JPH0679052B2 - Circuit board inspection method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit board inspection method for inspecting the quality of a circuit board on which electronic components and the like are mounted, and in particular, the impedance changes relatively greatly depending on the ambient temperature. The present invention relates to an inspection method suitable for a circuit board on which such elements are mounted.

[Conventional example]

Circuit board inspection devices called in-circuit testers have come to be used for inspection of circuit boards on which electronic components and the like are mounted.

An example thereof is shown in FIG. 6. For example, a circuit board to be inspected (hereinafter, referred to as “test board”) 3 which emits a measuring AC voltage of a predetermined frequency from a signal source 1 and passes through an amplifier 2.
In addition, a current that is inversely proportional to the magnitude of the impedance Z _X flows through the substrate 3. This current is taken in and detected by, for example, the current detector 4, converted into a voltage here, and then applied to the measurement unit 5. The measuring section 5 digitally converts the input voltage and uses the converted data to calculate the impedance Z _X of the test board 3 and compares it with a reference value Z _S stored in the memory in advance. In this case, as shown in the following formula, if it is within the predetermined tolerance ± α, it is judged as good, and if it is outside the tolerance, it is judged as bad, and the judgment result is displayed on the display 6 or the like. There is.

In determining Z _S -αZ _X Z _S + α determined Z _X <Z _S -α or Z _X> Z _S + α [Problems to be Solved by the Invention] The conventional circuit board inspection method for "bad" in the "good" is Generally, an AC voltage for measurement is applied from the signal source 1 to the test board 3, the average value of the voltage output from the current detector 4 is measured, and the impedance Z _X is obtained from the data.

However, when the object to be measured is, for example, a diode or the like, it has a temperature characteristic that its impedance changes significantly due to changes in ambient temperature. Therefore, in the conventional inspection method, it is difficult to set an appropriate tolerance for pass / fail judgment for a semiconductor element that is greatly affected by temperature.

The present invention has been made in view of the above circumstances, and its object is to automatically adjust the tolerance considering the change due to the influence of the ambient temperature to the tolerance for the original variation, especially for the low impedance in the forward direction such as the diode. It is to provide a highly accurate circuit board inspection method capable of accurately determining whether the quality is good or bad.

[Means for Solving the Problems]

Referring to FIG. 1, which illustrates an embodiment of the present invention,
In order to solve the above problems, the following means (1) to (2) are provided.

I. For example, for n circuit boards that have been confirmed to be non-defective in advance, add one cycle of measurement AC signal from the signal source 10 for each measurement point, and integrate the response signals for the positive half-wave and the negative half-wave, respectively. Integrator 17,18.

B. For example, the impedance Z _S i (i = 1, 2, ... N) for the positive half-wave and the negative half-wave of the measuring AC signal is obtained by the digital conversion data of the integrated voltage obtained from the integrators 17 and 18.
A reference data memory 21c for measuring the average value Zμ to obtain the average value Zμ thereof and holding this value as the reference impedance Z _{S at the} measurement point.

C. N pieces of impedance data Z _{S at the} above measurement points
i and its average value Zμ (reference impedance Z _S ) are respectively calculated to obtain a mean square value σ, and for example, based on the mean square value σ, a tolerance α ₁ for the variation of the measured impedance Z _X is set and the ambient temperature is set. Tolerance setting means 21a for setting the tolerance α ₂ or α ₃ with respect to the change in the impedance Z _X due to.

D. When the impedance of the test board 13 is measured, if the ambient temperature rises or falls below the ambient temperature during measurement of the good board, for example, the tolerance α ₂ or α ₃ is automatically added to the tolerance α ₁ according to the temperature difference, Comparing means 21b for judging the quality of the test board 13 by performing impedance comparison according to the following equation.

(Judgment Formula) When ambient temperature rises (Zμ−α ₁ ) −α ₂ Z _X (Zμ + α ₁ ) −α ₂ ...
(1a) When the ambient temperature decreases (Zμ-α ₁ ) -α ₃ Z _X (Zμ + α ₁ ) + α ₃ ...
(1b) (α ₂ = α ₃ = 0 when there is no change in ambient temperature) [Operation] By including the above-mentioned means, even if the impedance measurement value of the test board includes an amount that changes with temperature, The quality is judged correctly.

