JPH011940A - Through-hole void inspection method and device for multilayer printed circuit boards - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for automatically inspecting through-hole voids in a multilayer printed circuit board. The present invention relates to a method for inspecting through/hole voids in a multilayer printed circuit board suitable for inspection.

[Conventional technology]

In multilayer printed circuit boards, in order to make electrical connections between layers, a layer of steel or the like is formed on the inner wall of the through hole by electroless plating, and a solder layer is further formed on this layer by electroless plating. are laminated.

However, as the density of components mounted on a multilayer printed circuit board increases and the number of layers increases, the diameter of the through hole becomes smaller and the depth becomes deeper. It becomes difficult to form a plating layer on the surface of the metal, which causes conduction defects.

It is difficult to detect such a defect at first, and if the defect is not discovered, it will appear as a failure of the device during actual operation, which often leads to extremely serious consequences.

Therefore, it is important to detect and eliminate defects in which the copper plated on the inner surface of the through-hole does not adhere properly (this is called a through-hole void).

In the conventional through-hole void inspection method, as described in Japanese Patent Application Laid-Open No. 60-85596, a photosensitive plate or film is prepared in advance on one side of the board to check at least all the through-holes to be inspected. After pasting the board so as to cover it and irradiating light from one side of the board so as to pass through all the through holes to be inspected, conduct through-hole inspection using the photosensitive plate or film as a light-shielding mask. something is proposed.

Also, for example, the Proceedings Technical Program Natsutoel, the Electron Packaging Production Conference (Proceeding Technic)
al Program Natl, E, gectron
Pa-ckaging Production Co.
nferenc'e) VoL.

As described in 1974 PP56-62 (1974), a method has been published in which a fluorescent substance is mixed into a base material, irradiated with ultraviolet rays, and fluorescence generated from through-hole voids is observed. This method applies 35% to the print substrate.
A fluorescent dye that generates F)472 fluorescence by Chnx excitation light is mixed in. An ultraviolet lamp is placed below, and a filter that passes light at 350 mm is placed above it. Next, the print base material is placed in a horizontal state on top of the filter, and a 35 on ultraviolet ray absorbing filter is placed on top of the print base material. It should be noted that these lower filters, the print base material, and the upper filters have appropriate intervals, respectively.

In this state, if ultraviolet light is irradiated from the lower ultraviolet lamp toward the lower filter, if there is a through-hole void in the through-hole, the ultraviolet light will hit the through-hole void and the print substrate will be exposed to the light. Fluorescence occurs. When an operator looks at this from above, the inside of the through-hole where the through-hole void is located appears to be shining, and this is used to inspect the through-hole void.

Therefore, the above method seems to be capable of inspecting relatively large through-hole voids.

[Problem that the invention seeks to solve]

The former prior art described above has the following problems.

That is, in the former prior art, since the illumination light should illuminate only the periphery of the through hole and not enter the through hole, it is necessary to completely cover one side of the through hole. This is because if one of the through holes is incompletely covered, it will be mistakenly recognized that there is a through hole void, even though there is actually no through hole void. Furthermore, when there are a large number of through holes, it is necessary to open and close one or more of the through holes one by one, and the opening and closing operations are troublesome.

In the former prior art, it is necessary for the light irradiated around the through-hole to pass through the inside of the substrate and the through-hole void to reach the photosensitive plate or film on one side of the through-hole.

However, depending on the type of printed circuit board, the number of layers may be large, and depending on the shape of the inner layer pattern, the illumination light may be blocked midway, and the circuit pattern may already be formed on the outermost surface of the printed circuit board. The illumination light may be blocked and may not pass through the through-hole void to reach the photosensitive plate or film on one side of the through-hole, making it impossible to inspect the through-hole void.

Furthermore, in the former prior art, when illuminating light is irradiated onto the opposite side of the printed circuit board to which the photosensitive plate or film is attached, it is difficult to prevent the illuminating light from being irradiated into the through holes.

Next, in the latter conventional technique, although it was announced more than 10 years ago, it has not become a common method. I think the reason is as follows.

That is, the first reason is that the printing substrate must be mixed with a fluorescent substance, which increases the number of steps required for mixing, which is expected to increase costs and have a negative impact on the reliability of the printing substrate. Ru.

The second reason is that the ratio of the hole diameter and depth of the through-hole was made for process-grade printed circuit boards, and like today's printed circuit boards, the hole diameter is 0.5 to 0.3 m+.
If the thickness of the printed circuit board is a few centimeters with respect to a, it is difficult to inspect through-hole voids using the above method.

