JP2015007552A - Method for inspecting printed circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To specify a disconnected spot when whichever of a through-hole and an electronic component connection part provided within a range of position detection resolutions of TDR measurement is disconnected.SOLUTION: In a method for inspecting a printed circuit board 20 according to one embodiment, when either a through-hole 22 or a connection part 11 of an electronic component 10 close to each other is determined to be disconnected by TDR measurement, a conductive substance 61 is injected into the through-hole 22. Then, after the conductive substance 61 is injected, a disconnected portion of the printed circuit board 20 is measured again by a TDR method, and determination is made on the basis of the result of re-measurement as to which of the through-hole 22 and the connection part 11 of the electronic component 10 is disconnected.

Description

  The present invention relates to an inspection method for electrically inspecting a defective portion of a printed circuit board.

  Currently, printed circuit boards are indispensable in the electronics field. Usually, a through hole having a plated surface is formed on a printed circuit board in order to connect multiple wiring patterns. If the through hole continues to be subjected to thermal shock or the like, it may be affected by thermal stress, resulting in disconnection and failure. Similarly, the solder connection location of the electronic component is also easily affected by thermal stress, and may be disconnected and defective.

  ICT (In-Circuit Tester) such as boundary scan and TDR method (Time Domain Reflectometry) are generally known as methods for electrically inspecting such defective parts of printed circuit boards. ing. The inspection method using the TDR method does not need to incorporate a detection circuit in advance compared to ICT, and further has an advantage that a disconnection position can be specified (for example, JP 2010-164427 A). Patent Document 1) and Japanese Patent Laid-Open No. 9-061486 (Patent Document 2).

JP 2010-164427 A Japanese Patent Laid-Open No. 9-061486

  The position detection by the TDR measurement has an error of about 1 to 2 mm, and this error range is the position detection resolution. Therefore, when there are a plurality of disconnection points within the resolution range, it is difficult to specify where the disconnection is made by the conventional TDR method.

  For example, when a BGA (Ball Grid Array) package is mounted so as to cover the through hole, it is difficult to identify whether the through hole is broken or the BGA is broken as long as the conventional TDR method is used. In this case, since the disconnection part is covered with the BGA package, the disconnection part cannot be specified visually or by a resistance measuring instrument (electric tester) unless the BGA package is removed.

  The present invention has been made in consideration of the above-mentioned problems, and the object thereof is to select one of a through hole provided within the position detection resolution range of TDR measurement and a connection part of an electronic component. It is an object of the present invention to provide a printed circuit board inspection method capable of easily specifying which one is disconnected when there is a possibility of disconnection.

  In the printed circuit board inspection method according to the embodiment, when it is determined by TDR measurement that one of the through hole and the connection part of the electronic component that are close to each other is disconnected, the conductive material in the through hole Is injected. Then, after the conductive material is injected, the disconnection portion of the printed circuit board is measured again by the TDR method, and it is determined which of the through hole and the connection portion of the electronic component is disconnected based on the remeasurement result.

  According to the above-described embodiment, by applying the TDR measurement, it is determined which of the through hole provided in the range of the position detection resolution by the TDR measurement and the connection part of the electronic component is broken. It becomes detectable.

It is a block diagram which shows the whole structure of the test | inspection apparatus used with the test method of the printed circuit board by Embodiment 1. FIG. It is a figure which shows schematic structure of the interface part of the printed circuit board inspection apparatus of FIG. 1, and the printed circuit board to be examined. FIG. 3 is an enlarged view of a part of the printed circuit board of FIG. 2. It is a figure for demonstrating the injection | pouring method of the electroconductive substance to a through hole. It is a figure for demonstrating the case where an electroconductive substance is excessively injected into a through hole. It is a figure for demonstrating the method to inject | pour a conductive gel into a through hole. 3 is a flowchart illustrating a printed circuit board inspection procedure according to the first embodiment. It is the figure which represented typically the equivalent circuit of a printed circuit board. It is a figure for demonstrating the relationship between the output voltage of a TDR measuring device, and a detection voltage. It is an equivalent circuit diagram of the impedance of the device under test. It is a figure which shows the example of the synthetic | combination waveform synthesize | combined from the measurement waveform of TDR in the case of a good printed circuit board, and the TDR measurement waveform at the time of the complete disconnection of a through hole. 6 is a flowchart illustrating a printed circuit board inspection procedure according to the second embodiment.

  Hereinafter, each embodiment will be described in detail with reference to the drawings. The same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

<Embodiment 1>
[Overall configuration of printed circuit board inspection equipment]
FIG. 1 is a block diagram showing the overall configuration of an inspection apparatus used in the printed circuit board inspection method according to the first embodiment.

