet inserted between the enclosure and the metal spacer does not have selectivity of a resistance value. Noise currents with a wide band of several MHz to several GHz may therefore not be sufficiently suppressed. When the resistance sheet is inserted between the enclosure and the screw spacer, the selectivity of the resistance value is of importance since the cut-off frequency band width of noise is determined by the resistance of the resistance sheet and the inductance component possessed by the screw, the enclosure and the PCB or PWB. 
     On the other hand, patent document 6 proposes a method of measuring a current flowing through the joint portion between the PCB or PWB and the enclosure, by use of a probe with a coil for detecting flux caused by the current flowing through the screw portion. However, an end of the coil of the probe in patent document 6 also has sensitivity to factors such as an exogenous magnetic field. In particular, the intensity of the magnetic field emitted by an electrostatic gun used in an electrostatic test is significantly large. Therefore in the immunity test such as the electrostatic test, it is required that sensitivity to the exogenous magnetic field is suppressed while sensitivity to the current to be measured flowing in the screw portion is maintained. 
     Accordingly, an object of the present invention is to provide a noise suppression component in which a loss component can be inserted in series with respect to a wide band noise current such as an electrostatic discharge current flowing into a joint portion between a PCB or PWB and an enclosure, a screw or a spacer, and that can purposefully control a cut-off frequency band width. Another object of the present invention is to provide a component that can be replaced with a common component by use of a noise suppression component without increasing costs for countermeasures: the addition of a new component or the modification of a PCB or PWB wiring pattern or the like. 
     Another object of the present invention is to provide a current measuring probe that is less susceptible to an exogenous magnetic field emitted from a testing machine etc. during an immunity test and a measuring method using the current measuring probe. 
     Means for Solving the Problem 
     (1) An electronic device in which an enclosure and a PCB or PWB with a circuit component mounted thereon are secured by a screw and a metal spacer: wherein a noise suppression component of which base material is an insulating material is disposed between the PCB or PWB and the metal spacer; and a first conducting portion is formed on the metal spacer side of the noise suppression component, a second conducting portion is formed on the PCB or PWB side of the noise suppression component, and a resistance member is disposed between the first conducting portion and the second conducting portion. 
     (2) The electronic device according to (1): wherein the resistance member is disposed on a first surface of the noise suppression component. 
     (3) The electronic device according to (1): wherein the resistance member is embedded in the inside of the noise suppression component. 
     (4) The electronic device according to (1): wherein a plurality of the resistance members are formed parallel to each other. 
     (5) An electronic device in which an enclosure and a PCB or PWB with a circuit component mounted thereon are secured by a screw having a plate portion and a shank portion and a metal spacer: wherein the plate portion of the screw includes a first conductive portion connected to the shank portion, an insulator portion surrounding the first conductive portion, and a second conductive portion surrounding the insulator portion and electrically connected to the PCB or PWB; and a resistance member is disposed between the first conductive portion and the second conductive portion. 
     (6) The electronic device according to (5): wherein the resistance member is disposed on a surface of the insulator portion. 
     (7) The electronic device according to (5): wherein the resistance member is embedded in the inside of the insulator portion. 
     (8) The electronic device according to (5): wherein a plurality of the resistance members are formed. 
     (9) A method of measuring a noise current in an electronic device: wherein the electronic device has a configuration such that an enclosure and a PCB or PWB with a circuit component mounted thereon are secured by a screw and a metal spacer; a noise suppression component of which base material is an insulating material is disposed between the PCB or PWB and the metal spacer; a first conducting portion is formed on the metal spacer-side of the noise suppression component, a second conducting portion is formed on the PCB or PWB-side of the noise suppression component and a resistance member is disposed between the first conducting portion and the second conducting portion; and voltage between two ends of the resistance member is measured via a matching resistor connected in series with the resistance member. 
     Effect of the Invention 
