The present invention relates to a laser beam machining apparatus used in processing a printed circuit board or the like.
Conventionally available laser beam machining apparatuses are based on either a method in which processing is effected at a focal position of a laser beam by merely focusing the beam or a method in which processing is effected by inserting a mask into a laser beam transmission path and by reducing an image of the mask and forming it on a processing surface after setting the beam diameter to a desired size.
FIG. 27 is a diagram illustrating a cross section of a multilayered printed circuit board. The multilayered printed circuit board is a printed circuit board in which layers of a printed circuit board are insulated from each other by a resin and are superposed one on top of another. To interconnect wiring portions (copper foils) of one layer and another layer, blind via holes or through holes are formed in the substrate, and the cross sections of the holes are plated, thereby interconnecting the wiring portions between the layers. The diameters of the blind via holes or the through holes are becoming smaller due to the trend toward a higher degree of the packaging density of electronic components. In recent years, hole diameters on the order of several hundred microns have come to be required, and the processing of holes having different diameters have been required for one substrate. With the hole diameters of such a measure, it is difficult to cope with them by the conventional drilling, so that processing by using a laser beam is required. In addition, as the increasing trend toward a greater number of layers in the printed circuit board, the processing of blind via holes having different hole depths is required for one substrate.
Further, due to the higher degree of the density of the multilayered printed circuit board, cut processing has also come to be effected by a laser beam.
To effect the fine hole processing by using a laser beam, it is necessary to reduce the size of the laser beam to the size of the diameter of the hole to be processed. As such methods, the following two methods are known: a method in which the laser beam is focused by a lens (the system based on this method will be hereafter referred to as a focusing optical system) and a method in which a mask is provided in the optical path of the laser beam (laser beam transmission path) and an image of the mask is reduced and formed on a processing surface by means of a lens (the system based on this method will be hereafter referred to as an image-transfer optical system).
Next, a description will be given of the focusing optical system and the image-transfer optical system.
FIG. 28 is a diagram illustrating a conventional focusing optical system. In the focusing optical system, a laser beam is focused by a lens, and processing is effected in the vicinity of the focal position of the laser beam. At that time, a spatial filter such as the one shown in FIG. 29 is sometimes used to eliminate aberration components of the laser beam for the purpose of improving the laser-beam focusing characteristic.
In FIG. 28, reference numeral 1 denotes a laser oscillator; 2, a laser beam; 5, a focusing optical system; 6, an XY table; 7, a workpiece; 8, a reflecting mirror; and 9, a numerical controller (hereafter referred to as the NC device). The focusing optical system 5 is shown in FIG. 29. Numeral 51 denotes a lens; 52, a spatial filter; and 53, a focusing lens. The spatial filter 52 is disposed at a focal position of the lens 51, i.e., at the position where the beam is subjected to Fourier transform. The spatial filter 52 is used to enhance the beam focusing performance by eliminating the aberration components of the beam, and a detailed description is given in, for example, Walter Koechner, "Solid-State Laser Engineering," Springer-Verlag, 1992, pp. 174-180.
Next, a description will be given of the operation.
A laser beam emitted from the laser oscillator 1 is subjected to Fourier transform onto the spatial filter 52 by the lens 51, and the spatial filter 52 transmits only low-frequency components of the spatial frequencies of the laser beam. The beam with its aberration components eliminated by the spatial filter 52 is focused on the surface of the workpiece 7 by the focusing lens 53.
A plurality of processing conditions, i.e., such conditions as a laser output and the like, are stored in advance in a storage device of the NC device 9. Optimum conditions suited to the material, the thickness; and the processing shape of the workpiece 7 are selected, and the oscillator 1, the XY table 6, and the like are controlled on the basis of the selected conditions.
FIG. 30 is a diagram illustrating a conventional image-transfer optical system. A mask is provided in the laser beam transmission path, and an image of a pin hole in the mask is reduced and formed on a processing surface by means of a lens so as to obtain at the processing surface the laser beam having a diameter defined by the mask.