  Referring to FIG. 1, a printed circuit board inspection apparatus 1 inspects an electrical abnormality of a printed circuit board, in particular, a disconnection of a solder connection part of an electronic component and a disconnection of a through hole. The printed circuit board inspection apparatus 1 includes a TDR measuring device 40, an interface unit 30, and a computer 50 as a control unit.

  The TDR measuring device 40 includes a signal generator 41 that outputs a step voltage (pulse wave) having a steep rise, and an oscilloscope 46 that measures a reflected waveform of a rising portion of the step voltage (pulse wave) output from the signal generator 41. Including. The signal generator 41 and the oscilloscope 46 are connected to the connector 35 of the interface unit 30 by a high frequency cable 37. The step voltage (pulse wave) output from the signal generator 41 is applied to the printed circuit board (reference numeral 20 in FIG. 2) via the interface unit 30 and the probe 32. The oscilloscope 46 detects a reflected wave generated by the step voltage (pulse wave) being reflected by the printed circuit board 20 via the probe 32 and the interface unit 30. The reflected wave detected by the oscilloscope 46 is digitally converted and input to the computer 50.

  FIG. 2 is a diagram illustrating a schematic configuration of the interface unit 30 of the printed circuit board inspection apparatus 1 of FIG. 1 and the printed circuit board 20 to be inspected. In FIG. 2, the cross-sectional portion is hatched.

  Referring to FIG. 2, printed circuit board 20 is obtained by mounting a large number of electronic components on insulating substrate 21 (that is, a printed wiring board) on which a wiring pattern is formed. A through hole 22 whose surface is plated is provided in the middle of the wiring pattern. The multilayer wiring patterns are electrically connected by the through holes.

  FIG. 2 further shows a state in which an IC (Integrated Circuit) package 10 (also referred to as a BGA package 10) is mounted in the vicinity of the through hole 22. The IC package 10 is solder-connected to the wiring pattern by solder balls 11 formed in a lattice pattern on the back surface of the package. Such a mounting method is referred to as BGA (Ball Grid Allay). In BGA, since the solder connection portion is on the back surface of the package, it is difficult to visually inspect the solder connection state. Further, in the case of FIG. 2, since the through hole 22 is covered with the IC package 10, it is difficult to inspect whether the through hole 22 is broken by visual inspection or an electric tester.

  The interface unit 30 includes an insulating substrate 33, a wiring pattern 34 formed on the substrate 33, a plurality of probes 32 for contacting the wiring pattern near the inspection site of the printed circuit board 20, the through holes 22, and the like. And an insulating block (not shown) in which a plurality of probes 32 are embedded. A plurality of probes 32 are provided in order to inspect a plurality of inspection parts of the printed circuit board 20 at a time, but a configuration in which one probe is sequentially brought into contact with a plurality of locations to perform inspection is also possible. One of the plurality of probes 32 may be used to connect to the ground of the printed circuit board 20.

  Referring to FIG. 1 again, the interface unit 30 further includes a plurality of switches 36 such as relays. Each switch 36 corresponds to each probe 32 individually, and is provided in the path of the wiring pattern 34 that connects each probe 32 and the connector 35. The computer 50 sequentially switches the probes 32 connected to the TDR measuring device 40 by switching the connection of the switch 36. In this way, the printed circuit board inspection apparatus 1 sequentially inspects a plurality of inspection locations of the IC package 10 of FIG.

  Next, the configuration of the computer 50 will be described. The computer 50 includes a central processing unit (CPU), a memory circuit, an interface circuit for signal input / output, and the like. Viewed functionally, the computer 50 includes a measurement control unit 53, a storage unit 52, and a determination unit 51. These functions are realized by the computer 50 executing a printed circuit board inspection program.

  The measurement control unit 53 controls the timing at which the signal generator 41 outputs a step voltage (pulse wave). Furthermore, the measurement control unit 53 selectively connects the examination site and the TDR measuring device 40 by switching the switch 36 of the interface unit 30. Thus, the pulse wave output from the signal generator 41 sequentially passes through the wiring pattern 34 and the probe 32 of the interface unit 30 and reaches the inspection site of the printed circuit board. The reflected wave from the printed board returns to the signal generator 41 along the reverse path and is detected by the oscilloscope 46.

  The storage unit 52 stores a voltage waveform of a reflected wave measured in advance using a normal reference printed circuit board as a reference waveform. The reference waveform is preferably an average value of reflected waveforms measured on a number of reference printed boards. Further, the storage unit 52 preferably stores a waveform obtained by converting the reference waveform into an impedance when the printed circuit board side is viewed from the signal generator 41, and a differential waveform of those waveforms.

