The present invention relates to an apparatus and method for inspecting electrical continuity of a circuit board, for example, having a fine wiring pattern. The present invention also relates to a jig for use in such an inspection.
As a system for inspecting a circuit board, there have been known a pin-contact system and a non-contact system. As shown in FIG. 1, the pin-contact system is configured to inspect electrical continuity between both ends of a conductive pattern as an inspection object by bringing two pin probes directly into contact with the ends, respectively, and applying a current to one of the pin probes so as to determine a resistance value of the conductive pattern from a detected voltage at the other pin probe.
This pin-contact system has an advantage of a high signal-to-noise (SN) ratio because of the pin probes contacted directly with the conductive pattern.
On the other hand, in case of inspecting a fine-pitch board, it is fundamentally difficult to set up the pin probes only to a conductive pattern as an inspection object, and it is increasingly hard to secure an adequate positioning for bringing the pin probes into contact with the aimed pattern. Due to the necessity for keeping in the contact state, it is also difficult to maintain the initial accuracy of the pin probes themselves, resulting in undesirably increased running cost arising from replacements of the pin probes.
As shown in FIG. 2, the other non-contact/contact combined system is configured to apply an inspection signal including an alternating current (AC) component with making one pin probe contact directly with one end of a conductive pattern as an inspection object (or through a capacitive coupling in a non-contact manner) and detect the inspection signal through a capacitive coupling at the other end.
The non-contact/contact combined system allows at least one of the pin probes not to be contacted with a pattern wire or the conductive pattern. This provides a relatively wide acceptable range of positioning accuracy for the pin probe, which makes it possible to use the pin probe commonly for a plurality of pattern wires, and thereby to reduce the number of pin probes. Furthermore, since the pin prove is free from any wear, the combined system is effective for a board with a fine wiring pattern.
However, the non-contact/contact combined system has a small value of coupling capacitance and a high impedance (from several Mxcexa9 to several Gxcexa9). Thus, this system cannot detect any defective portion having a resistance ranging from about 10xcexa9 to about 100xcexa9.
As a result, due to the property including high impedance despite having many advantages, the non-contact/contact combined system has been actually implemented only for a board with an extremely narrow pitch not to allow pin probes to be adequately set up thereon. Thus, the required high accuracy in the pin probes and a jig thereof has been an obstacle in the effort to facilitate the cost reduction in the non-contact/contact combined system.
It is therefore an object of the present invention to provide a continuity inspection apparatus capable of inspecting any electrical conductivity not only under a high resistance but also under a low resistance by making a capacitance provided in the non-contact system generate a resonance in oscillations of a circuit formed on a board to reduce the impedance of the circuit.
According to the present invention, an electrode is disposed close to one of ends of a pattern as an inspection object to form a capacitance C between the end and the electrode, and an inductive element L is connected to the capacitance C. An inspection signal (frequency f) including an AC component is applied to the other end of the pattern wire through a pin probe.
When the impedance of a resonance circuit is reduced by appropriately adjusting the value L, or when the value L is adjusted, for example, so as to satisfy the following formula (1),
2fxc2x7L=(1/2)fxc2x7Cxe2x80x83xe2x80x83(1)
the following formula is derived from the formula (1).
L=(1/42)xc3x97f2xc3x97Cxe2x80x83xe2x80x83(2)
In other words, the impedance of the circuit can be zero by adjusting the value L in the formula (2), and then an output voltage V exhibits a maximum value. Given that VR is the output voltage V in case of using a reference circuit board (i.e. a circuit board in which no disconnection has been verified) and applying a resonance frequency fR thereto, the output voltage Vx in case of using an actual circuit board as an inspection object would indicate a larger value because the circuit is expected to come close to a resonance state.
As one example, when the value of the coupling capacitance is 10 fF, the relationship between the working frequency fR and the inductive element L which can cooperatively generate the resonance state is shown as follows,
when fR=10 kHz, L=25.3 kH, or
when fR=10 MHz, L=25 mH, or
when fR=50 MHz, L=1 mH, or
when fR=100 MHz, L=250 mH.
A parameter for controlling resonance includes the frequency f of the input inspection signal, the coupling capacitance C, and the inductance L of the inductive element. For example, when the electrode has a fixed size and the measuring is carried out with keeping the distance between the electrode and the pattern constant, the capacitance C would be, for example, about 15 fF. Then, by adjusting the value of the inductive element L in the range of about 250 mH to about 1 mH, and providing an AC signal source having a frequency ranging from about 50 MHz to about 100 MHz, the impedance can be substantially zero.
Based on the above knowledge, according to a first aspect of the present invention, there is provided a continuity inspection apparatus for inspecting electrical continuity between first and second terminals of a pattern wire formed on a board, comprising:
capacitive coupling means to be capacitively coupled with the first terminal in a non-contact manner to provide a coupling capacitance therebetween;
an inductive element connected to the capacitive coupling means to form a resonance circuit in conjunction with the capacitance yielded by the capacitive coupling means;
a first lead wire connected to the inductive element;
probe means connected to a second lead wire and to be contacted with the second terminal;
signal inputting means for inputting an inspection signal including an AC component into one of the first and second lead wires; and
signal detecting means for detecting an output of the inspection signal at the other of the first and second lead wires.
The arrangement of the inductive element may be variously modified. Thus, according to a second aspect of the present invention, there is provided a continuity inspection apparatus for inspecting electrical continuity between first and second terminals of a pattern wire formed on a board, comprising:
probe means to be directly contacted with the first terminal;
an inductive element connected to the probe means;
a first lead wire connected to the inductive element;
capacitive coupling means connected to a second lead wire and to be capacitively coupled with the second terminal in a non-contact manner to provide a coupling capacitance therebetween;
signal inputting means for inputting an inspection signal including an AC component into one of the first and second lead wires; and
signal detecting means for detecting an output of the inspection signal at the other of the first and second lead wires.
