Abstract:
There is provided a laser energy measuring unit whose laser energy measuring range is widened. The laser energy measuring unit has a filter provided within an optical path of laser to attenuate energy of the laser, a calculating section for measuring the energy of the laser passing through the filter and a condenser lens provided on one side of the filter for condensing the laser. The filter has a shading portion for blocking a center part of the laser beam from transmitting through the filter at position coincident with the center of the laser. The center part of the laser where its energy is large is cut by the shading portion and the calculating section measures the part where its energy is not so large. Thereby, it is possible to widen the laser energy measuring range.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a laser energy measuring unit and a laser machining apparatus having the laser energy measuring unit. 
         [0003]    2. Related Art 
         [0004]    Hitherto, in case of machining a blind hole (hereinafter referred to simply as a bore) that connects layers of a build-up type printed circuit board by the laser machining apparatus, an insulating layer formed by resin containing fillers such as fiberglass reinforced resin is removed by irradiating laser through an etching window formed by removing an outer copper layer by etching in advance in a conformal mask method or by directly irradiating laser to the insulating layer having no outer copper layer in a direct method. 
         [0005]    In this case, machining results vary when an output of the laser oscillator fluctuates. Then, in order to improve machining reliability, the laser to be supplied to the machining section is partly branched (1 to 2% of the laser to be supplied to the machining section) to measure energy of the branched laser and to machine while confirming that magnitude of the energy (referred to simply as “energy” hereinafter) of the laser is within a predetermined range. 
         [0006]      FIG. 6  is a schematic section view of a prior art laser energy measuring unit along a measuring laser beam. 
         [0007]    A condenser lens  17  and an energy measuring section  19  are disposed on an optical path of the measuring laser beam (called as a “monitoring beam” hereinafter)  2   b  branched from the laser oscillated from the laser oscillator. The energy measuring section  19  is disposed so that a center of a sensor face of a sensor  23  substantially coincides with a focal point of the condenser lens  17 . 
         [0008]    Next, an operation of the laser energy measuring unit will be explained. 
         [0009]    The monitoring beam  2   b  is condensed by the condenser lens  17  and enters the sensor  23 . A calculating section  19   a  of the energy measuring section  19  compares an output of the sensor  23  with a value set in advance. When the energy of the monitoring beam  2   b  deviates out of the preset range, the energy measuring section  19   a  outputs an alarm signal to a control section  30  of the laser machining apparatus. Receiving the alarm signal from the energy measuring section  19 , the control section  30  memorizes coordinates of a hole machined when it received the alarm signal and conducts a process specified in advance (sounds an alert buzzer or sometimes stops machining for example). 
         [0010]    By the way, a diameter of a machining laser beam varies corresponding to a diameter of a bore to be machined. A diameter of the monitoring beam  2   b  entering the sensor  23  also changes and the energy changes along with that. Therefore, it is necessary to use a sensor whose energy detecting range is wide as the sensor  23 . However, the sensor having a wide detecting range is expensive. 
         [0011]    Then, there has been proposed a technology of enabling even a sensor whose detecting range is narrow to measure the energy of laser by attenuating the energy of the monitoring beam  2   b  as disclosed in Japanese Patent Application Laid-open No. 2003-136267 for example. According to this technology, an attenuating member in which through holes of very small diameter are formed in mesh through a plain plate of a material that does not transmit the monitoring beam  2   b  is disposed on an incident side of the condenser lens  17  to attenuate the energy of the monitoring beam  2   b . When the diameter of the monitoring beam  2   b  changes, the attenuating member can attenuate the energy corresponding to the diameter of the monitoring beam  2   b . Accordingly, even the low cost sensor  23  can measure the energy of the monitoring beam  2   b.    
         [0012]    However, an energy attenuation rate of the attenuating member described above is uniform. Therefore, when the attenuation rate is once set by deciding the diameter and density of the through holes disposed on the plate by arranging to a maximum diameter of the monitoring beam  2   b , energy detecting precision drops when the diameter of the monitoring beam  2   b  is minimum, causing variations in the measured results. 
         [0013]    Accordingly, an object of the invention is to provide a laser energy measuring unit whose laser energy measuring range is wide and whose measuring precision is high. 