In FIG. 30, reference numeral 1 denotes the laser oscillator; 2, the laser beam; 3, an image-transfer optical system; 6, the XY table; 7, the workpiece; 8, the reflecting mirror; and 9, the NC device. The image-transfer optical system 3 is shown in FIG. 31. Reference numeral 31 denotes a lens; 32, a mask; and 33, a lens. The positional relationship among the mask 32, the lens 33, and the workpiece 7 is in such a positional relationship that the image of the mask is formed on the workpiece 7 at a certain magnification. As for image formation, a detailed description is given in, for example, K. Iizuka, "Engineering Optics," Springer-Verlag, 1985, pp. 145-164.
Next, a description will be given of the operation.
A laser beam emitted from the laser oscillator 1 is made incident upon the image-transfer optical system 3, and is focused by the lens 31 so as to be applied to the mask 32. The reason for focusing is to reduce the energy loss in irradiating the mask having a small diameter. As for the beam which is transmitted through the mask 32, an image of the mask is formed on the surface of the workpiece 7 at a certain reduction ratio by the lens 33.
A plurality of processing conditions, i.e., such conditions as a laser output and the like, are stored in advance in the storage device of the NC device 9. Optimum conditions suited to the material, the thickness, and the processing shape of the workpiece 7 are selected, and the oscillator 1, the XY table 6, and the like are controlled on the basis of the selected conditions.
In the image-transfer optical system, to change the beam diameter on the workpiece 7, a plurality of different masks are prepared in advance, and the mask is replaced by a different one when the beam diameter on the workpiece 7 is to be changed.
Important parameters in the fine-hole processing by using a laser beam are a minimum critical hole diameter R and a maximum critical hole depth DOF. The minimum critical hole diameter R and the maximum critical hole depth DOF can be expressed by the following formulae denoted by the singular reference number (1): EQU R=k1.times.l.times.F EQU DOF=k2.times.l.times.(F.sup.2) (1)
where
F=D/f PA1 k1 (focusing optical system)&gt;k1 (image-transfer optical system) PA1 k2 (focusing optical system)&gt;k2 (image-transfer optical system) PA1 R (focusing optical system)&gt;R (image-transfer optical system) PA1 DOF (focusing optical system)&gt;DOF (image-transfer optical system)
D is an effective diameter of the lens, f is a focal length of the lens, l is a wavelength of the laser beam, k1 and k2 are values which are determined by the material of the workpiece 7 and the state of the laser beam, i.e., the amount of aberration and the mode. A comparison of the values of k1 and k2 between the focusing optical system and the image-transfer optical system is shown below. However, it is assumed that the material of the workpiece 7 is the same, and that the state of the beam is also the same.
That is,
In processing, the image-transfer optical system is capable to processing a hole having a smaller diameter than the focusing optical system, but the depth of the hole which can be processed is shallower in the case of the image-transfer optical system than in the case of the focusing optical system.
FIG. 32 shows processable regions in the focusing optical system and the image-transfer optical system with respect to a certain material subjected to hole processing.
Accordingly, the processing of holes of small diameters is conventionally effected by a laser beam machining apparatus provided with an image-transfer optical system, while the processing of holes of large depths is conventionally effected by a laser beam machining apparatus provided with a focusing optical system. In addition, in the cut processing of multilayered printed circuit boards, since a beam having a large focal depth rather than a beam having a small diameter at the processing surface is required, cut processing is conventionally effected by a laser beam machining apparatus provided with the focusing optical system.
For this reason, in the processing of one multilayered printed circuit board, in order to efficiently and consistently effect the cut processing as well as the processing of through holes and blind via holes having different diameters and depths, it is essential to change over the system between the focusing optical system and the image-transfer optical system in one laser beam machining apparatus.