  The determination unit 51 determines whether or not a disconnection has occurred in the inspection printed circuit board by comparing the TDR measurement result for the inspection printed circuit board with the TDR measurement result for the reference printed circuit board. When a disconnection occurs at a certain inspection part on the printed circuit board for inspection and the impedance of the inspection part is increased, the voltage of the reflected wave detected after the time when reflection from the inspection part is detected is Increased compared to the case of the reference printed circuit board.

  The reflected waveform measured by TDR is converted into an impedance waveform when the printed circuit board side to be inspected is viewed from the signal generator 41, and the converted impedance waveform is compared with the impedance waveform of the reference printed circuit board for inspection. It is also possible to detect disconnection of the printed circuit board.

Specifically, if the output voltage (step voltage value) of the signal generator 41 is Va, the output impedance of the signal generator 41 is Z0, and the measured reflected voltage is Vb (t) as a function of time t, then printing is performed. The impedance Z1 (t) viewed from the substrate side is
Z1 (t) = Vb (t) × Z0 / (Va−Vb (t)) (1)
It is represented by The output impedance Z0 of the signal generator 41 is set equal to the characteristic impedance of the high frequency cable 37.

  In the above case, after the time when the reflected wave from a certain inspection site is detected, if the impedance on the printed circuit board for inspection is larger than the impedance on the printed circuit board for reference, the inspection site is disconnected. Can be determined. In this case, for example, the difference between the impedance value detected on the inspection printed circuit board and the reference impedance value is 3σ (σ is a standard deviation) of measured values measured for a large number of reference printed circuit boards. When this reference value is exceeded, it is desirable to determine that the wire is disconnected.

  The printed circuit board for inspection and the printed circuit board for reference may be compared with a differential waveform of a reflected wave or an impedance waveform by TDR measurement. When a certain inspection site is disconnected, the voltage waveform or impedance waveform increases as a whole as compared with the case of the reference printed circuit board after the time when reflection from the inspection site is detected. On the other hand, in the case of a differential waveform, a difference occurs between the inspection printed circuit board and the reference printed circuit board only near the time when reflection from the inspection region is detected, so it is easy to determine the position of the disconnected region. become. In particular, in the case of poor probe contact, an abnormality occurs in the differential waveform only near the time when reflection from the probe is detected, so that it is easy to distinguish and detect probe contact failure and printed circuit board disconnection.

  By using the differential waveform, it is possible to detect a failure other than a disconnection such as a short-circuit failure separately from the disconnection failure. When a short-circuit failure has occurred in the printed circuit board, the incident step voltage (pulse wave) propagates through the short-circuit path, and the reflected waveform is combined with a normal reflected waveform and measured with a TDR measuring instrument become. For this reason, the differential waveform after the time when the short-circuit portion is detected is completely different between the inspection printed circuit board and the reference printed circuit board. For example, since the time when the extreme value is taken becomes completely different, a failure due to disconnection and a short-circuit failure can be easily identified.

[How to determine whether a through hole is broken]
FIG. 3 is an enlarged view of a part of the printed circuit board 20 of FIG. In FIG. 3, the cross-sectional portion is hatched.

  Referring to FIG. 3, many of the causes of disconnection on a printed circuit board are solder connection portions and through holes in the printed circuit board that are susceptible to fatigue due to thermal stress. For example, FIG. 3 shows a case where a disconnection 23 occurs in the plated portion of the inner wall surface of the through hole 22 and a case where a disconnection 12 occurs in the solder connection portion (solder ball) 11 of the BGA package 10. .

  Here, in recent BGA packages, the number of narrow pitch pitches with an interval between solder balls of about 0.6 mm is increasing. Therefore, the distance between the through hole 22 and the solder ball 11 tends to be short, and the distance is about 1 mm.

  On the other hand, the position detection by TDR has an error of about 1 mm. That is, about 1 mm is the resolution (position detection accuracy) of TDR measurement. Therefore, when there is a possibility that two places are broken within about 1 mm, it is difficult to determine the broken place. In the example of FIG. 3, when one of the through hole 22 and the solder connection portion 11 of the BGA package 10 is broken, it is difficult to distinguish which one is broken by TDR measurement. .

  In particular, in the case of FIG. 3, since the IC package 10 is mounted on the upper part of the through hole 22, it is difficult to visually check the disconnection point, and an electric tester probe is attached to both ends of the through hole 22. In addition, it is difficult to detect the presence or absence of disconnection by measuring the resistance value of the through hole 22. Such a problem is a problem common to many printed circuit boards on which a BGA package is mounted.

  In the printed circuit board inspection method of the present embodiment, in order to determine whether the disconnection point is the through hole 22 or the solder connection part (solder ball) 11, liquid, gel (gel), sol, paste, etc. A conductive material having a shape is injected into the through hole 22. As a result, when the through hole 22 is disconnected, the through hole 22 temporarily becomes conductive. Then, TDR measurement is again performed on the printed circuit board for inspection after the injection of the conductive material, and if the impedance of the inspection site is lower than before the injection of the conductive material, it can be determined that the through hole 22 is disconnected, If there is no change in impedance, it can be determined that the solder connection portion (solder ball) 11 is disconnected (or, although the possibility is small, both are disconnected).

