Abstract:
A laser machining apparatus is capable of improving workpiece machining precision. The laser machining apparatus has a movable table for supporting a workpiece to be machined, and a camera for detecting the position of the workpiece by reflection light from an alignment mark formed through the workpiece. The apparatus also includes a jig plate provided between the table and the workpiece, and has light-receiving holes that overlap with the alignment mark (through-hole) and that are larger than the alignment mark (through hole).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a workpiece machining apparatus for machining workpieces by detecting the position thereof by reflection light from position-detecting through holes formed through the workpiece. 
   2. Description of Related Art 
   Hitherto, positioning of a printed circuit board, i.e., a workpiece to be machined by a workpiece machining apparatus, on a table has been carried out based on an alignment mark provided on a surface or in an inner layer of the printed circuit board. Then, there has been a method of irradiating light from the under (bottom) side of the printed circuit board to detect the alignment mark of the printed circuit board whose insulating layer is transparent or semitransparent in silhouette as disclosed in Japanese Patent No. 3023320. 
   Lately, it has become possible to machine a blind hole for connecting an upper layer with an inner layer by laser even in a printed circuit board whose surface layer (uppermost layer) is a copper layer. In this case, a surface treatment of forming a black CuO layer for example is often implemented on the surface of the uppermost copper layer to prevent the laser from being reflected by the copper layer. Then, a through hole is adopted as an alignment mark because the copper layer does not transmit light and it is hard to dispose an alignment mark on the surface by printing or the like. 
     FIG. 6  shows a prior art method for detecting the alignment mark by scanning an image thereof and  FIG. 7  shows a printed circuit board P seen from a direction of an arrow K in  FIG. 6 . 
   An uppermost layer of the printed circuit board P is a copper layer and a black CuO layer whose thickness is about 2 μm is formed on the surface thereof. A thickness of the printed circuit board P is 1 mm to 3 mm. A plurality of through holes  18  (referred to as “alignment marks” hereinafter) is formed through the printed circuit board P. A diameter d 1  of the alignment mark  18  is 1 mm to 3 mm. A ringed (annular) light  22  is disposed at an outer periphery of a camera  20 . The camera  20  is connected with an image processing device not shown. 
   When the alignment mark  18  is illuminated aslant, a profile of the alignment mark  18  becomes ambiguous because a shadow S is made due to the illumination within the alignment mark  18 . Because the image processing device calculates center coordinates of the alignment mark  18  by detecting the profile of the alignment mark  18 , its precision for detecting the center coordinates drops when the profile detecting precision drops. Then, machining precision of the workpiece machining apparatus has dropped as a result. The profile detecting precision drops when a scratch is made around the alignment mark  18  in particular. 
   It may be possible to solve the abovementioned problem by adopting the technique of Japanese Patent No. 3023320. However, a degree of freedom of disposition of the alignment marks decreases when a number of illuminators is reduced. When the number of illuminators is increased to increase the degree of freedom of disposition of the alignment marks, there arise problems in that not only does the structure of the apparatus become expensive, but also a space for disposing vacuum chucking holes becomes narrow. It thus limits a number of holes for chucking the printed circuit board P and lowers a workpiece holding performance. 
   Accordingly, an object of the invention is to provide a workpiece machining apparatus whose workpiece machining precision is improved. 
   SUMMARY OF THE INVENTION 
   According to the invention, a workpiece machining apparatus ( 100 ) has a movable table ( 9 ) for supporting a workpiece (P) to be machined and a position detecting device ( 20 ) for detecting a position of the workpiece (P) by reflection light from a position-detecting through hole ( 18 ) formed through the workpiece (P). The apparatus further includes a plate-like intermediate member ( 10 ) provided between the table ( 9 ) and the workpiece (P) and having light-receiving openings ( 12 ) that overlap with the position-detecting through holes ( 18 ) and whose opening area is wider than that of the position-detecting through hole ( 18 ). 
   Light entering to the position-detecting through hole ( 18 ) reflects on the table ( 9 ) by passing through the light-receiving opening ( 12 ) and illuminates the position-detecting through hole ( 18 ) from the side of the table ( 9 ). Therefore, the whole position-detecting through hole ( 18 ) is illuminated and no shadow of the through hole ( 18 ) is generated. The position detecting device ( 20 ) detects the position-detecting through hole ( 18 ) which causes no shadow and whose profile is not ambiguous, it can detect center coordinates of the through hole ( 18 ) accurately. 
   It is noted that the reference numerals within the parentheses above are denoted for the purpose of collating with parts in the drawings and do not limit the structure of the invention by any means. 
   Accordingly, the workpiece machining apparatus of the invention can improve its workpiece machining precision by steadily detecting the position of the workpiece because it is capable of accurately detecting the center coordinates of the position-detecting through hole. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of a laser machining apparatus as a workpiece machining apparatus of an embodiment of the invention; 
       FIG. 2  is a section view of a table of the laser machining apparatus shown in  FIG. 1 ; 
       FIG. 3  is a plan view of a jig plate of the invention; 
       FIG. 4  is a section view of a light-receiving hole and an alignment mark; 
       FIG. 5  shows a printed circuit board P seen in a direction of an arrow K 1  in  FIG. 4 ; 
       FIG. 6  shows a prior art method for scanning an image of an alignment mark; and 
       FIG. 7  shows a printed circuit board P seen in a direction of an arrow K in  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A preferred embodiment of the invention will be explained below. 
     FIG. 1  is a front view of a laser machining apparatus as a workpiece machining apparatus of an embodiment of the invention,  FIG. 2  is a section view of a table of the laser machining apparatus shown in  FIG. 1  and  FIG. 3  is a plan view of a jig plate of the invention. 
   The laser machining apparatus for machining workpieces by laser is shown in the embodiment. However, because the invention relates to the workpiece machining apparatus arranged so as to be able to accurately machine the workpieces by accurately detecting position of the workpieces, the invention includes not only the light receiving space but also machining apparatuses that machine workpieces by tools such as a drill and end mill. Still more, although a printed circuit board is referred to as a workpiece in the embodiment, the workpiece is not limited to be a printed circuit board. 
   An overall structure of the laser machining apparatus will be explained first. 
   In  FIG. 1 , an X-table  2  of the laser machining apparatus  100  is supported by guides  2 G and is arranged so as to move on a base  1  in a direction perpendicular to a surface of the sheet (in a direction obverse and reverse of the sheet or in a direction of an arrow X in  FIG. 3 ). A Y-table  8  is supported by guides  8 G and is arranged so as to move on the X-table  2  in a direction of an arrow Y. A table  9  is fixed on the Y-table  8 . A jig plate (intermediate member)  10  is fixed on the table  9  by means of bolts not shown. A printed circuit board P is placed on the jig plate  10  and is chucked by air as described later. 
   Mirrors  5 , a pair of optical mirrors  4  and an Fθ lens  3  are disposed on an optical path of a beam  6   a  outputted out of a laser oscillator  6 . The optical mirrors  4  and the Fθ lens  3  are disposed on a head  40  movable in a vertical direction (in a direction of an arrow Z. A camera  20  and illuminants  22  are disposed on a side of the head  40 . A center axis of an X-table  21  of the camera  20  is in parallel with a center axis of the Fθ lens  3 . The illuminants  22  are disposed so as to surround the camera  20 . 
   The table  9  will be explained below. 
   As shown in  FIG. 2 , a space  9   i  formed within the table  9  is connected to a vacuum pump not shown through a joint  9   j . A plurality of vacuum chuck holes  9   h  communicating with the internal space  9   i  is formed on the surface of the table  9 . Thus, the table  9  has an excellent workpiece holding performance, contributing to reduction of a running cost. 
   Next, the jig plate  10  will be explained. 
   As shown in  FIG. 3 , the jig plate  10  has mount holes  11 , the same number of light-receiving holes  12  with alignment holes formed through the printed circuit board P and the vacuum chuck holes  13  formed therethrough. Bolts not shown for fixing the jig plate  10  to the table  9  penetrate through the mount holes  11 . The chuck holes  13  are disposed so that their centers coincide with the other chuck holes  9   h  among the chuck holes  9   h  formed through the table  9  as shown in  FIG. 2 . Thus, the jig plate  10  also has an excellent workpiece holding performance, contributing to the reduction of the running cost. 
   The four light-receiving holes  12  are formed at positions where their centers coincide with the alignment marks (through-holes)  18 . A diameter d 2  of the light-receiving hole  12  is formed so as to meet the following equation 1, where a diameter of the alignment mark (through-hole)  18  is d 1  and positioning precision in setting the printed circuit board P on the jig plate  10  is A (normally about 1 mm). A plate having a thickness of T that meets the following equation 2 is used as the jig plate  10 .
 
