Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2000-12-12
2003-10-14
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06633376
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus for inspecting a printed circuit board using laser beam. More particularly, this invention relaters to an apparatus for detecting unremoved material remaining in the bottom of blind via hole or processed groove provided in a laminated printed wiring board, or measuring the thickness, optically, using laser beam.
BACKGROUND ART
Along with the recent advancement in performance of electronic appliances, higher density of wiring is being demanded. To satisfy this demand, the printed circuit board is laminated in multiple layers and down-sized.
A laminated printed wiring board requires formation of ultra-fine stop holes for conduction and connection between layers. These holes have a diameter of about 150 &mgr;m and are called blind via hole (BVH). With present day technology, it is difficult to drill holes or process stop holes of ø0.2 mm or less. Furthermore, the thickness of the insulating layer is generally less than 100 &mgr;m in a high density printed circuit board, and depth control is difficult in such thin layer. accordingly, it is impossible to form ultra-fine BVH by drilling.
A method using laser beam is being noticed as a BVH forming method replacing the drilling process. This processing method makes use of the difference in the absorption of light energy in the insulating material for forming the printed circuit board such as resin and glass fiber, and in the conductive layer which is made of copper.
As the light source of laser beam, carbon dioxide laser is partly put in use, because, copper reflects carbon dioxide laser almost completely. As shown in FIG.
22
(
a
), a copper foil removal part b of desired diameter is formed at a specified position of copper foil a by etching or the like. Then, and by irradiating this copper foil removal part b with laser beam L, the insulating substrate part c of resin or glass fiber is decomposed and removed. Thus, an ultra-fine hole d is formed as shown in FIG.
22
(
b
).
As shown in
FIG. 23
, moreover, a copper foil e may be previously laminated inside of the hole processing part (inside of the insulating substrate). Accordingly, decomposition and removal of the insulating member stops at this copper foil e, so that a stop hole f stopping securely at the copper foil e can be formed.
However, as shown in
FIG. 23
, if the stop hole f stopping at the copper foil e is machined using carbon dioxide laser, even if the laser beam is sufficiently emitted, the resin of the insulating member of thickness of 1 &mgr;m or less is left over on the copper foil e. This remaining resin after must be removed completely in a later process by etching with permanganic acid or the like.
Some times these holes are very small in diameter. Therefore, the etchant hardly goes inside the hole. If the thickness of the residual resin exceeds 1 &mgr;m due to defective laser machining condition or the like, then the residual resin may not be removed completely in some holes. If the BVH is formed by plating in this state, the resin is partly remaining between the plating film and the inner layer copper foil, and if stress is applied by heat cycle or the like, the plating film may be peeled off from the portion of the residual resin. Accordingly, it is required to inspect the thickness of the residual resin of the stop hole after laser machining.
As an apparatus for such inspection, an optical microscope as shown in
FIG. 24
is used. In this inspection apparatus, white light from a light source
100
is emitted to a printed circuit board W through an objective lens
102
from a beam splitter
101
. The light reflected from the printed circuit board W is magnified by the objective lens
102
, and an inverted real image Ea is formed ahead of a focusing lens
103
, and its real image Eb is taken by a CCD camera
104
.
When the white light is emitted to the surface of the residual resin, it is partly reflected, but the rest passes through the residual resin to reach the copper foil in the bottom. This light that falls on the copper layer is reflected therefrom. Therefore, when white light is emitted to the thin resin layer on the copper foil as illuminating light, the majority of reflected light returns from the copper foil. Thus, it is hard to discriminate the presence of residual resin.
Accordingly, in the inspection apparatus using optical microscope, although a residual resin of more than 10 &mgr;m may be detected, the precision of detection is poor as for residual resin of about several &mgr;m. Thus, this apparatus is not suited to inspection in mass production line, but can only be used to inspect the thickness of the residual resin after cutting and grinding the processing part after plating and observing the section. Further, this apparatus takes too much time in inspection, and all pieces cannot be inspected.
An optical inspection apparatus that uses ultraviolet laser beam is disclosed in Japanese Patent Application Laid-Open No. 7-83841. This optical inspection apparatus comprises, as shown in
FIG. 25
, an ultraviolet laser source
200
, a collimator lens
201
, a mirror
202
, a beam splitter
203
, a motor-driven rotating polyhedral mirror
204
, a scan lens
205
, a re-focusing lens
206
, a pin hole member
207
, and a photomultiplier
208
.
In this optical inspection apparatus, the laser beam generated by the ultraviolet laser source is magnified by the collimator lens
201
, the magnified laser beam is guided into the rotating polyhedral mirror
46
by way of the mirror
202
and beam splitter
203
, and it is scanned by the rotating scanning mirror
46
and focused on the printed circuit board W to be inspected by the scan lens
205
.
The ultraviolet ray generated from the printed circuit board W by irradiation with laser beam is returned recursively in the reverse route of the incident route, and is guided into the retroreflection detecting system by the beam splitter
203
disposed in the optical path. This ultraviolet reflected light is focused by the re-focusing lens
206
. On the focusing plane of the re-focusing lens
206
, the image near the irradiation point of the laser beam of the printed circuit board W to be inspected is observed. By the pin hole member
207
disposed on this focusing plane, only the central part is separated, and detected by the photomultiplier
208
.
This optical inspection apparatus inspects by scanning over the entire printed circuit board to detect the stop holes and grooves. Accordingly, it takes time. This problem arises because the rotating polyhedral mirror is used, and the laser beam cannot be brought up to the commanded position.
Besides, because the inspection makes use of the reflected light, the inspection result may vary depending on the inclination of the printed circuit board to be inspected. Furthermore, since the reflected light is detected by disposing a mask (pin hole member) on the focusing plane and extracting the light in its central part, the luminance is too low and reliable detection of residual resin is difficult.
The invention is devised to solve these problems, and it is hence an object thereof to present an apparatus for inspecting a printed circuit board of high reliability capable of detecting the residual resin on a copper foil nondestructively, securely, and at high precision and high speed, and moreover an inspection apparatus of printed circuit board having a re-processing function for re-removing the unremoved material.
DISCLOSURE OF THE INVENTION
The invention presents an apparatus for inspecting printed circuit board. This apparatus comprises a laser oscillator, a scanning device for positioning the irradiation position of laser beam emitted from the laser oscillator to a position of arbitrary coordinates in the mutually orthogonal X-axis direction and Y-axis direction being commanded, and a detector for detecting the light generated from the printed circuit board irradiated with the laser beam.
Therefore, the position of arbitrary coordinates commanding the irradiation position of laser beam can be instantly set by t
Kurosawa Miki
Mizuno Masanori
Moriyasu Masaharu
Nishida Satoshi
Takeno Shozui
Mitsubishi Denki & Kabushiki Kaisha
Rosenberger Richard A.
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