  Since the conductive material is in the form of liquid, gel (gel), sol, or paste, the conductive material can be removed after TDR measurement. That is, the printed circuit board inspection method of the present embodiment is a non-destructive inspection method that preserves a defective part. Therefore, physical analysis such as cross-sectional analysis is possible after removing the conductive material.

[Method of injecting conductive material]
FIG. 4 is a diagram for explaining a method of injecting the conductive material 61 into the through hole 22. FIG. 4 shows an example in which the fluidity of the conductive material is high (viscosity is low). Examples include ionic liquids and conductive silver pastes.

  As shown in FIG. 4, a liquid conductive substance 61 is injected into the through hole 22 from the surface of the printed board 20 where the BGA package 10 is not mounted by the fine needle syringe 60. The conductive state (that is, the resistance value of the through hole 22) changes depending on the degree to which the conductive material 61 affects the disconnected portion.

  Since a very small amount of the conductive material 61 is injected, the TDR measurement result is influenced by how the conductive material 61 is filled in the through hole 22. Therefore, it is preferable to inject the conductive material 61 from a small amount, gradually increase the amount, and determine the maximum injection amount in advance. This is to prevent the conductive material 61 from leaking from the through hole 22.

  The maximum injection amount cannot be determined uniformly because it differs depending on the thickness of the printed circuit board to be inspected, the diameter of the through hole, and the manufacturing method of the through hole. It is desirable to measure in advance the amount that the conductive liquid does not leak from the through hole by using the through hole at the same location on the unmounted printed board. Sometimes the injection volume is very small. For example, when the through-hole radius is 0.3 mm and the length is 1 mm, the injection amount is 0.00028 cc.

  FIG. 5 is a diagram for explaining a case where the conductive material 61 is excessively injected into the through hole 22A.

  Referring to FIG. 5, when liquid conductive material 61 is excessively injected into through hole 22A, conductive material 61 leaks from through hole 22A, and other circuits (for example, adjacent through holes 22B). ) May occur. Also in this case, since the impedance of the examination site may be reduced, if it is simply determined that the through-hole is disconnected because the impedance of the examination site is lower than before the injection of the conductive material 61, there is a possibility of erroneous determination. is there. Therefore, it is necessary to determine the criteria for determining the disconnection location in more detail.

  Specifically, in the case of the liquid conductive material 61, it is desirable to inject the conductive material 61 into the through hole 22 little by little and perform TDR measurement each time it is injected. It is observed that the TDR measurement waveform on the reference printed circuit board approaches the TDR measurement waveform on the reference printed circuit board by improving the disconnection state of the through hole 22 with the injection amount of the conductive material 61. If it is, it can be determined that the through hole 22 is disconnected.

  However, the resistance value of the broken through-hole 22 hardly coincides with the resistance value of the through-hole 22 in a non-defective reference printed circuit board, and a slight resistance component often remains. Therefore, it is possible to determine whether or not the through hole 22 is disconnected depending on whether or not the TDR measurement result of the printed circuit board for inspection after injecting the conductive material matches the TDR measurement result of the printed circuit board for reference. Note that you can't.

  More preferably, the reference printed circuit board and the inspection printed circuit board after injecting the conductive material are compared using a waveform obtained by differentiating the reflected wave of the step voltage or a waveform obtained by differentiating the impedance waveform. As a result of excessively injecting the conductive material 61 into the through hole 22, when a short circuit occurs with another circuit, the differential waveform is completely different (for example, the time at which the extreme value is given is completely different), so This is because it can be easily determined whether or not the conductive material 61 is excessively injected.

  FIG. 6 is a view for explaining a method of injecting the conductive gel 62 into the through hole 22. 6A shows a state before the conductive gel 62 is injected into the through hole 22, and FIG. 6B shows a state after the conductive gel 62 is injected into the through hole 22. As shown in FIGS. 6A and 6B, the conductive gel 62 is processed into a cylindrical shape in accordance with the shape of the through hole 22 and is pushed into the through hole 22.

  As the conductive gel 62, for example, the same material as that marketed as a medical electrode such as for electrocardiogram measurement can be used. If the conductive gel 62 is used, the possibility of leakage from the through hole 22 after injection into the through hole 22 is low.