     According to the present invention, electrostatic resistance characteristics or like immunity tolerance of an electronic device such as an information processor, an information appliance, or a storage device can be improved to enhance reliability of the electronic device, without adding a countermeasure component to a printed PCB or PWB mounted on the electronic device. In addition, the present invention can suppress EMI from the electronic device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a device on which a noise suppression component is mounted between a metal spacer and a PCB or PWB. 
         FIG. 2  is a cross-sectional view of the device on which the noise suppression component is mounted between a screw and the PCB or PWB. 
         FIG. 3  is a cross-sectional view of the device on which the noise suppression component is mounted between the spacer and the PCB or PWB/screw. 
         FIG. 4A  is a plan view of a noise suppression component provided with eight resistors. 
         FIG. 4B  is a cross-sectional view of a noise suppression component provided with eight resistors. 
         FIG. 5A  is a plan view of a noise suppression component provided with five resistors. 
         FIG. 5B  is a cross-sectional view of a noise suppression component provided with five resistors. 
         FIG. 6A  is a plan view of a noise suppression component configured such that eight resistors are embedded. 
         FIG. 6B  is a cross-sectional view of a noise suppression component configured such that eight resistors are embedded. 
         FIG. 7A  is a plan view of a noise suppression component configured such that five resistors are embedded. 
         FIG. 7B  is a cross-sectional view of a noise suppression component configured such that five resistors are embedded. 
         FIG. 8A  is a cross-sectional view of a screw having a noise suppression structure in which resistors are arranged on a screw plate. 
         FIG. 8B  is a cross-sectional view of a screw having a noise suppression structure in which resistors are arranged in a screw plate. 
         FIG. 9  is a cross-sectional view of a device on which a screw having a noise suppression structure is mounted. 
         FIG. 10A  is a plan view of a resistance detection screw current probe. 
         FIG. 10B  is a cross-sectional view of a resistance detection screw current probe. 
         FIG. 11  is a cross-sectional view of when a noise current flowing between a metal enclosure and a PCB or PWB is measured. 
         FIG. 12  is a block diagram of noise current measurement using a resistance detection screw current probe. 
         FIG. 13  is a graph of detected voltage and frequency characteristics of the resistance detection screw current probe. 
         FIG. 14  is an image diagram of noise current pathway control based on how to install noise suppression components. 
         FIG. 15  is an image diagram of measurement using a plurality of the resistance detection screw current probes. 
         FIG. 16  is an image view of a metal enclosure and a PCB or PWB secured to each other in a conventional manner. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments are hereinafter described along with their effects in detail with reference to the drawings illustrating the present invention. Incidentally, elements having the same function are denoted with like reference numerals in all of the drawings for explaining the embodiments and their repeated explanations are omitted. 
     Embodiment 1 
       FIG. 1  is a cross-sectional view of a device on which an electrostatic noise suppression component is mounted, illustrating a typical embodiment of the present invention. 
     A problem in a conventional electrostatic test is described using  FIG. 16  before the description of the embodiments of the present invention.  FIG. 16  is a cross-sectional view of an electronic device in which a metal enclosure  102  and a PCB or PWB  103  are secured via a metal spacer  108  by means of a screw  104 . The PCB or PWB  103  has a ground trace  106  and a power trace  107 , and an LSI (Large Scale Integrated Circuit)  109  and a bypass capacitor  110  are mounted thereon. 
     As mentioned in “Background Art”, in the electrostatic test specified in IEC61000-4-2, electrostatic noise  112  is applied to a portion of an electronic device where people touch, a metal enclosure in  FIG. 16  for example, by use of an ESD (Electrostatic Discharge) gun  111 . The noise current  105  occurring at this time flows from the metal enclosure  102  via the metal spacer  108  and a metal washer  901  or via the screw  104  and the metal washer  901  into the ground trace  106  of the PCB or PWB  103 . 
     The noise current  105  flowing into the ground trace causes a voltage fluctuation in the ground trace  106 . This causes malfunctions such as error reset of an active component (e.g. LSI  109 ) and unlocking of a transmitter. The noise current  105  flowing into the ground trace  106  induces noise voltage/current in the power trace  107 , a signal line or the like due to capacitive/inductive coupling, thereby causing the malfunctions in the LSI  109  as above. These matters consequently lead to a failure of the overall electronic device system and degrade reliability. 
