Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
2000-07-18
2004-02-10
Glick, Edward J. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S559360, C356S237400
Reexamination Certificate
active
06690024
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a laser inspection apparatus, for example, having an inspection function of detection, thickness measurement, etc., of an unremoved material left on the bottom face of a blind via hole during blind via hole machining in a multilayer wiring board called a printed wiring board.
2. Description of the Related Art
Making denser wiring has been demanded with recent high performance of the electronic machines. To meet the demand, multilayering and miniaturization of printed wiring boards are advanced. In one of the arts, it is indispensable to make a fine blind hole for interlayer conduction connection having a hole diameter of about 150 &mgr;m, called a blind via hole (SVH). However, it is difficult to drill a hole of &phgr;0.2 mm or less and machine a blind hole by the current drilling technique and in addition, an insulating layer of a high-density printed wiring board is 100 &mgr;m or less thick and it is difficult to control the depth with the accuracy, thus it is impossible to make a fine BVH by drilling.
Attention is focused on a method of irradiating a laser beam as a BVH making method replacing the drilling. The method uses the light energy absorption difference between a resin or glass fiber of an insulating material forming a part of a printed wiring board and copper of a conductor layer. Carbon dioxide laser is put to some use as a light source of laser beam. As shown in
FIG. 11
, if inner layer copper foil
24
is previously deposited on the inside of the machined part, dissolving and removal of an insulating member are stopped on the inner layer copper foil
24
, so that a blind hole
6
stopped reliably on the inner layer copper foil
24
can be made. Such a machined hole is particularly called a direct image hole. As shown in
FIG. 12
, on a board comprising copper foil
24
a
on a surface, a copper foil removal part of a necessary hole diameter is formed by etching, etc., and laser beam
20
having a beam diameter larger than the hole diameter of the removal part is applied, whereby a machined hole
6
can also be made. Such a machined hole is particularly called a conformal image hole.
If the blind hole stopped on the copper foil is machined by carbon dioxide laser as shown in
FIGS. 11 and 12
, the resin of the insulating member 1 &mgr;m or less thick is left on the inner layer copper foil if a sufficient laser beam is applied. Thus, after laser machining, it is necessary to etch the residual resin in a permanganic acid, etc., for completely removing the residual resin. At this time, if the blind hole is lessened to about 100 &mgr;in diameter, the etching liquid becomes hard to spread all over the inside of the hole, thus if the residual resin becomes thick exceeding a thickness of 1 &mgr;m because of a failure of the laser machining condition, etc., a hole where the residual resin cannot completely be removed occurs. In this state, if plating is applied and a BVH electrode is formed, the resin remains left in a part between the plate film and the inner layer copper foil. Here, if a stress is exerted by a heat cycle, etc., with it as the start point, the plate film peels off. Thus, it becomes necessary to inspect the thickness of the remaining resin after laser machining.
FIG. 13
a residual resin distribution when the number of shots is changed. It is known that the residual resin is a little observed in the vicinity of the center of a hole and is easily left in the vicinity of the wall faces of the hole. Like a hole made with the number of shots, five pulses, the resin is a little observed at the center position, but is much in the surroundings as a defective piece. Therefore, to inspect the residual resin for thickness, it is necessary to inspect a wide range from the center to the periphery.
An inspection apparatus in a related art uses an optical microscope to inspect a machined part as shown in FIG.
14
. If a resin is left about 10 &mgr;m or more thick, it can be detected under the optical microscope in the related art described on page 45 of Nikkei Science October 1990 issue, but the optical microscope is poor in detection accuracy of the residual resin about several &mgr;m thick as described above and is hard to apply in mass production. The post-plated machined part must be cut and ground, then the residual resin must be inspected for thickness by observing the cross section; the inspection takes time and 100% inspection cannot be conducted; this is a problem.
The reason why the residual resin cannot be detected under the optical microscope in the related art is as follows:
The optical microscope in the related art has a configuration as shown in FIG.
15
. Illumination white light
38
is applied through an object lens
5
to a printed wiring board
21
by a beam splitter
25
. Reflected light from the printed wiring board
21
forms an inverted real image
27
enlarged by the object lens
5
forward of an image formation lens
9
, and the real image is detected by a CCD camera
11
.
As shown in
FIG. 16
, when the while light
38
of illumination light of the optical microscope is applied to the surface of residual resin
22
, some is reflected and other light passes through the residual resin
22
and reaches copper foil
24
on the bottom and is reflected thereon. Therefore, if the white light
38
is applied to thin resin on the copper foil as illumination light, most reflected light is returned from the copper foil
24
and thus the residual resin
22
becomes invisible.
FIG. 17
shows an inspection apparatus described as one embodiment in JP-A-7-83841. In the figure, numeral
43
denotes an ultraviolet laser light source, numeral
45
denotes a collimation lens, numeral
44
denotes a mirror, numeral
25
denotes a beam splitter, numeral
46
denotes a rotary polyhedral mirror, numeral
21
denotes a printed wiring board to be inspected, numeral
9
denotes an image reformation lens, numeral
48
denotes a pin hole, and numeral
47
denotes a photomultiplier (photomultiplier tube).
Next, the operation of the related art example is as follows: Laser beam generated by the ultraviolet laser beam source
43
is enlarged using the collimation lens
45
. The enlarged laser beam is scanned using the rotary polyhedral mirror
46
and is condensed on the printed wiring board
21
through the object lens
5
.
The ultraviolet light generated from the printed wiring board
21
by irradiating the laser beam reversely traces the incident path, is fed back recursively, and is guided into a recursive reflection sense system by the beam splitter
25
placed in the optical path. The ultraviolet reflected light is formed through the image formation lens
9
. An image in the proximity of the application point of the laser beam to the printed wiring board to be inspected is observed on the image formation face. Only the center portion is separated through the pin hole
48
placed in the image formation face and is detected by the photomultiplier
47
.
Since the inspection apparatus in the related art shown in
FIGS. 15 and 16
is configured as described above, if the residual resin is thin, the reflected light is strong and the residual resin cannot be detected as described above; this is a problem.
The inspection apparatus shown in
FIG. 17
executes laser scanning on the rotary polyhedral mirror. The laser beam scan line may shift from the center line of the blind hole because of a position shift at the blind hole machining time, worsening of precision of the scanner, etc. For example, if the residual resin is a little observed in the vicinity of the hole center at a good level, but the periphery is at a defective level like the blind hole made with the number of shots, five pulses, a good piece may be erroneously determined a defective piece because of a scan line shift; this is a problem.
To prevent this, it is necessary to make the scan line spacing sufficiently smaller than the hole diameter and scan the full hole bottom face, but it takes enormous time in inspection; this is a prob
Funaoka Koji
Sakamoto Masahiko
Sasai Hiroshi
Takeno Shozui
Glick Edward J.
Kao Chih
Mitsubishi Denki & Kabushiki Kaisha
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