  The diameter of the conductive gel 62 is desirably formed larger than the inner diameter of the through hole 22. For example, the diameter of the conductive gel 62 is set to the inner diameter of the through hole plus 10 μm. As a result, the conductive gel 62 is inserted while being elongated and deformed while being in close contact with the inner wall surface of the through hole 22, so that the disconnected portion 23 of the through hole 22 can be more reliably conducted. In order to facilitate insertion into the through hole 22, it is also desirable to process the tip of the conductive gel 62 into a conical shape.

  Usually, the through hole has an inner diameter of about 1 mm to 0.3 mm and is manufactured in increments of 0.1 mm. The length of the through hole (that is, the thickness of the printed circuit board) is often 1 mm, 1.2 mm, 1.6 mm, and 2 mm. Therefore, if a conductive gel processed product is prepared in advance with its size, it is applicable in most cases.

[PC board inspection procedure]
FIG. 7 is a flowchart showing a printed circuit board inspection procedure according to the first embodiment. Hereinafter, with reference to FIG. 1, FIG. 2, and FIG.

  First, TDR measurement is performed in advance on a number of non-defective reference printed circuit boards by the TDR measuring device 40, and the average value of the measured reflected wave waveforms is stored in the storage unit 52 as a reflected waveform for reference. (Step S100). The storage unit 52 also stores an impedance waveform corresponding to the reference reflected waveform and a differential waveform thereof.

  Next, TDR measurement is performed on the printed circuit board for inspection by the TDR measuring device 40 (step S110). The computer 50 converts the voltage waveform of the measured reflected wave into an impedance waveform, and further generates a differential waveform thereof (step S115). The conversion to the impedance waveform follows the aforementioned equation (1).

  Next, the determination unit 51 of the computer 50 determines the presence or absence of disconnection of the inspection printed circuit board based on the comparison between the TDR measurement result on the reference printed circuit board and the TRD measurement result on the inspection printed circuit board. Specifically, until the time when reflection from a certain inspection site is detected, the determination unit 51 does not make the difference between the impedance of the reference printed circuit board and the impedance of the printed circuit board for inspection within the reference value Vr1, and after that time. If the impedance of the printed circuit board for inspection increases beyond the reference value Vr1, it is determined that the inspection site is disconnected (step S120).

  As a result of the above determination, a disconnection has occurred in one of the through hole 22 and the solder connection portion 11 of the electronic component (IC package 10) adjacent to the through hole 22, but it is difficult to determine the disconnection. (Yes in step S125). In this case, a step of injecting a conductive material into the through hole 22 is executed (step S130). Thereafter, the TDR measurement device 40 performs TDR measurement on the printed circuit board for inspection after injecting the conductive material (step S135). In this case, the contact position of the probe is the through hole 22 or a wiring pattern in the vicinity thereof. Next, the computer 50 converts the measured voltage waveform of the reflected wave into an impedance waveform, and further generates a differential waveform of the reflected wave and / or a differential waveform of the impedance (step S140).

  Next, the determination unit 51 of the computer 50 determines the differential waveform or impedance of the reflected wave based on the generated differential waveform or differential waveform of the reflected wave on the inspection printed board and the TDR measurement result on the reference printed board. The differential waveform is compared (step S145). When the time when the reflected waveform or the differential waveform of the impedance takes an extreme value after the time when the reflection from the through hole 22 is detected, the determination unit 51 greatly differs between the inspection printed board and the reference printed board. (YES in step S145), it is determined that the conductive material 61 or 62 is excessively injected into the through hole 22 (step S150). In this case, since the through-hole 22 and other components are short-circuited, a large difference occurs in the differential waveform between the reference printed board and the inspection printed board after the injection of the conductive material.

  When the conductive material injection amount is appropriate (NO in step S145), the determination unit 51 performs before and after the conductive material injection (that is, the TDR measurement result in step S110 and the TDR measurement in step S135). As a result, the impedance of the printed circuit board for inspection at the time when the reflection from the through hole 22 is detected is compared (step S150). The determination unit 51 determines that the solder connection portion 11 is disconnected when the difference in impedance of the printed circuit board for inspection before and after the injection is within the reference value Vr2, and the inspection after the injection of the conductive material is performed. When the impedance of the printed circuit board is decreased beyond the reference value Vr2, it is determined that the through hole 22 is disconnected.

  When the conductive material 61 is in a liquid state, the conductive material 61 is injected little by little into the through hole 22 in step S130, and steps S135 to S155 are executed each time it is injected. It is desirable to observe whether the TDR measurement waveform on the printed circuit board for inspection approaches the TDR measurement waveform on the printed circuit board for reference.

[Effect of Embodiment 1]
As described above, according to the printed circuit board inspection method of the first embodiment, disconnection occurs in either the through hole provided in the range of the position detection resolution by TDR measurement or the connection part of the electronic component. In addition, when it is difficult to identify a disconnection point by visual inspection or an electric tester, it is possible to specify which one is causing the disconnection by applying the TDR method. As another effect, since the conductive material injected into the through hole can be removed after the disconnection location is specified, the disconnection location can be maintained, and subsequent physical analysis can be performed.