〔Example〕

According to FIG. 1, the signal source 10 has a frequency of 1 k
An oscillator that outputs a sinusoidal AC voltage for one cycle per Hz at one measurement item is provided.
An alternating voltage for measurement, which is emitted from the device, is applied to the test board 13 via the current amplifier 12.

As a result, a current that is inversely proportional to the magnitude of the impedance Z _X flows through the substrate 13, is taken into, for example, the current detector 14, is detected, and is then converted into a voltage. In this embodiment, the current detector 14 is provided with a range setting circuit 14a for switching the current / voltage conversion magnification, and the switching operation of the range setting circuit 14a is performed by a control signal from the CPU 21, for example. There is. Ie CP
The U21 monitors the magnitude of the digital conversion data of the A / D converters 19 and 20 so that the converted analog voltage applied to the A / D converters from the integrators 17 and 18 respectively falls within a predetermined level range. Above range setting circuit 14
Switch a. As a result, in the current detector 14, the detected current is converted into a voltage at the specified magnification of the range setting circuit 14a, and for example, the full-wave rectification type absolute value circuit 15 provided in the next stage has one of positive or negative polarity. After being set to the pulsating current voltage, the switch 16 driven by the switching controller 11 is applied.

In this embodiment, the signal source 10 is composed of, for example, a D / A converter and a voltage amplifier (not shown).
The quantized sine waveform data of 1kHz, 1kHz from 0 ° to 360 ° emitted from PU21 is converted into an analog voltage and output as an AC constant voltage for measurement. The switching controller 11 includes, for example, a zero-cross comparator and a flip-flop (not shown), and the signal source 10
The switch 16 is driven to the contact A side and the contact B side, respectively, in synchronization with the positive half cycle and the negative half cycle of the AC voltage generated from the switch.

Therefore, for example, the switch 16 is driven to the contact A side during the half cycle period from 0 to π of the measuring AC voltage, and from π to 2
Assuming that the half cycle period up to π is driven to the contact B side, of the pulsating voltage output from the absolute value circuit 15, the voltage of the previous half cycle period is applied to the integrator 17, for example,
The voltage in the latter half cycle period will be applied to the integrator 18 and integrated respectively. These integrated voltages are, for example,
Digitally converted by A / D converters 19 and 20 and sent to CPU 21. The CPU 21 calculates the impedance Z _X of the test board 13 in the positive half-wave and the negative half-wave of the measuring AC voltage based on these integrated voltage data, respectively, and calculates the corresponding reference value Z _S according to the above judgment formula (1). The quality and the judgment result are compared with the result and the measurement data is sent to the recording / display unit 22, for example.

Next, a supplementary explanation of the above operation will be given with reference to FIG. In addition, in FIG. 1, (a) to (h) in FIG.
The same reference numerals are attached to the portions where the operation of (1) is performed. In this circuit board inspection method, as described above, first, the circuit board confirmed as a good product is measured and the data is stored in the memory, and then the test board is measured under the same conditions as the non-defective product board and both data are taken. Are to be compared.

That is, for example, as shown in FIG.
When an AC voltage for measurement of 10 cycles is generated from 10 cycles, a current as shown in (b) of the same figure is inversely proportional to the magnitude of the impedance Z _S of the non-defective substrate. This current is detected by the current detector 14 and then converted into a voltage, which is converted into, for example, a negative pulsating current voltage by the absolute value circuit 15 as shown in FIG.

The switch 16 is driven by the output of the switching controller 11 to the contact A side during the positive half-wave period of 0 to π, and the contact B during the negative half-wave period of π to 2π, as shown in FIG. It is designed to be driven to the side. Therefore, the voltage between 0 and π applied from the absolute value circuit 15 is integrated by the integrator 17 as shown in (e) in the figure, and the voltage between π and 2π is integrator as shown in (f). It is integrated at 18. Assuming that the integrated voltages of these non-defective substrates are V _SA and V _SB , these two integrated voltages are digitally converted by A / D converters 19 and 20, respectively, as shown in (G) and (H), and sent to the CPU 21. To be The CPU21 calculates the reference impedances Zμ _A and Zμ _B of the non-defective board for the positive half-wave and the negative half-wave of the measuring AC voltage based on the integration and voltage data, and the tolerance α _1A for the variation in the impedance of the test board. , Α _{1B is obtained} by the tolerance setting means 21a, for example.