         [0014]    It is another object of the invention to provide a laser machining apparatus that stops machining so as not to degrade its machining precision when magnitude of the energy of the laser measured by the laser energy measuring unit deviates out of a predetermined range. 
       SUMMARY OF THE INVENTION 
       [0015]    According to the invention, a laser energy measuring unit ( 32 ) having an attenuating member ( 21 ) provided within an optical path of laser ( 2   b ) to attenuate energy of the laser ( 2   b ) and measuring means ( 19 ) for measuring the energy of the laser ( 2   b ) passing through the attenuating member ( 21 ) includes a condenser lens ( 17 ) provided on one side of the attenuating member ( 21 ) for condensing the laser ( 2   b ) to the measuring means ( 19 ) and the attenuating member ( 21 ) has a shading portion ( 21   a ) for blocking a center part of the laser ( 2   b ) from transmitting through the attenuating member at position coincident with the center of the laser ( 2   b ). 
         [0016]    According to the invention, a laser machining apparatus ( 31 ) has branching means ( 16 ) for branching a laser beam ( 2 ) oscillated from a laser oscillator ( 1 ) into a machining laser beam ( 2   a ) for machining workpieces and a measuring (monitoring) laser beam ( 2   b ) for measuring energy of the laser beam ( 2 ) and a laser energy measuring unit for measuring the energy of the measuring laser beam ( 2   b ) and detecting an abnormal condition when the measured value of the measuring laser beam deviates out of a predetermined range. The laser energy measuring unit is the laser energy measuring unit ( 32 ) described above. 
         [0017]    The laser energy measuring unit ( 32 ) attenuates the laser beam ( 2   b ) by the filter ( 21 ) before or after the condenser lens ( 17 ) and condenses the monitoring beam ( 2   b ) by the condenser lens ( 17 ) to measure the energy of the laser beam ( 2   b ) by the energy measuring section ( 19 ). The attenuating member ( 21 ) has the shading portion ( 21   a ) for blocking the center part of the laser beam ( 2   b ) from transmitting through the attenuating member at the position coincident with the center of the laser beam ( 2   b ) to shade the center part of the laser beam ( 2   b ). Therefore, the laser energy measuring unit ( 32 ) blocks the energy of the center part that largely differs corresponding to the diameter of the laser beam ( 2   b ) and measures the energy of the part whose energy is not so large by the energy measuring section ( 19 ). 
         [0018]    The laser machining apparatus ( 31 ) of the invention branches the laser ( 2 ) oscillated from the laser oscillator ( 1 ) by the beam splitter ( 16 ) and the laser energy measuring unit ( 32 ) measures the branched measuring laser ( 2   b ) to detect the abnormal condition when the measured energy value deviates out of the predetermined range. The laser machining apparatus ( 31 ) stops machining when the laser energy measuring unit ( 32 ) detects the abnormal condition. 
         [0019]    It is noted that the purpose of the reference numerals within the parentheses above is to collate them with parts within the drawings and do not limit the structure of the invention by any means. 
         [0020]    Accordingly, the laser energy measuring unit of the invention brings about the following effects because it measures the energy of the part of the laser whose energy is not so large. 
         [0021]    That is, it becomes possible to measure the energy even by a low cost measuring means. It is also possible to widen the laser energy measuring range. Still more, it is possible to enhance the laser measuring precision and to set an attenuation rate of the attenuating member without linking to the diameter of the laser. 
         [0022]    The laser machining apparatus of the invention may be also arranged so as not to degrade its machining precision because it stops machining when the magnitude of the energy of the laser measured by the laser energy measuring unit deviates out of the predetermined range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is an exploded perspective view of a laser machining apparatus according to an embodiment of the invention; 
           [0024]      FIG. 2  is a schematic section view of a laser energy measuring unit of the embodiment of the invention along a measuring laser; 
           [0025]      FIG. 3  is a plan view of a filter of the laser energy measuring unit of the embodiment of the invention; 
           [0026]      FIGS. 4A and 4B  are graphs showing distributions of intensity of the laser entering a beam splitter on a plane perpendicular to an optical axis of the laser; 
           [0027]      FIGS. 5A and 5B  are graphs showing energy of a monitoring beam passing through the filter on the plane perpendicular to an optical axis of the laser beam; and 
           [0028]      FIG. 6  is a schematic section view of a prior art laser energy measuring unit along a measuring laser beam. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    A laser energy measuring unit according to an embodiment of the invention and a laser machining apparatus having this measuring unit will be now be explained. 