In the processing of a multilayered printed circuit board using a laser beam, to enhance productivity, an attempt is made to realize high-speed production by moving not only the XY table but also the beam by using a galvanomirror. FIG. 33 shows a laser beam machining apparatus using a galvanomirror.
In FIG. 33, reference numeral 1 denotes the laser oscillator; 2, the laser beam; 19, an image-transfer optical system; 6, the XY table; 7, the workpiece; 8, the reflecting mirror; and 9, the NC device. The image-transfer optical system 19 is shown in FIG. 34. Reference numeral 31 denotes the lens; 32, the mask; 36, an f.theta. lens; 37, a galvanomirror. The positional relationship among the mask 32, the lens 36, and the workpiece 7 is in such a positional relationship that the image of the mask is formed on the workpiece 7 at a certain magnification when the beam is directed vertically downward from the galvanomirror 37 to the f.theta. lens 36.
Next, a description will be given of the operation.
A laser beam emitted from the laser oscillator 1 is made incident upon the image-transfer optical system 19. The beam made incident upon the image-transfer optical system 19 is focused by the lens 31 so as to be applied to the mask 32. The reason for focusing is to reduce the energy loss in irradiating the mask having a small diameter. The beam which has been transmitted through the mask 32 is led to the transferring f.theta. lens 36 by the galvanomirror 37, and an image of the mask reduced at a certain magnification by the f.theta. lens 36 is formed on the surface of the workpiece 7. The galvanomirror 37 is capable of guiding the beam to an arbitrary position on the f.theta. lens 36, with the result that the beam can be scanned by the galvanomirror 37 to the extent corresponding to the region of the size of the f.theta. lens 36 on the workpiece 7.
A plurality of processing conditions, i.e., such conditions as a laser output and the like, are stored in advance in the storage device of the NC device 9. Optimum conditions suited to the material, the thickness, and the processing shape of the workpiece 7 are selected, and the oscillator 1, the XY table 6, the galvanomirror 37, and the like are controlled on the basis of the selected conditions.
The output of a laser oscillator is stable in the vicinity of a rated output, but at a low output the fluctuation width becomes large and the output becomes unstable. Since only several pulses are used in hole processing, variations in the laser output exerts a large effect on variations in the result of processing. Accordingly, the variations in the result of processing become large in a low-output region where the fluctuation width is large.
Since the conventional laser beam machining apparatus is constructed as described above, a number of problems that are listed below have been encountered.
The laser beam machining apparatus having only the focusing optical system is incapable of processing small-diameter holes since its minimum critical processing hole diameter is larger than that in the case of the laser beam machining apparatus having only the image-transfer optical system. Meanwhile, the laser beam machining apparatus having only the image-transfer optical system is incapable of processing deep through holes and blind via holes since the depth of the holes which the apparatus can process is shallower than that in the case of the laser beam machining apparatus having only the focusing optical system. In addition, in cut processing, if the thickness of the printed circuit board becomes large, a beam having a large focal depth at the processing surface is required, so that processing cannot be effected by the laser beam machining apparatus having only the image-transfer optical system.
Further, in the image-transfer optical system, the mask 31 must be changed in order to change the beam diameter on the workpiece 7. Accordingly, productivity declines by the portion of the time need for mask replacement. Additionally, even if a number of kinds of masks with different hole diameters are prepared, it is impossible to cope with the processing of holes with continuously changing diameters.
If a comparison is made between a case where the beam is directed vertically downward from the galvanomirror 37 to the f.theta. lens 36 and a case where it is directed diagonally downward, the distance between the mask 32 and the f.theta. lens 36 differs by L1-L0, as shown in FIG. 35. Consequently, in the case where the beam is directed diagonally downward from the galvanomirror 37 to the f.theta. lens 36, the image of the mask 32 on the workpiece 7 becomes blurred, so that satisfactory processing cannot be effected.
In the low-output region where the fluctuation width is large, variations in the result of processing become large.