That is, since the amount of fluorescence that passes through small through-hole voids is very small, it is difficult to inspect through-hole voids using the above method.

In addition, when the above method is applied to a board before a circuit pattern is formed on the outermost surface of the printed circuit board, that is, a board whose outermost surface and through-hole inner surface are coated with copper foil, the copper foil causes fluorescence. There is a problem with being blocked.

The purpose of the present invention is to inspect through-hole voids using fluorescence, but it is possible to inspect small through-hole voids without mixing fluorescent dye into the printing substrate by making various improvements to the optical system. Another object of the present invention is to provide a through-hole void inspection method that enables through-hole void inspection with high accuracy without being affected by optical interruption within the through-hole or the shape of the inner layer pattern of a printed circuit board.

[Means for solving problems]

The above purpose is to provide a photodetection optical system that detects light in a wavelength band different from the light generated from the target object when the target object is irradiated with light in a specific wavelength band, and Using light that illuminates the through-holes of the printed circuit board and changes into light of different wavelength bands when it passes through the through-hole voids and hits the base material of the printed circuit board,
This is achieved by detecting through-hole voids by detecting light in a wavelength band different from the illumination light generated in the through-hole when the light is illuminated into the through-hole from the light detection optical system. ,ru.

[For production]

The photodetection optical system used in the present invention emits high-intensity excitation light from a lamp such as an ultra-high-pressure mercury lamp, and the excitation light is focused by a flexor lens and an excitation filter generates fluorescence from the printed circuit board base material. Only light in the optimal wavelength band is passed through to reach the microscope objective lens. The microscope objective lens used has a large numerical aperture and a convergence angle of 10° or more when irradiating the through hole. The excitation light focused by such a microscope objective lens is placed on a scattering surface that reflects the light.
When irradiated into the formed through-hole, the excitation light reflects the inner surface of the through-hole and diffuses, illuminating the entire area.

If there is a through-hole void on the inner surface of the through-hole, excitation light enters the base material of the printed circuit board, and fluorescence is emitted from the base material onto the inner surface of the through-hole. This fluorescence generally has a long wavelength band (h), but is not directional, so it is emitted in the surrounding area with a similar amount of light, and the inner surface of the through hole is emitted towards the photodetection optical system side. The reflected fluorescence reaches the entrance of the through hole and is imaged on the detector by the microscope objective lens, but at this time, it is imaged by the dichroic mirror installed between the microscope objective lens and the detector. The detector reflects the excitation light and transmits only the fluorescence, and absorbs the light in the wavelength range of the excitation light with an absorption filter, allowing only the fluorescence to pass. Therefore, if there is a through-hole void, the detector is detected brightly, but if there is no through-hole void, the entrance of the through-hole will be completely dark and no light can be detected.

Therefore, it is possible to easily inspect whether or not there is a through-hole void in the through-hole.

Further, when the size of the through-hole void is small, the amount of fluorescence from the through-hole void is extremely small, and may be on the order of lo'-2 to 10-'' lux on the surface of the detector. Therefore, a normal TV camera and a linear image sensor cannot be used as a detector.Therefore, in the present invention, for example, a silicon intensifier target (5ilicon intensif
iar Targat) (hereinafter referred to as SIT) TV camera, image intensifier (Image I
A TV camera such as a TV camera (hereinafter referred to as II) that can detect weak light of 10-3 or less is used. In addition, as a linear image sensor, an image intensifier (I
mage Intensi-fier) tube,
A commercially available ultra-high sensitivity linear image sensor combined with a normal linear image sensor is used. In this way, the present invention includes a light detection optical system having a microscope objective lens with a large numerical aperture and an ultra-high sensitivity detector;
If there is a through-hole void inside the through-hole, the through-hole void can be inspected by using excitation light that emits fluorescence with different wavelength bands on the inside of the through-hole and detecting the fluorescence from the entrance of the through-hole. Therefore, through-hole voids can be easily inspected with high reliability, thereby making it possible to prevent the production of defective printed circuit boards and to prevent failures of computers and the like.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be explained in detail below with reference to FIG. 1 and FIG. 2.