     Accordingly, to enhance the reliability of the overall electronic device system, the above-mentioned malfunctions of the LSI  109  due to the electrostatic noise  112  is required to be reduced. In order to achieve this, a countermeasure component such as a bypass capacitor  110  or the like is mounted on the PCB or PWB  103 . Other examples of the countermeasure component include a varistor, an ESD protection diode and an EMI filter. 
     However, it is difficult to predict where (which line) the noise current  105  dominantly flows on the PCB or PWB  103  in the electrostatic test and the mechanism thereof is not understood. It is therefore very difficult to dispose the above-mentioned countermeasure component against the current path of the noise current  105  accurately and effectively. For such reason, PCBs or PWBs are designed in a manner that every portion possible to be problematic is mountable with the countermeasure component, which in turn increases the proportion of a countermeasure component cost to a PCB or PWB size and raises component costs and PCB or PWB costs. 
     A basic countermeasure such that prevents the noise current  105  from flowing into the LSI  109  on the PCB or PWB  103 , the ground trace  106  and the power trace  107  is needed to avoid this. The present invention provides a component that controls the noise current  105  flowing from the metal enclosure  102  via the metal spacer  108  into the PCB or PWB  103 . 
     A component embodying the present invention is next described in detail with reference to  FIG. 1 . As described earlier, electrostatic noise  112  is applied to a metal enclosure  102  in  FIG. 1  by use of an ESD gun  111  in electrostatic tests. A noise current  105  then flows from the metal enclosure  102  via a metal spacer  108  or a screw  104  into a PCB or PWB  103 . 
     In the present invention, a noise suppression component  100  is inserted between the metal spacer  108  and the PCB or PWB  103  as shown in  FIG. 1 . The noise suppression component  100  includes resistors  101  arranged on an insulating base material having wiring on the front surface thereof. Inserting the noise suppression component  100  allows the resistors  101 , serving as loss components with respect to the pathways of the noise current  105  flowing from the metal enclosure  102  into the PCB or PWB  103 , to be disposed purposefully. 
     As described, by inserting the loss component into the pathway of the noise current, the noise current  105  flowing into the PCB or PWB  103  can be reduced. Therefore it is not necessary to dispose the countermeasure components at all the positions possible to be problematic on the PCB or PWB  103  as described above. This suppresses an increase in PCB or PWB cost. 
     The noise suppression component  100  is inserted in place of the metal washer  901  in  FIG. 16 ; therefore, a new component is not added. A new component such as the resistance sheet or the magnetic body inserted between the metal spacer  108  and the PCB or PWB  103  which are employed in the conventional art is not added. In addition, since an intended resistant value can be inserted, a reduction amount and a frequency band width intended to be reduced of the noise current  105  can be controlled easily. 
     Another embodiment different from the embodiment shown in  FIG. 1  is described with reference to  FIG. 2 . In the embodiment shown in  FIG. 2 , the spacer connecting a metal enclosure  102  with a PCB or PWB is a metal-embedded plastic spacer  201  of which surface is of plastic and the inside is of metal. The metal inside the spacer is not in direct contact with the PCB or PWB  103 . 
     In this case, when the electrostatic noise  112  is applied to the metal enclosure  102  by an ESD gun  111 , the component of the noise current  105  which flows into the PCB or PWB  103  via the metal space  108  is very small in  FIG. 2  unlike in  FIG. 1 . This is because the ground trace  106  of the PCB or PWB  103  and the enclosure  102  are not electrically connected to each other due to the presence of the metal-embedded plastic spacer. The current flowing from the enclosure  102  to the PCB or PWB  103  via the metal spacer and the screw can be thus controlled. 
       FIG. 3  shows another embodiment of the present invention. In the embodiment of  FIG. 3 , the noise suppression component  100  is inserted between the metal spacer  108  and the PCB or PWB  103 , and between the screw  104  and the PCB or PWB  103 . With the arrangement of the noise suppression components  100  as shown in  FIG. 3 , both the noise current  105  flowing from the metal enclosure  102  to the PCB or PWB  103  via the metal spacer  108  and the noise current  105  flowing from the metal enclosure to the PCB or PWB  103  via the metal spacer  108  and via the screw  104  can be controlled. An effect of further reducing the noise current  105  can be achieved compared with that in the embodiment shown in  FIG. 1 . 
