Abstract:
A method for inspecting an electronic circuit formed on a board with a peripheral circuit includes steps of providing a terminal for inputting and outputting an electronic signal, providing an impedance increase means for increasing an impedance of an electrical connection between the electronic circuit and the peripheral circuit and providing an inspection means for inspecting the electronic circuit. The impedance of the electronic circuit is increased to prevent influence of the peripheral circuit before and during inspection of the electronic circuit, and the increase of the impedance is removed after the inspection.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and claims the benefit of priority of Japanese Patent Application No. 2005-116295 filed on Apr. 13, 2005, the disclosure of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to a circuit inspection method.  
       BACKGROUND OF THE INVENTION  
       [0003]     Conventionally, electronic circuits on a substrate are implemented by using a packaged (resin sealed) ICs and microcomputers because of their ease of handling and resistivity to environmental influence. However, demand for high functionality and more complicated system in electronic control units (ECUs) urges increase of the scale of the electronic circuits, in spite of a limited space reserved for the ECUs.  
         [0004]     The limitation imposed on the ECUs drives reduction in volume of the electronic circuits by using smaller ICs (packaged parts, chip-size packages, bare chips [IC chips without resin seal] or the like). In this case, reduced spacing between the terminals and/or decreased size of the terminals on the smaller ICs make it more difficult to perform quality inspection by contacting the terminals.  
         [0005]     Japanese Patent Document JP-A-H5-72280 discloses an electronic circuit inspection method for quality assurance that inspects the electronic circuits in a state of implementation on a board. However, the electronic circuit implemented on the board has peripheral circuits connected thereto, thereby receiving influence from the peripheral circuits during inspection and having inaccuracy in inspection result. That is, critical inspections in terms of product quality such as leakage current inspection, function inspection or the like cannot be performed on the electronic circuit in the state of implementation on the board.  
       SUMMARY OF THE INVENTION  
       [0006]     In view of the above-described and other problems, the present invention provides a method for inspecting an electronic circuit in a state of implementation on a board without having influence from other circuits that are not included in inspection.  
         [0007]     The inspection method of the present invention for inspecting an electronic circuit disposed on a board includes steps of providing a terminal for inputting and outputting an electronic signal to/from the electronic circuit , providing a peripheral circuit, providing an impedance increase unit for increasing an impedance of an electrical connection between the electronic circuit and the peripheral circuit, and providing a inspection unit for inspecting the electronic circuit. The impedance increase unit performs inflow prevention process of an electric current from the peripheral circuit into the electronic circuit and/or outflow prevention process of the electric current from the electronic circuit to the peripheral circuit before performing inspection process of the electronic circuit with the inspection unit. The inspection unit performs the inspection process of the electronic circuit by electrically coupling the terminal with the inspection unit and either providing inspection signal for or receiving inspection signal from the electronic circuit. The impedance increase unit performs electrical coupling process between the electronic circuit and the peripheral circuit after the inspection process.  
         [0008]     The subject electronic circuit to be inspected can avoid influence of other circuits by increasing impedance of the subject electronic circuit itself relative to the peripheral circuit. In this manner, leak current inspection or function inspection of the electronic circuit can be performed in a state of implementation on the board.  
         [0009]     The method for inspecting the electronic circuit formed on a board can also be described as conditions of electronic circuits in relation with the peripheral circuit. That is, the inflow/outflow of the electric current to/from the electronic circuit is prevented by increasing impedance of the electronic circuit with the impedance increase unit as an initial condition, and electric current shutoff condition is kept maintained during the inspection. The inspection signal is provided to the terminal from the inspection unit during the inspection. The impedance of the electronic circuit is lowered after the inspection to have the electrical coupling between the electronic circuit and the peripheral circuit.  
         [0010]     The third version of the description of the inspection is more direct than the former ones. The inspection of the electronic circuit is executed by increasing the impedance of the electronic circuit with the impedance increase unit in the first place, and inspection signal is provided to the circuit with the increased impedance maintained in the same level. After the inspection, the impedance is lowered to resume the electrical connection between the electronic circuit and the peripheral circuit.  
         [0011]     In another aspect of the present invention, the impedance increase unit is provided as a switch. For example, a MOS switch may be used to securely increase the impedance of the electronic circuit by using a small control current in a simple circuit structure without causing a leak current.  
         [0012]     In still another aspect of the present invention, the impedance increase unit may be provided by omitting a circuit wiring between the electronic circuit and the peripheral circuit for electrical coupling. That is, the impedance of the electrical coupling between pads is controlled without using the switch or the like when the circuit wiring between the pads is not disposed.  