As shown in FIG. 1, the printed circuit board 1 has a multi-layered interior and a through hole 2 and wiring butter/lb. The inner surface of the through hole 2 and both the front and back surfaces of the printed circuit board 1 are made of It is covered with foils 3a and 3b. Note that the copper foil 3a covering the inner surface of the through hole 2 is formed into a scattering surface so as to reflect excitation light 13 from a photodetection optical system 5, which will be described later. Reference numeral 4 denotes a through-hole void, which is a missing portion of a part of the copper foil 3a on the inner surface of the through-hole 2. 5 is a photodetection optical system,
A lamp 6, a collector lens 7, an excitation filter 8, a dichroic mirror 9, a microscope objective lens 10,
It consists of an absorption filter 11 and a detector 12. The lamp 6 is formed to emit excitation light 13 of high intensity, such as an ultra-high pressure mercury lamp. The collector lens 7 collects the excitation light from the lamp 6. The excitation filter 8 is selected so as to pass the excitation light 13 having the optimum wavelength for hitting the base portion 1a of the printed circuit board 1 and generating fluorescence 14.

For example, as shown in FIG. 2(a), the dichroic mirror 9 is formed to pass only the excitation light 13 having a wavelength restricted to a certain wavelength band.
As shown in , it is formed to reflect excitation light 13 and pass fluorescence (light with a longer wavelength than the wavelength of excitation light 13) 14. The microscope objective lens 10 has a large numerical aperture, that is, a convergence angle θ of 10' or more for the excitation light 13 and the fluorescence 14 shown in FIG. I try to make it shine. The absorption filter 11 absorbs light in the wavelength band of the excitation light 13, as shown in FIG. 2 tc+, and allows only the fluorescence 14 to pass through. The detector 12 is 5ITTV
Camera II 10-2 to 10 like a TV camera etc.
- It is formed with ultra-high sensitivity that can detect fluorescence of weak light of 3 lux or less, and the linear image sensor uses a commercially available ultra-high-performance linear image sensor that combines a II tube and a normal linear image sensor. ing. Note that the fluorescence 14 generated upon hitting the base material portion 1a has no particular directionality and flows from the through hole void 4 to the through hole 2.
It is emitted with a similar amount of light.

Since the through-hole void inspection apparatus according to the present invention is configured as described above, the through-hole void inspection method will be explained next.

The excitation light 13 from the lamp 6 is collected by the collector lens 7, and the excitation filter 8 passes only the excitation light 13 having a wavelength restricted to a certain wavelength band as shown in FIG. Through the microscope objective lens 10
When the excitation light 13 is focused inside the through hole 2, it is sufficiently diffused inside the through hole 2 and illuminates the entire surface.

When there is a through-hole void 4 in the through-hole 2, the excitation light 13 passes through the through-hole void 4 and enters the base portion 1a of the printed circuit board 1, so that the excitation light 13 passes through the through-hole void 4 and enters the base portion 1a of the printed circuit board 1. Fluorescent light 14 is emitted into the through hole 2 through the hole void 4. Among these fluorescent lights 14, the upwardly directed fluorescent lights 14 reach the entrance 2a while being reflected from the inner surface of the through hole 2, and are imaged on the detector 12 by the microscope objective lens 10. At this time, the excitation light 13 is reflected by the dichroic mirror 9 as shown in FIG. 2(b), and only the fluorescence 14 is transmitted. In addition, absorption filter 8
As shown in FIG. 2(C), the detector 12 absorbs light in the wavelength band of the excitation light 13 and passes only the fluorescence 14.
When there is a through hole void 4 on the inner surface of the through hole 2, the entrance 2 of the through hole 2 is detected brightly,
If there is no through-hole void 4, the entrance 2& of the through-hole 2 will be completely dark and no light will be detected. Furthermore, since the detector 12 is formed to have ultra-high sensitivity and is formed to have ultra-high sensitivity as a linear image sensor,
Fluorescence of weak light of 3 lux or less can be detected.

Next, some other embodiments according to the present invention will be described.

First, FIG. 3 shows a case where the lamp 6 is disposed at a position opposite to the detection 112 with respect to the through-hole cover 2 of the printed circuit board 1. In this embodiment, as compared to FIG. 1, the dichroic mirror 9 is naturally unnecessary, but in its place a condenser lens 15 for condensing the excitation light 13 is required. This condenser lens 15 has a large numerical aperture similar to that of the microscope objective lens 10'.

Next, FIG. 4 shows a case where a laser beam 16 is used in place of the excitation light 13 in FIG. 1.

Since the laser beam 16 emits a single wavelength or multiple wavelengths, when only light in the excitation light band is emitted,
The excitation filter 8 shown in FIG. 1 becomes unnecessary.