 d 2&gt;(1.2 d 1+ A )  Eq. 1
 
T&gt;1.5d2  Eq. 2
 
   The following equation 3 may be found from the equations 1 and 2:
 
( T/ 1.5)&gt; d 2&gt;(1.2 d 1+ A )  Eq. 3
 
   Accordingly, preferably, the diameter d 2  of the light-receiving hole  12  is set as follows:
 
T&gt;d2&gt;d1  Eq. 4
 
   Next, an operation for detecting the alignment mark  18  will be explained.  FIG. 4  is a section view of the light-receiving hole  12  and the alignment mark (through-hole)  18  and  FIG. 5  shows the printed circuit board P seen in a direction of an arrow K 1  in  FIG. 4 . 
   When light sources (illuminants)  22  are lit after positioning a center axis of the alignment mark  18  with the designed center of the alignment mark  18 , part of light illuminated from the light sources  22  enter within the light-receiving hole  12 . The light entering to the inside of the light-receiving hole  12  illuminates the alignment mark  18  from underneath thereof by being reflected by the surface of the table  9 . As a result, no shadow of the alignment mark  18  occurs, and the profile of the alignment mark  18  becomes clear as shown in  FIG. 5 . 
   An image processing device not shown binarizes image data around the alignment mark  18  scanned by the camera  20  (the alignment mark  18  is discriminated as black and the surface of the printed circuit board P is discriminated as white in case of the figure) to detect the profile of the alignment mark  18  and calculates center coordinates of the alignment mark  18  directly from the round profile or from the center thereof. 
   It is noted that laser machining carried out after determining the center coordinates of the alignment mark  18  is the same with the prior art technique, so that its explanation will be omitted here. 
   The invention improves the detecting precision because it allows a width of threshold values in binarizing the image data to be increased (i.e., roughed) as compared to the prior art and discriminate the scratches on the surface of the printed circuit board P around the alignment mark  18  as white. 
   Still more, it becomes possible to detect the center coordinates of the alignment mark  18  accurately, and to improve the machining precision because the laser machining apparatus can detect the profile of the alignment mark  18  accurately. 
   Furthermore, according to the present embodiment of the invention, the printed circuit board P is steadily chucked because the chuck holes  13  that communicate with the chuck holes  9   h  formed through the table  9  are provided on the jig plate  10   
   It is noted that the thickness T of the jig plate  10  is arranged to be 1.5 times or more of the diameter d 2  of the light-receiving hole  12  to make the contrast more clear, it may be around one time. 
   Still more, the material of the jig plate  10  may be metal or synthetic resin, as long as it readily reflects light.