     In the description of embodiment 1 using  FIGS. 1-3 , only one of the portions connecting the metal enclosure  102  with the PCB or PWB  103  was described. However, if the noise suppression component  100  of the present invention is inserted into all of the portions connecting the metal enclosure  102  with the PCB or PWB  103 , a further high effect can be obtained. 
     As shown in  FIG. 14 , of the portions connecting the metal enclosure  102  with the PCB or PWB  103 , a portion to which the noise suppression component  100  is inserted may be selected and the value of the resistance  101  of the noise suppression component  100  may also be arbitrarily changed. The pathway of the noise current  105  flowing from the metal enclosure  102  to the PCB or PWB  103  can be thereby controlled. In  FIG. 14 , reference numerals  100   a - 100   c  denote noise current suppression components and  104   a - 104   d  denote screw components. 
     For example, if the resistance of the noise suppression component  100   a  or  100   c  is increased, noise currents indicated with arrows of dotted lines in  FIG. 14  can be checked, and a noise current flowing in the direction indicated with a solid line can be increased. That is to say, the noise current flowing toward the LSI can be reduced by making the noise current flow as indicated with the arrow of the solid line in  FIG. 14 . 
     In the description of the present embodiment, the electrostatic test which is one of the immunity tests is especially referred to. It goes without saying that the present invention provides the same effect for other tests specified by IEC such as a radiation immunity test emitting electromagnetic waves or an immunity test applying pulse noise. 
     As described heretofore, the noise suppression component  100  of the present invention can reduce the noise current  105  flowing from the metal enclosure  102  into the PCB or PWB  103  and control noise getting mixed into the LSI  109  mounted on the PCB or PWB  103 . Consequently, the malfunction of the LSI  109  is reduced and a more reliable electronic device can be provided by the present invention. 
     Embodiment 2 
     An embodiment of a noise suppression component  100  according to the present invention is described in detail with reference to  FIGS. 4A to 8B .  FIGS. 4A to 7B  shows noise suppression components configured to have substantially the same size and shape as those of the metal washer  901 .  FIGS. 8A and 8B  show a noise suppression component having the same configuration as that of the screw  104 . Each of them can be employed by replacing them with the conventionally used metal washer  901  and screw  104 . 
     Details of  FIGS. 4A to 5B  are first described.  FIGS. 4A and 5A  are plan views of the noise suppression component  100  and  FIGS. 4B and 5B  are cross-sectional views of the noise suppression component  100 . An upper portion and a lower portion of the noise suppression component  100  are respectively defined as a surface A and a surface B in the cross-sectional views of  FIGS. 4B and 5B . 
     The noise suppression component  100  is composed of a threaded hole  400  adapted to receive a screw therethrough, a surface-A connection pad  401   a , a surface-B connection pad  401   b  and resistors  101 . Resistors  101  mounted on the noise suppression component  100  electrically connect the surface-A connection pad  401   a  with the surface-B connection pad  401   b  at any resistance value. 
     The noise suppression component  100  is thereby configured to have an electric loss between the surface-A connection pad  401   a  and the surface-B connection pad  401   b  due to the resistors  101 . A via  402  is used to connect the surface-B connection pad  401   b  of the noise suppression component  100  at the surface A and the surface B. 
     Inserting the noise suppression component  100  having such configuration enables arbitrary resistance to be inserted between the metal spacer  108  and the PCB or PWB  103  or between the screw  104  and the PCB or PWB  103  shown in  FIGS. 1 to 3 . 
       FIGS. 4A and 4B  shows the configuration where eight of the resistors are mounted. If the resistance of the noise suppression component  100  is set as e.g. 10 ohms, eight of 80-ohm chip resistors are to be mounted.  FIGS. 5A and 5B  shows a configuration where five of the resistors are mounted. Similarly, if a resistance of e.g. 10 ohms is set as the resistance value of the noise suppression component  100 , five of 50-ohm chip resistors should be mounted. 
     In  FIGS. 5A and 5B , the resistors are disposed on one side of the threaded hole. Therefore, the form of the noise suppression component can be made small compared with the case where the resistors are arranged concentrically. Meanwhile, when the resistors are disposed only on one side, the flexibility of inductance controlling is limited compared with the case where the resistors are arranged concentrically. 