         [0013]     In still yet another aspect of the present invention, the impedance of the electronic circuit may be increased by disposing a diode in a manner that prevents the inflow of the electric current from the peripheral circuit into the electronic circuit and/or the outflow of the electric current from the electronic circuit to the peripheral circuit. In this manner, the electronic circuit can be shipped as a completed product after the inspection without any processing of wiring. Further, the characteristic of the diode disposed in an IC of the peripheral circuit can easily be changed after the inspection for controlling the impedance.  
         [0014]     In still yet another aspect of the present invention, the increase of the impedance may be provided by using a three-state circuit that selectively cuts off the electric current from either of a power supply, a ground (GND), or the peripheral circuit for the electronic circuit in a first, a second or a third state of operation. In this manner, the electronic circuit can be inspected as a completed product because the wiring is not necessarily processed after the inspection.  
         [0015]     In still yet another aspect of the present invention, the increase of the impedance is provided by not disposing wiring between a first pad and a second pad that are respectively connected to bundles of equipotential circuit wiring. In this manner, the number of pads for increasing the impedance is decreased.  
         [0016]     In still yet another aspect of the present invention, the electronic circuit and the peripheral circuit are disposed on a same board. In this manner, the object of the inspection, i.e., the electronic circuit, and the peripheral circuit are implemented on the same board to be inspected as an almost completed product for achieving an improved product quality. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings, in which:  
         [0018]      FIG. 1  shows a block diagram of a circuit configuration on a board regarding an embodiment of the present invention;  
         [0019]      FIG. 2  shows a schematic diagram of the circuit configuration on the board in a first embodiment;  
         [0020]      FIG. 3  shows a schematic diagram of the circuit configuration on the board in a second embodiment;  
         [0021]      FIG. 4  shows a schematic diagram of the circuit configuration on the board in a third embodiment;  
         [0022]      FIG. 5  shows a schematic diagram of the circuit configuration on the board in a fourth embodiment;  
         [0023]      FIG. 6  shows a schematic diagram of the circuit configuration on the board in a fifth embodiment;  
         [0024]      FIG. 7  shows a schematic diagram of the circuit configuration on the board in a sixth embodiment;  
         [0025]      FIG. 8  shows a schematic diagram of the circuit configuration on the board in a seventh embodiment;  
         [0026]      FIG. 9  shows a schematic diagram of the circuit configuration on the board in an eighth embodiment; and  
         [0027]      FIG. 10  shows a schematic diagram of the circuit configuration on the board in a ninth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Embodiments of the present invention are described with reference to the drawings.  
         [0029]      FIG. 1  shows a block diagram of a circuit configuration on a board  10  regarding an embodiment of the present invention. How impedance of a part of the circuit is increased by an impedance increase unit (HiZ unit)  11  in relation to an inspection of an IC on the board is a focus of the present invention.  
         [0030]     The board  10  has the IC 1   12  implemented thereon as a test object, and each terminal on the IC 1   12  is connected to the HiZ unit  11  through a wiring  13 . An IC 2   14  and a pull-down resistor or the like are also connected to the IC 1   12  through another wiring  13 . In addition, a power source  16  and a GND  17  are connected to the IC 1   12  and IC 2   14  for providing driving current. In this case, the IC 2   14 , the power source  16 , the GND  17 , electric elements such as the pull-down resistor and the like are categorized as a peripheral circuit  15 . The impedance of the IC 1   12  is increased by using the HiZ unit  11  on each of the terminals of the IC 1   12  for performing inspections such as leak tests and function tests. In this manner, an inflow/outflow of the electric current between the IC 1   12  and the peripheral circuit  15  is prevented. The HiZ unit  11  applied in a practical circuit is described in the following description.  
       First Embodiment  
       [0031]      FIG. 2  shows a schematic diagram of the circuit configuration on the board  10  in a first embodiment. In the present embodiment, the IC 1   12  as the test object is connected a switch  26  that constitutes the HiZ unit  11 , and the peripheral circuit  15  that includes the IC 2   14 , a pull-down resistor  20 , a condenser  21  on the board  10  as well as an insulation unit  24 , an ammeter  22  and a signal generator  23  for testing the test object.  
         [0032]     The pull-down resistor  20  and the condenser  21  are connected to the IC 1   12  through the wiring  13 , and the IC 2   14  includes an analog output circuit such as an operational amplifier (Op. Amp)  25  as shown in  FIG. 2 . The IC 1   12  is in connection with the pull-down resistor  20 , the condenser  21 , and the IC 2   14  by the switch  26  as the HiZ unit  11 . In the following embodiments, the pull-down resistor  20  and the condenser  21  are connected to the GND  17  unless otherwise designated. Also, the IC 1   12  has a TP terminal  27  for connecting external devices such as an inspection unit  24 .  