Next, FIG. 5 shows another embodiment of the excitation light condensing method, in which the excitation light illuminates a wide surface on the printed circuit board.

In this embodiment, the excitation light 13 from the lamp 6 is reflected downward by an ellipsoidal reflector 17, and an ellipsoidal reflector 18 is formed symmetrically opposite the ellipsoidal reflector 17, Since the microscope objective lens 10 and the light are focused in the through hole 2 of the blind substrate 1 from an oblique direction,
Intense excitation light 13 is irradiated into the through hole 2 in a diagonally downward direction. Since there is a through hole void (not shown) in the through hole 2, the fluorescence 1
4, this fluorescence 14 is reflected by two reflecting mirrors 1.
9a.

The fluorescent light 14 is imaged onto the detector 12 via the excitation filter 19 b, and only the fluorescence 14 is passed to the detector 12 by the excitation filter 11 . Note that in this embodiment, the dichroic mirror shown in FIG. 1 is not necessary.

Therefore, in this embodiment, the strong excitation light 13 is illuminated into the inside of the through hole 2 from an oblique direction, so the excitation light 13 is more effectively reflected in the oblique direction within the through hole 2, resulting in minute through holes. Hole voids can be illuminated.

However, when the excitation light 13 illuminates a wide surface of the printed circuit board 1 as in this embodiment, although it is extremely rare, when a foreign object is present on the printed circuit board 1 as shown in FIG. As a result, fluorescence 14 is generated, and there is a possibility that the through hole 2 may be mistakenly recognized as if there is a through hole void inside the through hole 2. This can be prevented by narrowing down the range over which the excitation light 13 illuminates the printed circuit board 1.

For example, as shown in FIG. 7(al(b)), using the photodetection optical system 5 shown in FIG. This problem can be solved by installing the Teruno diaphragm 21 so that the image is formed on the upper surface of the image.

That is, FIG. 8(al(b)) shows a case where a relatively large circular illumination diaphragm 21 & whose illumination diaphragm 21 is slightly small for the field of view 10 m of the microscope objective lens 10 is used. In this case, the excitation light 13 illuminates the entire field of view visible through the microscope objective lens 10.
1 is a case where a small circular illumination diaphragm 921b is used, and in this case, only one through hole 2 is illuminated by the excitation light 13. Furthermore, Figure 10 (al (bl is Teruno Shiboshi 21)
This is a case where a plurality of small perforated diaphragms are formed in e, and in this case, a plurality of through holes 2 are illuminated at the same time.

Therefore, Teruno diaphragm with the appropriate shape! l) By arranging 21, even if fluorescence 14 is generated due to foreign matter m existing on a surface other than through-hole 2 of printed circuit board 1, it is possible to prevent this from being mistaken as a through-hole void. can.

Nao, the above-mentioned Figure 8 Tal (bl to Figure 10 (al)
(b) has been described with the case in which a TV camera is used as the detection Bg12 in mind, but when a linear image sensor is used, the Teruno diaphragm fi21d is shaped into a rectangle as shown in Figures 11(a) and (b). There is a method to illuminate only the part that is formed and detected by the linear sensor and through hole 2
In the case of illuminating only the target area, a teruno diaphragm 21e with a plurality of small holes drilled at positions corresponding to the through-holes 2 is used, as shown in FIG.

In addition, the above-mentioned figures 8 (tal fb) to 12 (al
(b) Ic J6 I''C shows the case where the Teruno diaphragm 21 is formed into a circular shape and a rectangular shape, but the shape is not limited to this, and it is also possible to make it into a square shape or an oval shape, for example.

Furthermore, in FIGS. 1 to 12(a) and (b), before the outermost layer pattern of the printed circuit board 1 is formed (
before etching]. Before etching, the entire printed circuit board 1 is covered with copper foil, so unless the printed circuit board 1 has through-hole voids 4, the fluorescence 14 will not be detected by the detector 9. (The occurrence of fluorescence 14 due to the addition of foreign matter is an extremely rare exception.) If fluorescence 14 is detected, it can be determined that there is a through-hole void 4 within the through-hole 2.

Therefore, by targeting the printed circuit board 1 before etching, the through-hole voids 4 can be easily detected.