       FIGS. 6A-7B  show other embodiments of the noise suppression component  100  according to the present invention. Similarly to  FIGS. 4A-5B ,  FIGS. 6A and 7A  are plan views of the noise suppression component  100  and  FIGS. 6B and 7B  are cross-sectional views of the noise suppression component  100 . The noise suppression components  100  shown in  FIGS. 6A-7B  are different from the embodiments described earlier with  FIGS. 4A-5B  in that resistors  101  are embedded between the surface A and surface B of the noise suppression component  100 , that is, in an inner layer thereof. 
     In the embodiments shown in  FIGS. 4A-5B , due to restriction on the height of resistors  101  mounted on the surface A of the noise suppression component  100 , the noise suppression component  100  of the present invention may not be inserted into devices in which the metal spacer  108  and components are crowded. However, the configuration shown in each of  FIGS. 6A ,  6 B and  7 A,  7 B has no restriction on the height of components or the resistors  101 . The noise suppression components  100  shown in  FIGS. 6A-7B  can be inserted into such components crowded areas. 
     The resistance value of the noise suppression component  100  shown in each of  FIGS. 6A ,  6 B and  7 A,  7 B can be set similarly to that shown in each of  FIGS. 4A ,  4 B and  5 A,  5 B. While the resistors  101  shown in  FIGS. 4A-5B  can be mounted externally so as to arbitrary change the resistance value of the resistor  101 , it is difficult to arbitrarily change the resistance value of the resistors  101  shown in  FIGS. 6A-7B . 
     Incidentally,  FIGS. 6A-7B  show the configuration in which the chip resistors are embedded in the inner layer of the noise suppression component  100 . A resistant material may instead be embedded in place of the chip resistors to produce the same effect. 
     Additionally, while the embodiments shown in  FIGS. 4A-7B  illustrate the noise suppression components  100  on which the eight or five resistors  101  are mounted, the same effect can be produced even if the number of resistors  101  is one or ten. In this case, if it is assumed that the number of the resistors  101  mounted on the noise suppression component  100  is N and the resistance value of the noise suppression component  100  is R ohm, a resistor  101  having a value obtained by dividing R [ohm] by N should be mounted. 
       FIGS. 8A and 8B  are cross-sectional views illustrating further other embodiments of the noise suppression component in the present embodiment. The noise suppression components  100  shown in  FIGS. 8A and 8B  are noise suppression screws  800  each having a noise suppression structure. Resistors  101  are each provided between the noise suppression screw  800  and a corresponding one of conductors  802  of respective screw plates  801 . 
     Each of the screws in  FIGS. 8A and 8B  has a shank portion and a plate portion. The plate portion has a basic structure including a first conductive portion connected to the shank portion, an insulating portion surrounding the first conductive portion, and a second conductive portion surrounding the insulating portion. Thus, in this state, the first conductive portion of the plate portion and the second conducting portion of the plate portion are not electrically connected to each other. In the present embodiment, a resistance member is disposed between the first conductive portion and second conductive portion of the plate portion. 
       FIG. 8A  shows a structure in which the resistors  101  are mounted on the screw plate  801 .  FIG. 8B  shows a structure in which the resistors  101  are embedded inside the screw plate  801 . The ways for determining a resistance value and the number of the resistors  101  is the same as the contents described earlier with  FIGS. 4A-7B . As with  FIGS. 6A-7B , the resistors  101  embedded as shown in  FIG. 8B  may either be chip resistors or resistive materials and the same effect can be obtained. 
       FIG. 9  is a cross-sectional view showing an example in which the noise suppression screw  800  described with  FIG. 8  is applied to an actual device. If the metal-embedded plastic spacer  201  as described with  FIG. 2  is used, a noise current flowing into the PCB or PWB  103  via the embedded metal and the screw  104  can be suppressed. 
     In  FIG. 2 , the noise suppression component  100  is inserted in place of the metal washer  901 . Meanwhile, in the embodiment shown in  FIG. 9 , the metal screw  104  is replaced with the noise suppression screw  800  to secure the PCB or PWB  103  to the enclosure  102  and the metal-embedded plastic spacer  201 , thereby suppressing the noise current  105 . 
     Although the configuration in  FIG. 9  uses the spacer having metal therein as a spacer, the present embodiment is not limited to this configuration. For example, a metal spacer may be used as the spacer. In this case, it is predicted that resistance on the metal spacer side is reduced. Metal spacers or the like may be used in cases where the current on the screw plate side on the PCB or PWB is more intended to be controlled or suppressed than the noise current on the metal spacer side on the PCB or PWB. 