         [0033]     Test procedure for the IC 1   12  is described in the following description.  
         [0034]     The IC 1   12  is electrically isolated from the peripheral circuit  15  by operating the switch  26  as HiZ unit  11 . The influence from the peripheral circuit  15  is prevented in this manner. Then, the TP terminal  27  is used to connect the inspection unit  24  to the IC 1   12  for the leak inspection and the function inspection without having the influence from the peripheral circuit  15 .  
         [0035]     The switch  26  may be provided as a mechanical switch or may be provided as a semiconductor switch by using a semiconductor element such as a p-MOS, an n-MOS or an analog switch.  
       Second Embodiment  
       [0036]      FIG. 3  shows a schematic diagram of the circuit configuration on the board in a second embodiment. Difference between the present embodiment and a previous embodiment exists in that the HiZ unit  11  is provided by omitting the wiring. In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0037]     The increase of the impedance in the present embodiment is arranged by not disposing the wiring  13  between two pads  30  that are used in place of the switch  26  as shown in  FIG. 3 . In this manner, the impedance is increased without adding any extra circuit such as the switch  26  to have a same effect as the first embodiment. Further, the pads  30  are utilized as TP terminals  27  during the inspection. The two pads  30  are electrically connected after the inspection. In this manner, the number of TP terminals  27  is decreased.  
       Third Embodiment  
       [0038]      FIG. 4  shows a schematic diagram of the circuit configuration on the board in a third embodiment. Difference between the present embodiment and previous embodiments exists in that the pull-down resistor  20  and the condenser  21  are implemented on the board  10  after the inspection, and the switch  26  is disposed between the IC 1   12  and the Op. Amp  25  in the IC 2   14 . In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0039]     The IC 1   12  is isolated from the GND  17  by not disposing the condenser  21  and the resistor  20 , and the switch  26  in the IC 2   14  is used to isolate the Op. Amp  25  from the IC 1   12  to increase the impedance and shut the influence from the peripheral circuit  15  as shown in  FIG. 4 . Then, the TP terminals  27  are used to input inspection signal. After the inspection, the condenser  21 , the resistor  20  are implemented on the board  10 . In this manner, the influence from the peripheral circuit  15  including the IC 2   14 , the GND  17  or the like is prevented during the inspection of the IC 1   12 . Any extra circuitry or the like is not required on the board  10  for performing the inspection.  
         [0040]     In this case, the switch  26  may be disposed on a portion of the wiring toward a point A instead of an inside of the IC 2   14 . The parts used to isolate the IC 1   12  may not necessarily be the condenser  21  and the resistor  20 . That is, other electronic parts may be kept un-implemented on the board  10  during the inspection.  
       Fourth Embodiment  
       [0041]      FIG. 5  shows a schematic diagram of the circuit configuration on the board in a fourth embodiment. Difference between the present embodiment and a previous embodiment exists in that the switches  26  are disposed between the pull-down resistor  20 /the condenser  21  and the GND  17  respectively. In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0042]     The switch  26  between the resistor  20  and the GND  17 , and the switch  26  between the condenser  21  and the GND  17  are respectively turned off to isolate the resistor  20 /the condenser  21  from the GND  17  as shown in  FIG. 5 . The switch  26  being turned off during the inspection can increase the impedance, and the switch  26  is turned on to connect the IC 1   12  to the GND  17  after the inspection. In this manner, the influence of the peripheral circuit is prevented during the inspection. That is, the present embodiment has the same effect as the previous embodiment.  
         [0043]     The switch  26  may be provided as a mechanical switch or may be provided as a semiconductor switch by using a semiconductor element such as the p-MOS, the n-MOS or an analog switch.  
       Fifth Embodiment  
       [0044]      FIG. 6  shows a schematic diagram of the circuit configuration on the board in a fifth embodiment. Difference between the present embodiment and a previous embodiment exists in that the wiring  13  is not disposed between the pull-down resistor  20 /the condenser  21  and the GND  17  respectively. In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0045]     The switch  26  disposed between the resistor  20 /the condenser  21  and the GND  17  is replaced with the two pads  30 , and the isolation of the resistor  20 /the condenser  21  from the GND  17  is established by not connecting the two pads  30 . The two pads  30  are short-circuited by the wiring  13  or bonding after the inspection by using the TP terminals  27 . The wirings  13  having equipotential voltage are connected to a single pad  30  in a bundle as shown in  FIG. 6  to have a fewer number of the pads  30 . In this manner, the present invention achieves reduced steps of processes and parts cost for bonding or re-wiring after the inspection and reduced size of the circuit in addition to the effect expected in the fourth embodiment.  