<Embodiment 2>
The printed circuit board inspection method according to the second embodiment is different from the first embodiment in the method for specifying the disconnection location on the printed circuit board for inspection after the conductive material is injected. Specifically, a composite waveform is generated by combining a TDR measurement waveform on a non-defective reference printed circuit board and a TDR measurement waveform at the time of through-hole disconnection at an appropriate ratio. The TDR measurement waveform on the printed circuit board for inspection is compared. Hereinafter, it will be described in detail with reference to the drawings.

[How to identify the disconnection after injection of conductive material]
FIG. 8 is a diagram schematically showing an equivalent circuit of the printed circuit board 20.

  Referring to FIG. 8, a wiring pattern 71 on the mounting surface side of the electronic component and a wiring pattern 70 on the opposite side to the mounting surface are connected through a through hole 22 (resistance value R). An electronic component (IC package) 10 is connected to a wiring pattern 71 on the mounting surface side of the electronic component via a solder connection portion (solder ball) 11. A TDR signal (step voltage) is input to the wiring pattern 70 on the side opposite to the mounting surface at a position where it is connected to the through hole 22 or close to the through hole 22. The impedance when the wiring pattern 70 is viewed from the through hole 22 is a (t), and the impedance when the wiring pattern 71 is viewed from the through hole 22 is b (t).

  FIG. 9 is a diagram for explaining the relationship between the output voltage Va of the TDR measuring device 40 and the detection voltage Vb. Referring to FIGS. 8 and 9, the detected voltage Vb (t) detected by the TDR measuring device 40 includes the peak value (step voltage value) Va of the TDR signal, the output impedance Z0 of the TDR measuring device 40, It is represented by the following formula (2) by the impedance Z1 (t) of the measurement object.

Vb (t) = (Z1 (t) × Va) / (Z0 + Z1 (t)) (2)
Here, the TDR device originally replaces the difference in the impedance Z1 depending on the position on the electric circuit with the time information (that is, the arrival time of the TDR signal + the time until the reflected wave returns to the TDR device), and the impedance Z1. It is a device that measures as a time change. In the case of the present embodiment, the object to be measured is a circuit on a printed board, and the impedance Z1 varies depending on the position of the wiring. Therefore, the detection voltage Vb (t) and the impedance Z1 (t) change according to the elapsed time t from the application time of the step voltage.

  On the other hand, in the above formula (2), the fixed values of the step voltage Va and the output impedance Z0 are determined in advance according to the TDR measuring device 40.

  Specifically, since a commercially available TDR device is connected to a 50Ω coaxial cable or the like and measures an object to be measured, 50Ω is often used as a standardized impedance. Therefore, the output impedance Z0 is almost 50Ω. However, it is not necessarily 50Ω. For example, if a 100Ω wiring is used, there is no problem even if the output impedance Z0 is 100Ω.

  In most cases, the step voltage Va is a low voltage (for example, 0.1 V to 0.4 V) that does not adversely affect the electric circuit, components, and the like mounted on the printed circuit board 20.

  Next, a method for calculating the impedance Z1 (t) of the object to be measured according to the resistance value R of the through hole 22 will be described. If the impedance Z1 (t) of the measurement object can be calculated, the detection voltage Vb corresponding to it can be calculated. As shown in FIG. 8, the TDR signal is input to the through hole 22 on the side opposite to the mounting surface of the electronic component, or is input to the wiring pattern 70 in the vicinity of the through hole 22. The through hole 22 is in the middle of the circuit wiring, and the TDR step voltage signal is transmitted to the two wiring patterns connected to the through hole 22 (in the case of FIG. 8, the wiring pattern 71 on the mounting surface side and the wiring on the opposite side of the mounting surface). Propagate to both). In this case, the impedance Z1 (t) of the device under test is determined by the combination of the impedances of both wiring patterns. Since the impedances a and b of both wiring patterns also change depending on the position, the respective position information is replaced with time information and expressed as a (t) and b (t).