     The description has been given thus far focusing on the control of the noise current  105  flowing into the PCB or PWB  103  from the metal enclosure  102  during the electrostatic test with reference to  FIGS. 1-9 . However, it goes without saying that the noise suppression component  100  proposed by the present invention has the same effect for the noise current  105  reversely flowing into the metal enclosure  102  from the LSI  109  via the ground trace  106  in the PCB or PWB  103 , the screw  104  and the metal spacer  108 . 
     Embodiment 3 
     Embodiment 3 describes a probe for noise measurement and a measuring method of the measurement. The washer-type noise suppression components  100  shown in  FIGS. 4A-7B  can be utilized as noise measurement probes by modifying the configurations thereof. The use of the noise measurement probe makes it possible to measure the noise current  105  flowing into the PCB or PWB  103  from the metal enclosure  102  via the metal spacer  108  and the screw  104 . 
     One example of the embodiment of the probe structure for measuring an electric current flowing into a joint portion between the metal enclosure  102  and the PCB or PWB  103  is described by use of  FIGS. 10A and 10B .  FIG. 10A  is a plan view of a resistance detection screw current probe  1000  and  FIG. 10B  is a cross-sectional view of the resistance detection screw current probe  1000 . 
     The resistance detection screw current probe  1000  shown in  FIGS. 10A and 10B  is provided by modifying and transforming the noise suppression component  100  shown in  FIGS. 4A and 4B . The resistance detection screw current probe  1000  includes a threaded hole  400 , a surface-A connection pad  401   a , a surface-B connection pad  401   b , a detection resistor  1001 , a surface-A connection pad extension line  1002   a , a surface-B connection pad extension line  1002   b , a voltage detection terminal  1003  and a matching resistor  1004 . 
       FIG. 11  shows a state where the resistance detection screw current probe  1000  is used to measure a noise current  105  flowing between the metal enclosure  102  and the PCB or PWB  103  during an electrostatic test. As shown in  FIG. 11 , when the electrostatic noise  112  is applied to the metal enclosure  102  by the ESD gun  111 , the noise current  105  flows from the metal enclosure  102  and the metal spacer  108  to the PCB or PWB  103  via the resistance detection screw current probe  1000 . 
     The noise current  105  flows into the detection resistor  1001 , so that a voltage difference occurs between the surface-A connection pad  401   a  side and surface-B connection pad  401   b  side of the detection resistor  1001 . This voltage difference is observed by a measuring instrument  1100  such as an oscilloscope, a spectrum analyzer or the like via the surface-A connection pad extension line  1002   a  and the surface-B connection pad extension line  1002   b . The noise current  105  can be thus measured. Similarly to the explanation for the resistance value of the noise suppression component  100  in  FIG. 4A ,  4 B, if the resistance value of the resistance detection screw current probe  100  is 10 ohms, eight of the chip resistors of 80 ohms should be mounted as detection resistors  1001 . 
     A detailed description is here given of the measurement principle of the noise current  105  using the resistance detection screw current probe  1000  shown in  FIG. 11 .  FIG. 12  is a block diagram in which the metal enclosure  102  and the PCB or PWB  103  are blocked and the resistance detection screw current probe is drawn in further detail. As can be seen from the figure, the resistance detection screw current probe  1000  is inserted between the metal enclosure  102  and the PCB or PWB  103  via enclosure mating portions: metal spacer  108 , the screw  104  or the like in  FIG. 11 . The resistance detection screw current probe  1000  is connected from the voltage detection terminal  1003  via a cable to the measuring instrument  1100  and terminates at Rt=50 ohms. 
     A detailed explanation of the resistance detection screw current probe  1000  depicted in a circuit diagram of  FIG. 12  is next complemented. The detection resistor  1001  or a current detecting portion is assumed to have a resistance Rs, parasitic inductance is assumed to be Ls and parasitic capacitance is assumed to be Cp, and these are collectively defined as impedance Zp of a detecting portion. Further, the surface-A connection pad extension line  1002   a  is assumed to be Lcg, the surface-B connection pad extension line  1002   b  is assumed to be Lcs, a matching resistor  1004  is assumed to be Rd and voltage to be detected by the measuring instrument  1100  is assumed to be V 1 . 
     In  FIG. 12 , if the noise current  105  flowing when the electrostatic noise  112  is applied to the metal enclosure  102  by the ESD gun  111  is assumed to be I 1 , the voltage V 1  detected by the measuring device  1100  is represented as (expression 1). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     V 
                     1 
                   