Thus, the reference impedances Zμ _A (Z _SA ), Zu _B (Z _SB ), and the tolerances α _1A , α at each measurement point on the non-defective substrate
_{When 1B} is taken by the tolerance setting means 21a, for example, the impedances Z _XA and Z _{XB of} the test board 13 are measured by the same method as described above, and the data are compared by the equation (1).

In this case, the tolerance α ₁ for the variation of the impedance in the equation (1) and the tolerance α ₂ or α ₃ to be added to the impedance change due to the change of the ambient temperature are:
For example, the following equation α ₁ = k ₁ σ + k ₂ Zμ [Ω] (2) α ₂ (α ₃ ) = Zμk ₃ ΔT [Ω] (3) is used. In the above equation, k ₁ , k ₂ , k ₃
Is a constant and ΔT is a variable relating to the difference in ambient temperature.

First, the tolerance α ₁ will be described with reference to FIG. 3. Zμ on the horizontal axis is, for example, the average value of the impedances Z _s i measured at the same measurement points of n non-defective substrates,
That is, it represents the reference impedance Z _S at that measurement point, and both sides of it represent the deviation from the average value. The vertical axis shows the number of test substrates, that is, the number of measurement data, as a standard.

In the figure, the average value Zμ is When n is set to an appropriate value, the distribution state of the number of substrates for each deviation value is approximated to a normal distribution symmetrical with respect to the average value Zμ as shown by the solid line. Therefore,
The root mean square σ From the above, 99% or more of the number of test substrates falls within the range of ± 3σ, and the number outside this range is 1% or less as well known.

In this case, since all non-defective products are used as the test substrates, it is necessary to determine the number of 1% or less deviating from 3σ as a non-defective product. Therefore, in this embodiment, considering measurement error and the like, for example, k ₁ of the equation (2) is set to 3 and k _{2 is set} to 2%, and α ₁ = 3σ + 0.02Zμ [Ω] (2a) or α ₁ = It is set to (3σ / Zμ + 0.02) × 100 [%] (2b).

Here, for example, the impedance distribution of the test board is the third
Assuming that it is as shown by the dotted line in the figure, by applying this tolerance α ₁ , it is possible to inspect the quality of the variation with sufficient accuracy.

Next, the tolerance α on the temperature characteristic of impedance
₂ (α ₃ ) will be described with reference to FIG.
When a non-defective substrate and a test substrate are measured at the same ambient degree T, as shown in FIG. 3A, the distribution of the number of data corresponding to the center value Zμ of impedance and each deviation value is the same as that shown in FIG. It will be similar.

This test board is, for example, an ambient temperature T + ΔT,
Assuming that the distributions have center values Z _X ′ and Z _X ″ when measured by T−ΔT, in this embodiment, the tolerance α ₁ to the original variation shown in FIG. 3 is obtained. The deviation −α ₂ or + α ₃ from the center value Zμ is automatically added so that a good product is not erroneously determined to be defective even if the ambient temperature changes and the impedance changes.

This tolerance is calculated as follows. Fig. 4 (B)
Shows the voltage-current characteristics of the diode, the current flowing when the forward voltage V _F is applied to the diode is I _F
Then, this current _IF is generally expressed by the following equation.

I _F = I _S {exp (qV _F / kT) -1} (4) where, I _S : Saturation current when reverse voltage is applied [A] q: Electricity of electron 1.6 × 10 ^{− 19} [C] k: Boltzmann's constant 1.38 × 10 ⁻²³ [J / K] T: ambient temperature [K].

As is clear from equation (4), this current _IF is the applied voltage
When V _F is constant, it changes exponentially with the ambient temperature T as a variable, and an example at around room temperature is shown in FIG. 4 (C). In the figure, the ambient temperature T is T ₁ , T ₂ , T ₃ (T ₁ >
This is an example of voltage-current characteristics when T ₂ > T ₃ ). It has a negative temperature coefficient of about 2.0 to 2.5 mV per 1K change in ambient temperature, and the horizontal axis shows an interval proportional to the temperature difference. It is known to move almost parallel along it. It is known that the saturation current I _S of the equation (4) also changes depending on the temperature T, and that the current almost doubles when the temperature rises by 10 ° C. Therefore, if the change in current is replaced with the change in impedance, the relationship between ambient temperature and impedance can be obtained.