FIG. 10 is an equivalent circuit diagram of the impedance Z1 (t) of the device under test.
8 to 10, when viewed from the TDR measuring instrument 40, the impedance a (t) of the wiring pattern 70, the through hole 22 (resistance value R) connected in series with each other, and the impedance b (of the wiring pattern 71) ( t) are connected in parallel. Therefore, the impedance Z1 (t) of the device under test is
Z1 (t) = {a (t) × (b (t) + R)} / (a (t) + b (t) + R) (3)
It is represented by

When the through hole 22 is completely disconnected, the resistance value R becomes a very large value. At this time, the impedance Z1 (t) of the object to be measured is
Z1 (t) = a (t) (4)
Therefore, if the detection voltage at this time is Vb∞ (t), the impedance a (t) of the wiring pattern 70 can be expressed by using the equation (2).
a (t) = Vb∞ (t) × Z0 / (Va−Vb∞ (t)) (5)
It can be seen that

On the other hand, when the printed circuit board 20 is a non-defective product, the resistance value of the through hole 22 can be regarded as approximately 0Ω (R≈0). Assuming that the detection voltage at this time is Vb0 (t), from equations (3) and (2),
Z1 (t) = Vb0 (t) × Z0 / (Va−Vb0 (t)) (6)
b (t) = Z1 (t) × a (t) / (a (t) −Z1 (t)) (7)
It becomes. Therefore, the impedance b (t) of the wiring pattern 71 can be calculated by substituting a (t) and Z1 (t) obtained by the equations (5) and (6) into the equation (7).

  Thus, since the impedances a (t) and b (t) of the wiring patterns 70 and 71 are found, the detection voltage Vb (t) corresponding to the arbitrarily given resistance value R of the through hole 22 is expressed by the equation (2). And (3).

  In the above description, the detection voltage Vb∞ (t) when the through hole 22 is completely disconnected can be substituted with the detection voltage Vb (t) when the IC package 10 is removed. This is because the through hole 22 and the solder connection part 11 of the IC package 10 are located within the resolution range of the TDR measurement, so that there is no significant difference in the waveform detected by the TDR method.

  FIG. 11 is a diagram showing an example of a combined waveform 82 synthesized from a TDR measurement waveform 80 in the case of a non-defective printed circuit board and a TDR measurement waveform 81 when the through-hole is completely disconnected.

  Referring to FIG. 11, it is assumed that a step voltage is output to the printed circuit board at time t0 and reflection from the through hole is observed at time t1. A TDR waveform 82 corresponding to a given through-hole resistance value R is synthesized from a TDR measurement waveform 80 in the case of a non-defective printed circuit board and a TDR measurement waveform 81 when the through-hole is completely disconnected. The measurement waveform 81 when the through-hole is completely disconnected can be substituted by the TDR measurement waveform when the adjacent BGA package is removed.

  Actually, in order to specify whether the disconnection point is a through hole or a solder connection part of an adjacent BGA package, the resistance value R of the through hole is changed variously and a TDR waveform corresponding to each resistance value R is synthesized. To do. By comparing the TDR waveform corresponding to each synthesized resistance value R with the TDR measurement waveform after the injection of the conductive material, the TDR measurement waveform after the injection of the conductive material in the vicinity of time t1 can be obtained from any of the synthesized TDR waveforms. It is determined whether or not they match within a predetermined gap. If the deviation is small, it can be determined that the through hole is disconnected. On the other hand, if the TDR measurement waveform hardly changes even when the conductive material is injected, it can be determined that the solder connection portion is disconnected. The comparison of the reflected waveform may be performed using a voltage waveform, or may be performed by converting the impedance waveform into an impedance waveform.

  It should be noted that even if a conductive material is injected into the disconnected through-hole, the resistance value of the original non-disconnected through-hole is not completely restored. For this reason, it is necessary to synthesize a TDR waveform corresponding to each resistance value R and compare the synthesized TDR waveform with the TDR measurement waveform after the injection of the conductive material.

  Further, as described in the first embodiment, the use of the differential waveform of the voltage waveform or the impedance waveform makes it easy to identify the disconnection location. This is because the waveform tendency (differential waveform) is completely different when the through hole is short-circuited with another circuit due to excessive injection of the conductive material.

[PC board inspection procedure]
FIG. 12 is a flowchart showing a printed circuit board inspection procedure according to the second embodiment. In the inspection procedure of FIG. 12, step S105 is executed between steps S100 and S110 of FIG. 7, and steps S160 and S165 are executed instead of step S155. Since the other steps are the same as those in FIG. 7, the same reference numerals (step numbers) may be attached and the description may not be repeated.

  With reference to FIG. 1, FIG. 2 and FIG. 12, first, in step S100, TDR measurement is performed in advance on a number of non-defective reference printed boards by the TDR measuring device 40, and the waveform of the reflected wave thus measured is measured. The average value is stored in the storage unit 52 as the reference waveform A (step S100). The storage unit 52 also stores an impedance waveform corresponding to the reference waveform A and a differential waveform thereof.

  In the next step S105, TDR measurement is performed by the TDR measuring device 40 with respect to the BGA package 10 removed from the reference printed circuit board (actually, before the BGA package 10 is mounted and other components are mounted). TDR measurement is performed in a later state). The average value of the measured waveform of the reflected wave is stored in the storage unit 52 as the reference waveform B. The storage unit 52 also stores an impedance waveform corresponding to the reference waveform B and a differential waveform thereof.