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                         t 
                       
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                         Z 
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                         Z 
                         p 
                       
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                         ⁡ 
                         
                           ( 
                           
                             
                               L 
                               cs 
                             
                             + 
                             
                               L 
                               cg 
                             
                           
                           ) 
                         
                       
                       + 
                       
                         R 
                         d 
                       
                       + 
                       
                         R 
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Z p  is the impedance of the detecting portion and is represented as (expression 2). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     Z 
                     p 
                   
                   = 
                   
                     
                       
                         R 
                         s 
                       
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                         jω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           L 
                           s 
                         
                       
                     
                     
                       1 
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                         jω 
                         · 
                         
                           
                             C 
                             p 
                           
                           ⁡ 
                           
                             ( 
                             
                               
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                                 ⁢ 
                                 
                                     
                                 
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                   ( 
                   2 
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     Symbol ω is an angular frequency and is represented as (expression 3).
 
[Expression 3]
 
ω=2π· f   (3)
 
     In  FIG. 12 , when the noise current  105  flows at a constant amplitude of 10 mA in a frequency range of 1 MHz to 4 GHz, calculation results of the voltage V 1  detected by the measuring instrument  1100  are illustrated as  FIG. 13 . These results are obtained under the following conditions: Rs is set at 1 ohm and Rd is set at 0 ohm; Lcs and Lcg are both set at 3 nH, and Cp is fixed at 3 pF; and Ls is set at 0.2 nH and at 1.0 nH. 
     In the example of a measurement of the resistance detection screw current probe  1000  shown in  FIG. 13 , the voltage V 1  detected by the measuring instrument  1100  is constantly 10 mV at a frequency of 500 MHz or below. At a frequency of 500 MHz or higher, the voltage V 1  tends to increase with the frequency. This largely relates to the property of impedance Zp of the detecting portion represented in (expression 2). That is, when ω=0 (f=0) is substituted in (expression 1) and (expression 2), Zp=Rs holds, and the detected voltage V 1  is represented by a constant product of the noise current  105 : I 1  and the detected resistance  101 : Rs. A point of variation at which the V 1  is the constant product of I 1  and Rs is determined by a cutoff frequency: fc of the impedance Zp of the detecting portion. The cutoff frequency is represented by (expression 4). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     f 
                     c 
                   