Therefore, assuming that the operating temperature range of this device is, for example, 5 to 40 ° C., the saturation current I _S becomes twice (or 1/2) with respect to the change of the ambient temperature of 10 ° C. as described above. For example, it can be approximated as I _Sn = I _{S (n-1)} × 2 ^{(Tn-Tn-1) / 10} (5) (n = 1,2,3, ...).

Here, if the standard ambient temperature at the time of measuring a non-defective substrate is set to 23 ° C (300K) for simplification, the saturation current I at this time is
_{For S (23)} , the saturation current I _{S (40) at the} maximum operating temperature of 40 ° C (313K) is I _{S (40)} = I _{S (23)} × 2 ^{(40-23) / 10} ≈ I _{S (23 )} × 3.3 ……… (6) Saturation current I _{S (5) at the} lower limit of operating temperature 5 ° C (278K ₎ is I _{S (5)} = I _{S (23)} × 2 ^{(5-23) / 10} ≈ I _{S (23)} × 3.5 ……… (7).

Therefore, if the forward current ratio of the upper limit temperature of 40 ° C to the standard temperature of 23 ° C is ΔI _F1 , then ΔI _F1 = I _{F (40)} / I _{F (23)} , so the measurement voltage V _F is, for example, 0.35 V By substituting equation (6) into equation (4), ΔI _F1 = I _{F (40)} {exp (qV _F / 313k) -1} / I _{S (23)} {exp (qV _F / 300k) -1} ≈1.8 (8) Also, the current ratio of the lower limit temperature of 5 ° C to the standard temperature of 23 ° C is Δ.
If I _F2 , then ΔI _F2 = I _{F (5)} / I _{F (23)} , so formula (7) is substituted into formula (4), and ΔI _F2 = I _{F (5)} {exp (qV _F / 278k) -1} / I obtain _{S (23) {exp (qV} F / 300k) -1} ≒ 0.78 ......... (9).

Assuming that the impedances at the standard temperature, the upper limit temperature, and the lower limit temperature are Z ₍₂₃₎ , Z ₍₄₀₎ , and Z ₍₅₎ , respectively, the impedance ratio of the upper and lower limit temperatures with respect to the standard temperature is given by equation (8),
Since it is the reciprocal of the value obtained in (9), Z ₍₄₀₎ / Z ₍₂₃₎ = 1 / 1.8 ≈ 0.56 / 1 ……… (10) Z ₍₅₎ / Z ₍₂₃₎ = 1 / 0.78 ≈1.3 / 1 ………… (11) By the way, the impedance ratio between the lower limit temperature and the upper limit temperature is Z ₍₅₎ / Z ₍₄₀₎ ≈2.3 / 1 ………… (12).

Although the increase / decrease in impedance due to the change in ambient temperature changes in a curve substantially along an exponential function as described above, there is no particular problem even if this is approximated to a straight line in practice.

Therefore, k ₃ = (0.56-1) the added value k _3, for example per 1 ℃ the direction in which the ambient temperature increases from normal temperature in this embodiment / (40-23) ≒ -2.6 [% ] = -Zμ × 0.026 [Ω], and when there is a difference of ΔT in the ambient temperature, α ₂ = k ₃ × ΔT = −Zμ × 0.026 × ΔT [Ω] ……… (1
3a) Alternatively, α ₂ = −2.6 × ΔT [%] (13b) is added.

For example, the direction in which the ambient temperature drops below the standard temperature is 1
The addition value k ₃ per _{℃ k 3 = (1.3-1) /} (23-5) ≒ 1.7 [%] = Zμ × 0.017 [Ω] ......... and (14), there is a temperature difference of similarly ΔT In this case, α ₃ = k ₃ × ΔT = Zμ × 0.017 × ΔT [Ω] ……… (15
a) Alternatively, α ₃ = 1.7 × ΔT [%] ... (15b) is added.

If the value of 3σ representing the impedance variation is relatively large and the added value due to temperature change does not need to be set very finely, the value of equation (12) can be used to uniformly set k per 1 ° C, for example. ₃ = (2.3-1) / (40-5) ≈ 4 [%] = Zμ × 0.04 [Ω] ……… (16) and the tolerance α ₂ or α ₃ is α ₂ = −Zμ × 0.04 × ΔT [Ω] = -4 × ΔT [ %] ......... (17a) α 3 = -Zμ × 0.04 × ΔT [Ω] = 4 × ΔT [%] may be added as ......... (17b) . In this case, the standard temperature for measuring the non-defective substrate may be any number within the operating temperature range, which is practically convenient.