However, as a practical matter, the target printed circuit board 1
In some cases, the image after the outermost layer pattern is formed (after etching) is used. In this case, as shown in FIGS. 13 and 14, when the excitation light 13 hits the substrate portion 1a'lc of the printed circuit board 1' other than the through hole 2, fluorescence 14 is generated and the inside of the through hole 2 is illuminated. This will be mistaken for through hole void 4. Therefore, in the present invention, FIG.
As shown in FIG. 1, a shutter n is installed between the illumination diaphragm ν21 and the excitation filter 8. This shutter η works in conjunction with a drive mechanism (not shown) that moves the printed circuit board 1' horizontally and horizontally and vertically with respect to FIG. The shutter 21 is opened only when the position reaches the through hole 2 of the printed circuit board 1', and the shutter n is closed at other times.

Therefore, the opening and closing of this shutter B and the interlocking with the drive mechanism are as follows.
For example, this can be easily done by setting in advance the position of the through hole 2 with respect to the total amount of movement of the printed circuit board 1'.

Furthermore, the method for detecting through-hole voids 4 in FIGS. 13 and 14 can be easily understood from the embodiments already described, so a description thereof will be omitted.

〔Effect of the invention〕

According to the present invention, through-hole voids in through-holes can be easily inspected with high reliability, so production of defective printed circuit boards can be prevented, and
It is possible to prevent failures of the computer, etc.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a through-hole void inspection device showing an embodiment of the present invention, FIG. 2 (al is a transmittance curve diagram of an excitation filter, FIG. Figure (cl is the transmittance curve diagram of the absorption filter,
FIG. 3 is an explanatory diagram of a through-hole void inspection device showing another embodiment of the present invention, FIG. 4 is an explanatory diagram of a through-hole void inspection device showing another embodiment of the present invention, and FIG. 5 is an explanatory diagram of a through-hole void inspection device showing another embodiment of the present invention, FIG. 6 is an explanatory diagram showing a case where there is a foreign substance on a printed circuit board, and FIG. 7(a) is an explanatory diagram of another embodiment of the present invention. An explanatory diagram of a through-hole void inspection device showing an embodiment, FIG. 7 (bl
is a diagram showing the Teruno diaphragm shown in Fig. 7 (al), and Fig. 8 (al.
(BL to FIG. 12 (al) (BL is an explanatory diagram showing the relationship between the field of view of the microscope objective lens and the illumination diaphragm,
Figure fat is an explanatory diagram of a through-hole void inspection device showing another embodiment of the present invention, and Figure 13 (bl is Figure 13 (a).
FIG. 1 is an enlarged plan view of the vicinity of the through hole of the printed circuit board. 1... Printed circuit board, 2... Through hole, 4...
・Through hole void, 5... Photodetection optical system. Agent Patent Attorney Tadashi Akimoto Actual Figure 2 ((1) N) Raise 1) Letter (b) Dichroic Mirror (c) 裟-Mata 74 Ruta No. 31 Figure 4 1---Glint Group, Law, 10 -・1 month, 1! i1M objective lens 2 --- through hole
11--・Absorbing fill 7--Collegtalence" +2- & Shonen 9---Guiguro Ink Mifu Figure 5 1-m-2) Ruto 11 Nanae II-
J71 Koejima Fukureta 2-11 Through Hole
12-11st intake 6, -1 and F.
13-11 Encouragement L8-11 Waste Power Men: Fui I Leda 14-m-~(Power 210--
-M*1fLW'+R'rl) Ren K 17~18
---aFIaR'RIL Fig. 6 1-11 Print group 4th addition +3-J Kiyu 2
-m-through-ho d/14--rental light 9--itta ΔKloipkumihu 20-4? fi10--with control (
Figure 7 (a) (b) 1---Aldo steaming (9---Dichloie Kumirer 2--Through-hole) 10--One cut of one piece Shinsu 6 --- Run 11 --
One excitation filter 7 --- Reflector lens 12-
--Tarri turn 8-#I start 27 draw 21--Issutero・
Condition νFigure 8Figure 9

Claims (3)

[Claims] 1. A photodetection optical system detects light in a wavelength band different from the light emitted from a target object when light in a specific wavelength band is symmetrically irradiated; When the light illuminates the inside of the through-hole and hits the base material of the printed circuit board through the through-hole void,
When using light that changes into different wavelength bands and illuminating the through hole with the light from the photodetection optical system,
A through-hole void inspection method comprising: inspecting a through-hole void in a through-hole by detecting light in a wavelength band different from the illumination light generated in the through-hole. 2. Claim 1, wherein the light detection optical system is provided with a microscope objective lens having a large numerical aperture.
Through-hole void inspection method described in section. 3. 2. The through-hole void inspection method according to claim 1, wherein the photodetection optical system is provided with an ultra-high sensitivity detector.