                   = 
                   
                     
                       R 
                       s 
                     
                     
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                         · 
                         
                           L 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Accordingly, the value of the noise current  105  may be considered to be obtained by dividing the detected voltage V 1  by Rs at a cut off frequency fc or below. However, this is established when the matching resistor  104 : Rd=0 ohm holds. When the matching resistor  1004 : R 4  and the detection resistor  1001 : Rs are in a relation of R 4 +Rs=50, the value of the noise current  105 : I 1  can be obtained by doubling the detected voltage V 1  and dividing the doubled value by the detection resistor  1001 : Rs. 
     In the resistance detection screw current probe  1000 , the conversion from the detected voltage to current is very easy at a frequency such that is a constant product of the noise current  105 : I 1  and the detection resistor  1001 : Rs. Therefore, the resistance detection screw current probe  1000  has better property as the cutoff frequency is higher. 
     According to the present invention described above, the cutoff frequency can be controlled easily by appropriately selecting Ls. In other words, a flat area in  FIG. 13  can arbitrarily be varied. Further, according to the present invention, the flat area in  FIG. 13  can be broadened easily. According to an experiment, the flat area in  FIG. 13  can be broadened to approximately 4 GHz. 
       FIG. 10  shows the embodiment in which the eight detection resistors  1001  are mounted on the resistance detection screw current probe  1000 . However, the number of the detection resistors  1001  mounted may be five as shown in the noise suppression component  100  of  FIG. 5  and the same effect can be produced. Further, needless to say, also a structure similarly to those shown in  FIGS. 6 and 7  where the detection resistors  1001  are embedded between the A-surface connection portion  401   a  and the B-surface connection surface  401   b , i.e., in the inner layer can produce the same effect. Further, as with the structure in  FIG. 8 , the detection resistors  1001  may be mounted on or embedded in the screw plate  801  and voltage between both the ends of the detection resistor  1001  may be measured to obtain the noise current  105  flowing via the screw. 
     Even when the number of the detection resistors  1001  is one or ten, the same effect can be produced. When a number of N detection resistors  1001  are mounted on a resistance detection screw current probe of which resistance value is to be R ohm, the value of the N detection resistors  101  can be obtained by dividing R [ohm] by N [pieces]. 
     However in the resistance detection screw current probe  1000 , the inductance of the detected portion is smaller and the cutoff frequency represented by expression 4 is higher when a plurality of (e.g., five, eight or more) detection resistors  1001  are mounted than that of when one resistor  1001  is mounted. Thus, the resistance detection prove  1000  has satisfactory property. The reason for this is as below. 
     For example, it is assumed that a chip having a size of 1.0 mm×0.5 mm or 0.4 mm×0.2 mm is used for the resistance detection resistor  1001 . The parasitic inductance of the chip resistor of this size is approximately 1 nH. If the number of the detection resistors  1001  is one, Ls in  FIG. 13  is 1 nH. On the other hand, if the number of the detection resistors  1001  is five, the parasitic inductance is one-fifth, i.e., Ls=0.2 nH in  FIG. 13 . When the parasitic inductance Ls is able to be reduced by embedding a resistant material as shown in  FIGS. 6 and 7 , it is not necessary to arrange a plurality of the detection resistors. 
     Description for the embodiments of the resistance detection screw current probe  1000  according to the present invention has been made by taking as an example the electrostatic test using the discharged gun  111  as shown in  FIG. 11 . However, the resistance detection screw current probe  1000  of the present invention has the same effect for other immunity tests such as, for example, a radiation immunity test, a conductive pulse test or the like. 
     The description is given focusing on the measurement of the noise current  105  flowing into the PCB or PWB  103  from the metal enclosure  102  with reference to  FIG. 11 . It goes without saying that the present invention can be applied to the measurement of the noise current  105  reversely flowing from the LSI  109  via the ground trace  106  of the PCB or PWB  103 , the screw  104  and the metal spacer  108  into the metal enclosure  102 , producing the same effect. The probe  1000  can be utilized as a probe for identifying a source or a problematic portion of EMI. 
     Next, a description is given of a method of measuring a current flowing in joint portions between the metal enclosure  102  and the PCB or PWB  103  of the electronic device during an electrostatic test by use of a plurality of the resistance detection screw current probe.  FIG. 15  shows one example of such a method. Resistance detection screw current probes  1000   a - d  are secured by corresponding screws  104   a - d  at four joint portions between the metal enclosure  102  and the PCB or PWB  103 . Further, respective cables are pulled out from the resistance detection screw current probes  1000   a - d  and connected to a measuring instrument  1100 . 
     An ESD gun  111  is connected to a control PC. The control PC controls the discharge of electrostatic noise. Control signals of the ESD gun  111  are inputted into the control PC as triggers of the measuring instrument  1100 . The control PC is also connected to the LSI  109  to control the LSI  109  and to monitor the inside of the LSI  109  during the test. 
     In  FIG. 15 , the control PC emits a trigger signal and the ESD gun  111  applies electrostatic noise  112  to each of the screws  104   a - d . At this time, the noise current  105  flowing in each of the screws  104 - d  is observed by the measuring instrument  1100 . 
     The control PC monitors the inside of the LSI  109  during the application of the electrostatic noise  112 . A position where the electrical discharge causes the malfunction of the LSI  109  can be determined. In addition, it is also possible to analyze which one of the screws  104   a - d  that the noise current  105  acting as an obstacle flows through at that time. Thus, the noise current measuring method and measuring system can be provided such that is capable of elucidating the mechanism of the LSI  109  malfunctions during the electrostatic test, and in turn, elucidating the mechanism of the electronic device malfunctions. 
     EXPLANATION OF REFERENCE NUMERALS 
       100  . . . noise suppression component,  101  . . . resistor,  102  . . . metal enclosure,  103  . . . PCB or PWB,  104  . . . screw,  105  . . . noise current,  106  . . . ground trace,  107  . . . power trace,  108  . . . metal spacer,  109  . . . LSI,  110  . . . bypass capacitor,  111  . . . ESD gun,  112  . . . electrostatic noise,  201  . . . metal-embedded plastic spacer,  400  . . . threaded hole,  401   a  . . . surface-A connection pad,  401   b  . . . surface-B connection pad,  402  . . . via,  800  . . . noise suppression screw,  801  . . . screw plate,  802  . . . conductor,  901  . . . metal washer,  1000  . . . resistance detection screw current probe,  1001  . . . detection resistor,  1002   a  . . . surface-A connection pad extension line,  1002   b  . . . surface-B connection pad extension line,  1003  . . . voltage detection terminal,  1004  . . . matching resistor,  1100  . . . measuring instrument.