  Subsequent steps S110 to S150 are the same as those in FIG. In step S160, the determination unit 51 of the computer 50 generates TDR waveforms respectively corresponding to the resistance values R of the plurality of through holes based on the reference waveforms A and B as described in the equations (2) to (7). Synthesize. The synthesized waveform is stored in the storage unit 52. Note that step S160 can be executed any time after step S105.

  In the next step S165, the determination unit 51 of the computer 50 compares the TDR measurement waveform of the printed circuit board for inspection after injecting the conductive material with the plurality of synthesized TDR waveforms. As a result, the determination unit 51 confirms that the TDR measurement waveform after injecting the conductive material in the vicinity of time t1 at which reflection from the through hole 22 is observed matches any of the synthesized TDR waveforms within a predetermined deviation width. If it is, it is determined that the through hole 22 is disconnected. On the other hand, when the TDR measurement waveform after the injection of the conductive material in the vicinity of time t1 does not change within a predetermined deviation from the TDR measurement waveform before the injection, the determination unit 51 determines the solder connection portion of the BGA package 10. It is determined that 11 is disconnected.

  The embodiment disclosed this time must be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Printed circuit board inspection apparatus, 10 IC package, 11 Solder ball (solder connection part), 20 Printed circuit board, 22, 22A, 22B Through hole, 32 Probe, 70, 71 Wiring pattern, 37 High frequency cable, 40 TDR measuring machine, 41 Signal generator, 46 oscilloscope, 50 computer, 51 determination unit, 52 storage unit, 53 measurement control unit, 60 fine needle syringe, 61 conductive substance, 62 conductive gel.

Claims (6)

A method for inspecting a printed circuit board,
The printed circuit board is
A wiring pattern including a through hole;
An electronic component connected to the wiring pattern so as to cover the through hole,
The through hole and the connection part of the electronic component are within the resolution range of the TDR (Time Domain Reflectometry) method,
A first measurement step of measuring the printed circuit board by a TDR method;
A first determination step of determining whether or not a break has occurred in the printed circuit board based on the measurement result of the first measurement step;
A step of injecting a conductive material into the through hole when it is determined by the first determination step that one of the through hole and the connection part of the electronic component is disconnected; and
A second measurement step of re-measuring the through hole of the printed circuit board and the connection part of the electronic component by a TDR method after the injection of the conductive material;
A printed circuit board inspection method comprising: a second determination step of determining which of the through hole and the connection part of the electronic component is disconnected based on a measurement result of the second measurement step. Each of the first and second measurement steps includes:
Applying a step voltage generated by a signal generator to the wiring pattern via a probe;
The printed circuit board inspection method according to claim 1, further comprising: detecting the step voltage reflected from the printed circuit board through the probe. The first determination step includes
The result of the TDR measurement performed on the reference printed circuit board in advance and the measurement result of the first measurement step on the printed circuit board for inspection are compared until the first time when reflection from a certain inspection site is detected. The difference between the impedance of the reference printed circuit board and the impedance of the printed circuit board for inspection is within a first reference value, and the impedance of the printed circuit board for inspection is the first reference value after the first time. The method for inspecting a printed circuit board according to claim 2, comprising a step of determining that the inspection part is disconnected when the number of the inspection parts is larger than. The second determination step includes
At a second time when reflection from the through hole is detected, the impedance of the printed circuit board for inspection based on the measurement result in the first measurement step and the measurement result in the second measurement step When the difference from the impedance of the printed circuit board for inspection is within the second reference value, it is determined that the connection part of the electronic component is disconnected, and the inspection based on the measurement result in the second measurement step The printed circuit board inspection method according to claim 3, further comprising a step of determining that the through hole is disconnected when an impedance of the printed circuit board decreases beyond the second reference value. The second determination step includes
A plurality of resistance values of the through holes are assumed based on a reflection waveform detected from the reference printed circuit board by TDR measurement and a reflection waveform detected by the TDR measurement from the reference printed circuit board not connected to the electronic component. A step of synthesizing a plurality of reflected waveforms corresponding to each resistance value,
A step of determining whether a disconnection occurs in the through hole or the connection part of the electronic component based on a comparison between the reflected waveform detected in the second measuring step and a plurality of synthesized reflected waveforms. The printed circuit board inspection method according to claim 3, further comprising: The second determination step further includes:
Comparing the differential waveform of the reflected wave or the differential waveform of the impedance based on the measurement result in the second measurement step with the differential waveform of the reflected wave or the differential waveform of the impedance based on the TDR measurement result for the reference printed circuit board. The printed circuit board inspection method according to claim 4, further comprising: determining whether or not the conductive material is excessively filled.

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