The above values of k ₁ , k ₂ , k ₃ , α ₁ , α ₂ , α ₃ etc. are merely examples, and the user side of the apparatus may appropriately determine them according to the actual condition of the board inspection. Of course. For example above distribution characteristics of impedance changes is ambient temperature (variations in spreading width) alpha in the case where no change _2, alpha ₃
However, since it may actually change, it may be necessary to adjust the values of α ₂ and α ₃ depending on the degree. Also, as one of the application examples, by setting the allowable lower limit value and the allowable upper limit value (P, Q in Fig. 4 (A)) as appropriate, it is possible to detect shorts, opens, etc. of mounted parts due to temperature changes. Is.

It should be noted that ΔT is the difference in ambient temperature between the inspection of the non-defective board and the inspection of the test board as described above. In this embodiment, for example, the temperature detector 23 detects it and inputs data to the measurement unit 21. However, the value read from a room thermometer or the like may be manually input.

Incidentally, FIG. 5 is a flow chart showing an example of the case where the automatic setting of the tolerance is controlled by the CPU 21.

〔effect〕

As described above in detail, in the present invention, the impedance at each measurement point of the circuit board which is confirmed to be a non-defective product is measured, and the average value Zμ is set as the reference impedance Z _{S of the} measurement value, and the average value Zμ. 3 times the root mean square value σ of the above impedance measurement data, that is, 3
σ is calculated, and the value ± α ₁ = 3σ + Zμ × 2% obtained by adding 2% of the average value Zμ to this 3σ is set as the tolerance for the variation of the test substrate.

In addition, the difference ΔT between the ambient temperature at the time of inspecting the non-defective substrate and at the time of inspecting the test substrate is detected, and this variable ΔT and the operating ambient temperature T are detected.
A known constant k ₃ related to the above, and the product of the impedance mean value Zμ at the measurement point and −α ₂ (or + α ₃ ).
= Zμ × k ₃ ΔT is automatically added to α ₁ as the impedance change of the diode due to the change in ambient temperature.

Therefore, according to this circuit board inspection method, accurate pass / fail judgment can be automatically performed including not only the original variation in impedance of the test board but also the change in impedance due to the ambient temperature difference.

[Brief description of drawings]

1 to 5 relate to an embodiment of the present invention, FIG. 1 is a block diagram showing an example of a configuration of a circuit board inspection apparatus to which the present invention is applied, and FIG. 2 is an operation explanatory diagram of each part, Third
The figure is an explanatory view of the setting principle of the acceptability difference α ₁ for acceptability judgment, FIGS. 4 (A) to 4 (C) are explanatory views of the setting principle of the addition tolerance α ₂ (α ₃ ) with respect to temperature change, and FIG. FIG. 6 is a flowchart showing an example of the case where the tolerances α ₁ , α ₂ (α ₃ ) are automatically set by the CPU, and FIG. 6 is a block diagram showing the configuration of the conventional device. In the figure, 10 is a signal source, 13 is a circuit board to be inspected, 14 is a current detector, 14a is a range setting circuit, 17 and 18 are integrators, and 19 and 20 are A / D.
The converter, 21 is a CPU.

Claims (1)

[Claims] 1. A response signal obtained by applying a measuring AC signal from a signal source to a circuit board to be inspected is converted into a voltage in a predetermined range and integrated, and CP is converted by the digital conversion data.
A circuit that measures the impedance at a predetermined measurement point on the board at U, compares the measured value with a reference value that was previously taken from a non-defective board, and determines whether the board is good or bad depending on whether it is within a predetermined tolerance. In the board inspection method, a tolerance is set for the variation of the board impedance, which is a predetermined value added to 3σ of the impedance distribution characteristic of the non-defective board, and the impedance measurement is performed against the change of the board impedance with temperature. A circuit board inspection method, characterized in that a tolerance according to an ambient temperature difference between the time and the impedance measurement of the non-defective board is automatically added to the above-mentioned tolerance to make a pass / fail judgment.