         [0030]      FIG. 1  is an exploded perspective view of the laser machining apparatus of the embodiment of the invention,  FIG. 2  is a schematic section view of the laser energy measuring unit of the embodiment of the invention along a measuring laser and  FIG. 3  is a plan view of a filter of the laser energy measuring unit of the embodiment of the invention. 
         [0031]    The laser machining apparatus will be explained first. 
         [0032]    The laser machining apparatus  31  is arranged so as to make bores through a printed circuit board  8  by a laser beam  2   a . A pulse shaper  18 , a total reflection mirror  4   a , a mask  12 , a total reflection mirror  4   b  and a beam splitter  16  are disposed on an optical axis of the laser  2  outputted out of a laser oscillator  1 . The laser oscillator  1  outputs the pulse-wise laser  2 . The pulse shaper  18  is arranged so as to control intensity and pulse width of the laser  2  supplied to a machining section. It is noted that zero-order light  15  not supplied to the machining section is absorbed to a beam damper  14 . A plurality of mask holes  12   a  whose diameter is different (three in case of the figure) is formed through the mask  12  supported by a two-step cylinder  11 . The beam splitter  16  is arranged so as to transmit 2% of the incident laser  2  and to reflect the remainder. 
         [0033]    A total reflection mirror  4   c , scanner mirrors  5   b  and  5   a  and a fθ lens  3  on the reflection side of the beam splitter  16  and a condenser lens  17 , a filter  21  and an energy measuring section  19  are disposed on the transmission side of the beam splitter  16 . The condenser lens  17 , the filter  21  and the energy measuring section  19  compose a laser energy measuring unit  32 . 
         [0034]    The scanner mirrors  5   a  and  5   b  are positioned by being rotated centering on axes of rotation respectively by scanners  6   a  and  6   b . The total reflection mirror  4   c , the scanners  6   a  and  6   b  and the fθ lens  3  are mounted on a machining head  7 . The machining head  7  is arranged to be movable in a direction of an arrow Z by a Z-axis motor  13 . 
         [0035]    A printed circuit board  8  is fixed on a X-Y table  9  movable in directions of arrows X and Y while facing to the fθ lens  3 . An area of a machining area  10  defined by the diameter of the fθ lens  3  is about 50 mm×50 mm. 
         [0036]    Next, a machining operation of the laser machining apparatus  31  will be explained. 
         [0037]    Prior to machining, the fθ lens  3  is positioned in the direction of the arrow Z based on a surface of the printed circuit board  8  and a center of the machining area  10  to be machined is positioned on an optical axis of the fθ lens  3 . The two-step cylinder  11  is also operated to position the optimum mask hole  12   a  for the diameter of the bore to be machined on the optical axis of the laser beam  2 . 
         [0038]    The pulse shaper  18  sets energy (i.e., intensity and pulse width) of the laser beam outputted out of the laser oscillator  1 . The mask hole  12   a  of the mask  12  shapes an outline of the laser  2 . Then, the laser  2  enters the beam splitter 16.2% of the incident laser  2  transmits through the beam splitter  16  and enters the condenser lens  17  as the monitoring (measuring) beam  2   b . The remaining laser  2  reflected by the beam splitter  16  (called as a “machining beam  2   a ” hereinafter) is positioned by the scanner mirrors  5   b  and  5   a  and enters the fθ lens  3 . Then, the machining beam  2   a  is vertically irradiated to the printed circuit board  8  to create a bore. 
         [0039]    The energy of the monitoring beam  2   b  is measured by the laser energy measuring unit  32  described below. When a value of the measured energy is out of a predetermined range, the laser energy measuring unit  32  informs it to the control section  30  to stop the machining operation of the laser machining apparatus  31 . 
         [0040]    Next, the laser energy measuring unit will be explained in detail. 