The coupling capacitance may be formed at both the first and second terminals. Thus, according to a third aspect of the present invention, there is provided a continuity inspection apparatus for inspecting electrical continuity between first and second terminals of a pattern wire formed on a board, comprising:
first capacitive coupling means to be capacitively coupled with the first terminal in a non-contact manner to provide a coupling capacitance therebetween;
an inductive element connected to the first capacitive coupling means to form a resonance circuit in conjunction with the capacitance yielded by the first capacitive coupling means;
a first lead wire connected to the inductive element;
second capacitive coupling means connected to a second lead wire and to be capacitively coupled with the second terminal in a non-contact manner to provide coupling capacitance therebetween;
signal inputting means for inputting an inspection signal including an AC component into one of the first and second lead wires; and
signal detecting means for detecting an output of the inspection signal at the other of the first and second lead wire.
The above object of the present invention can also be achieved according to a fourth aspect of the present invention which provides a continuity inspection jig having first and second terminal groups spaced apart each other with leaving a given distance therebetween. The continuity inspection jig comprising:
a lead wire connected to all or part of first ends of the first terminal group so as to apply a continuity inspection signal thereto;
contact sections provided respectively at all or part of second ends of the first terminal group and to be contacted with a board as an inspection object;
one or more inductive elements connected to all or part of the second terminal group; and
electrodes provided respectively at all or part of the second ends of the second terminal group and for forming a coupling capacitance without any contact with a wiring pattern of the board as an inspection object.
The above object can also be achieved according to a fifth aspect of the present invention, which provides a continuity inspection method for inspecting electrical continuity between first and second terminals of a pattern wire formed on a board, comprising the steps of:
positioning a given electrode close to the first terminal to form a coupling capacitance, and connecting a given inductive element to the electrode, followed by connecting a first lead wire to the inductive element and connecting a second lead wire to the second terminal, so as to form a resonance circuit by the first lead wire, inductive element, electrode, coupling capacitance, first terminal, pattern wire, second terminal and second lead wire;
applying an inspection signal including an AC component to one of the first and second lead wires; and
detecting an output of the inspection signal at the other of the first and second lead wires.
In order to achieve the same object, according to a sixth aspect of the present invention, there is provided a continuity inspection method for inspecting electrical continuity between first and second terminals of a pattern wire formed on a board, comprising the steps of:
bringing a first lead wire directly into contact with the first terminal through an inductive element, and capacitively coupling a second lead wire with the second terminal in a non-contact manner to provide a coupling capacitance therebetween, so as to form a resonance circuit by the first lead wire, inductive element, first terminal, pattern wire, second terminal, electrode, coupling capacitance and second lead wire;
applying an inspection signal including an AC component to one of the first and second lead wires; and
detecting an output of the inspection signal at the other of the first and second lead wires.
In order to achieve the same object, according to a seventh aspect of the present invention, there is provided a continuity inspection method for inspecting electrical continuity between first and second terminals of a pattern wire formed on a board, comprising the steps of:
capacitively coupling an inductive element connected to a first lead wire with the first terminal through a first electrode in a non-contact manner, and capacitively coupling a second lead wire with the second terminal through a second electrode in a non-contact manner, so as to form a resonance circuit by the first lead wire, inductive element, first electrode, coupling capacitance, first terminal, pattern wire, second terminal, second electrode, coupling capacitance and second lead wire;
applying an inspection signal including an AC component to one of the first and second lead wires; and
detecting an output of the inspection signal at the other of the first and second lead wires.
Comparing the above construction with a conventional example having only a coupling capacitance, in case of no inductance L, for example, given that the coupling capacitance C is 10 fF and the working frequency is 10 kHz, the output impedance of the circuit is calculated as follows.                               1          /                      (                          2              ⁢              fC                        )                          =                ⁢                  1          /                      (                          2              xc3x97              3.14              xc3x97                              10                3                            xc3x97                              10                                  -                  15                                                      )                                                  =                ⁢                  1.6          ⁢                      xe2x80x83                    ⁢          G          ⁢                      xe2x80x83                    ⁢          Ω                    
Thus, it is almost impossible to measure a resistance of the pattern. Given that the frequency f is 100 MHz, the impedance can be reduced as follows.
1/(2xc3x973.14xc3x97106xc3x9710xe2x88x9215)=159 kxcexa9
However, in view of cost performance, it is impractical to increase the frequency up to such a value. That is, it is extremely important to select an optimum value of frequency.
Thus, in one embodiment of the present invention, the above method may further comprise the step of; previous to the step of applying an inspection signal, determining a resonance frequency for a pattern wire between first and second terminals of a given reference board by applying an inspection signal to the reference board while changing the frequency of the inspection signal, and in the step of applying an inspection signal, applying the inspection signal to one of the first and second lead wires with using said determined resonance frequency as a frequency thereof.
It is necessary to define the allowable changing range of the frequency in advance. Particularly, one embodiment of the present invention may include the step of; in the step of determining a resonance frequency, changing the frequency of the inspection signal for the reference board within a given range having a center frequency defined by a standard frequency determined based on the constant of the inductive element.
If the difference between the reference board and an actual board as an inspection object comes up, an apparent difference can be caused in the detected signal. In order to compensate this error, one embodiment of the present invention may include the step of; in the step of applying an inspecting signal, changing the frequency of the inspection signal for the board as an inspection object within a given range having a center frequency defined by the frequency determined in the step of determining a resonance frequency.