       Sixth Embodiment  
       [0046]      FIG. 7  shows a schematic diagram of the circuit configuration on the board in a sixth embodiment. Difference between the present embodiment and previous embodiments exists in that the IC 1   12  has an external power supply  16 . In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0047]     The IC 1   12  has the power supply  16  provided thereto from outside. In this case, the switch  26  is provided between the power supply  16  and the pull-up resistor  20  as the HiZ unit  11 . In this manner, a simple structure for shutting the electric current from/to the peripheral circuit  15  is provided. The switch  26  may be disposed in a section between the pull-up resistor  20  and a point B instead of a section between the power source  16  and the pull-up resistor  20 .  
       Seventh Embodiment  
       [0048]      FIG. 8  shows a schematic diagram of the circuit configuration on the board in a seventh embodiment. Difference between the present embodiment and a previous embodiment exists in that a zener diode  80  is used as the HiZ unit  11 . In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0049]     The zener diode  80  used in place of the switch  26  as the HiZ unit  11  is shown in  FIG. 8 . In this case, the zener diode  80  cannot be used if the inspection applies a voltage VTP that is greater than a threshold voltage Vf of the zener diode  80  to the TP terminal  27 , because the voltage VTP greater than the Vf causes a wrap-around current toward the power supply  16 . The inspection can appropriately be performed if the circuit configuration allows the voltage V 23  of the power supply  16 . That is, the inspection can be performed when the voltage V 23  is set to be greater than addition of the voltage of VTP and Vf.  
         [0050]     In this manner, the electric current from the power supply  16  (i.e., the peripheral circuit  15 ) is prevented by only using a single piece of the zener diode  80 .  
       Eighth Embodiment  
       [0051]      FIG. 9  shows a schematic diagram of the circuit configuration on the board in an eighth embodiment. Difference between the present embodiment and previous embodiments exists in that a three state digital output circuit is used in the IC  2   14 . In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0052]     The IC 1   12  is connected to condenser  21  and a transistor  90 , and the IC 2   14  has a digital output circuit of CMOS or the like formed therein as shown in  FIG. 9 . The condenser  21  and the transistor  90  are isolated from the IC 1   12  by the switch  26 . The impedance of the circuit of the present invention is increased in three states by using a circuit having a p-MOS  91  and an n-MOS  92  (i.e., a push-pull circuit). That is, an H state having an output voltage from the power supply  16 , an L state having an output voltage from the GND  17 , and a HiZ state that is categorized to neither of the previous states are the states being expected. The inspection of the IC 1   12  is performed in the HiZ state by connecting the inspection unit  24  to the TP terminals  27 . The HiZ state of the push-pull circuit is released and the switch  26  is turned on after the inspection. The three state push-pull circuit can be formed in the IC 2   14  without having any influence in the peripheral circuit  15  on the board  10 .  
         [0053]     The push-pull circuit may increase the impedance only when the push-pull circuit has a signal input (i.e., an inspection signal or the like). The switch  26  may be replaced with the pads  30  for isolation of the transistor  90  and the condenser  21  from the IC 1   12  as shown in  FIGS. 3 and 6 .  
       Ninth Embodiment  
       [0054]      FIG. 10  shows a schematic diagram of the circuit configuration on the board in a ninth embodiment. Difference between the present embodiment and previous embodiments exists in that the HiZ unit  11  having the pull-down resistor  20  and the zener diode  80  is formed in the IC 2   14 . In the present embodiment, like parts have like numerals as the previous embodiment and descriptions of the like parts are omitted.  
         [0055]     The IC 1   12  is connected to the IC 2   14 , and the pull-down resistor  20  is formed in the IC 2   14 . The zener diode  80  is disposed between the GND  17  and the pull-down resistor  20  for isolating the resistor  20  in the IC 2   14  from the GND  17  in the IC 2   14 . Disposing the zener diode  80  increases the impedance and prevents influence of the pull-down resistor  20  on the IC 2   14 . The inspection of the IC 1   12  is performed in this state of increased impedance by connecting the inspection unit  24  to the TP terminals  27 . The inspection cannot appropriately be performed if the voltage applied to the TP terminals  27  is greater than a breakdown voltage VB of the zener diode  80 . The zener diode  80  can be short-circuited after performing the inspection by zapping (adjustment process in terms of resistance/voltage characteristic applied for a semiconductor in a manufacturing process by using a laser), thereby enabling removal of the influence of the zener diode  80  on a normal operation of the IC 2   14 .  
         [0056]     The zener diode  80  of the present embodiment may be replaced with the switch  26  that is shown in  FIG. 7 .  
         [0057]     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.