         [0041]    The laser energy measuring unit  32  measures the energy of the monitoring beam  2   b  and informs the control section  30  to stop the machining operation of the laser machining apparatus  31  carried out by the machining beam  2   a  when the value of the measured energy is out of the predetermined range. 
         [0042]    The laser energy measuring unit  32  includes the condenser lens  17 , the filter  21 , the energy measuring section  19  and others as shown in  FIG. 2 . 
         [0043]    The filter  21  is made from a copper plate for example and through holes  22  having a small diameter (diameter d=0.5 mm for example) with a pitch of P=1 mm for example are formed in grid in the directions of arrows X and Y, except of a circular shading portion  21   a  that is formed centering a center position O and that blocks the transmission of the laser as shown in  FIG. 3 . It is noted that (Ds) in  FIG. 3  indicates a range into which the monitoring beam  2   b  enters when the diameter of the mask hole  12   a  is small and (DL) indicates a range into which the monitoring beam  2   b  enters when the diameter of the mask  12  is large. 
         [0044]    The monitoring beam  2   b  transmitting through the beam splitter  16  ( FIG. 1 ) enters the sensor  23  of the energy measuring section  19  by being condensed by the condenser lens  17  and by being attenuated by the filter  21  as shown in  FIG. 2 . A calculating section  19   a  of the energy measuring section  19  compares an output of the sensor  23  with a value set in advance. Then, when the energy of the monitoring beam  2   b  deviates out of the range set in advance, it outputs an alarm signal that informs of an abnormal condition to the control section  30  that controls the whole laser machining apparatus  31 . 
         [0045]    Next, a role of the filter  21  will be explained. 
         [0046]      FIGS. 4A and 4B  are graphs showing distributions of intensity of the laser entering the beam splitter  16  on a plane perpendicular to the optical axis of the laser  2  and  FIGS. 5A and 5B  are graphs showing energy of the monitoring beam  2   b  passing through the filter  21  on the plane perpendicular to the optical axis the monitoring beam  2   b . Axes of abscissa of  FIGS. 4 and 5  represent distances from the optical axis and axes of ordinate represent the energy. Then, areas of regions  24  and  25  denoted by slant lines represent amounts of energy. 
         [0047]    While the diameter of the laser  2  transmitting through the mask hole  12   a  gradually increases, the rate of increase of the diameter and the distribution of intensity (i.e., distribution of energy) on the plane perpendicular to the optical axis differs depending on the diameter of the mask hole  12   a . That is, when the diameter of the mask hole is large, the intensity of the center part is large and the diameter of the beam does not increase (is not widened) so much as shown in  FIG. 4A . In contrary to that, when the diameter of the mask hole is small, although the intensity of the center part does not increase so much, the diameter of the beam becomes large (widened) as shown in  FIG. 4B . 
         [0048]    Because the filter  21  has the shading portion  21   a  at its center, the monitoring beam  2   b  having the distribution of energy of  FIG. 4A  will have a distribution of energy as shown in  FIG. 5A  when it transmits through the filter  21 . The monitoring beam  2   b  having the distribution of energy of  FIG. 4B  will have a distribution of energy as shown in  FIG. 5B  when it transmits through the filter  21 . 
         [0049]    Then, there is almost no difference between the slanted area of the region  24   a  and that of the region  25   a  (i.e., the amount of energy detected by the sensor  23 ) in  FIGS. 5A and 5B . That is, the energy of the monitoring beam  2   b  entering the sensor  23  is substantially the same regardless of the diameter of the mask holes  12   a.    
         [0050]    Accordingly, no saturation of output signal nor burning damage of the sensor  23 , that may otherwise occur due to excessive energy of the monitoring beam  2   b , occur. The sensor is not also affected by noise that may otherwise occur when the energy of the monitoring beam  2   b  is too small. Still more, a general purpose sensor may be used as the sensor  23 . 
         [0051]    It is noted that the shape of the filter  21  shading the center part of the monitoring beam  2   b  is not limited to be circular and it may have another shape such as a polygonal shape. 
         [0052]    Still more, it is also possible to prepare a plurality of filters whose diameter d and pitch P of the through holes  22  are changed to replace corresponding to the output of the laser  2 .