Abstract:
Apparatus for drilling a via-hole in a printed circuit board (PCB) includes a carbon monoxide laser deliver laser radiation pulses. The pulses have a relatively broad wavelength-range, and slow rising and falling edges. The rising and falling edges of the pulses are clipped using and acousto-optic modulator. A dispersion-compensator compensates for dispersion in the clipped pulses introduced by the AOM. Achromatic focusing optics focus the dispersion-compensated, clipped pulses on the PCB for the via-hole drilling.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to laser drilling of via holes in printed circuit boards (PCBs). The invention relates in particular to laser drilling PCBs using long wavelength infrared radiation from a gas discharge laser. 
         [0002]    DISCUSSION OF BACKGROUND ART 
         [0003]    Sealed off radio-frequency (RF) excited carbon dioxide (CO 2 ) lasers are currently favored for drilling via-holes (via-drilling) in PCBs. These lasers are relatively compact in relation to available output power. By way of example, a laser less than one meter (m) in length can deliver a beam of long-wavelength infrared (IR) radiation at wavelength around 10.6 micrometers (μm) with an average power of 400 Watts (W) or greater. While CO 2  laser via-drilling is fast and efficient, there will be an eventual limit to the smallest hole-size that can be drilled due to the long wavelength of the laser beam. Demands on minimum hole-size and spacing are increasing rapidly for PC boards used in so called “smart phones”, which, practically considered, are hand-held portable computer devices with many more functions than simply making and receiving phone calls. In a current state-of the-art smart phone, a PCB may have as many as 30,000 via-holes. As more functions are added to the smart phones, more complex circuitry with more via holes will be required, and, eventually a shorter drilling wavelength will be required to achieve smaller hole-size and closer spacing. 
         [0004]    It has been postulated that a preferred via-hole drilling wavelength would be a so-called mid-IR (MIR) wavelength between about 2 μm and about 8 μm. Apart from the potential for smaller hole-size and closer spacing, MIR wavelengths have a higher absorption coefficient in PCB materials than at the longer, CO 2  laser radiation wavelengths. This would allow a more rapid absorption of heat into the PCB, which could lead to holes with cleaner walls and less collateral thermal damage in general. 
         [0005]    MIR wavelengths could be generated from commercially available near-IR (NIR) emitting, solid-state lasers, fiber-lasers, or optically pumped semiconductor lasers, using sum-frequency generation or optical-parametric generation. This however would require apparatus having many times the cost-per-watt achievable in a CO 2  laser system. 
         [0006]    Arguably, the only potentially viable candidate MIR laser for replacing a CO 2  laser for via-drilling is a CO laser. Recent investigations of sealed off CO-lasers have led to a sealed off CO-laser which is, with only a simple modification, a sealed-off CO 2  laser but with a different lasing-gas mixture. A power output of about 80% of that of a corresponding CO 2  laser has been achieved. 
         [0007]    In via-drilling operations, CO 2  lasers are driven in a pulsed mode. A problem with CO lasers is that when driven in a pulsed mode, pulse rise and fall times are relatively long. Long rise and fall times of pulses can create unacceptable collateral thermal damage around laser-drilled via holes. In theory at least, rise and fall times of laser pulses can be shortened by modulation “clipping” of the laser-pulses using an acousto-optic modulator (AOM). 
         [0008]    This is complicated, however, by a broad wavelength range of CO laser output. CO laser output occurs at range of laser wavelengths between about 4.5 μm and about 6.0 μm. An AOM functions by virtue of a refractive index grating induced in a susceptible crystal such as a germanium (Ge) crystal by applying a high RF voltage to the crystal. Inducing the grating diverts a laser beam from one path through the crystal with no grating induced, into an alternate path at an angle to the un-diverted (RF applied) path. The angle of the diverted path, of course, is wavelength dependent. With a collimated beam having the full bandwidth of the CO laser, a diverted beam would be spread into a fan of rays which would complicate focusing the beam onto a PCB for drilling. 
         [0009]    This beam-spreading by an AOM could be mitigated by limiting the CO laser bandwidth, for example, by using a spectrally selective device such as an etalon or grating within the laser-resonator. The nature of the CO laser, however, is such that output power would be reduced in direct proportion to the degree of spectral-bandwidth reduction. Reducing the spectral bandwidth to proportions compatible with the AOM could reduce the CO laser power to as low as one fifth of that of a comparably sized and pumped CO 2  laser. This and related problems must be overcome for a CO laser to become a commercially viable replacement for a CO 2  laser for via-drilling. 
       SUMMARY OF THE INVENTION 
       [0010]    In one aspect, apparatus in accordance with the present invention comprises an acousto-optic modulator (AOM) and a carbon monoxide (CO) laser. The CO laser emits laser-radiation pulses, with radiation in the pulses having a plurality of wavelengths in a wavelength range between about 4.5 micrometers and 6.0 micrometers. The radiation pulses having a temporal rising edge and a temporal falling edge. The laser radiation pulses are incident on the AOM in an incidence direction. The AOM is arranged to receive the radiation pulses, disperse a central temporal portion of the pulses, excluding a portion of the rising edge and a portion of the falling edge, in a first range of wavelength-dependent dispersed directions at an angle to the incidence direction. Residual portions of the pulses are transmitted by the AOM along the incidence direction. A dispersion-compensator is arranged to receive the central temporal portion of the pulses and reduce the range of dispersed directions to a second range less than the first range. At least one optical element is arranged to achromatically focus the temporal pulse portions from the dispersion-compensator onto the work-piece. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0012]      FIG. 1  is a block diagram schematically illustrating via-drilling apparatus in accordance with the present invention including a CO laser of  FIG. 1  operated in a pulsed manner, with pulses having a broad wavelength range characteristic of the CO laser, an acousto-optic modulator (AOM), for selecting and clipping pulses emitted by the CO laser, a dispersion-compensator for correcting dispersion introduced in the selected and clipped pulses by the AOM, and achromatic focusing optics for focusing the dispersion corrected pulses onto a printed circuit board (PCB) for via drilling. 
           [0013]      FIG. 2  schematically illustrates a preferred embodiment of the apparatus of  FIG. 1 , wherein the dispersion-compensator is a prism, and the achromatic focusing optics include a concave minor and the AOM is driven by a plurality of sequentially switched RF-generators. 
           [0014]      FIG. 2A  is a graph schematically illustrating an example of dispersion-compensation as a function of wavelength in the apparatus of  FIG. 2  wherein the AOM is a germanium-crystal AOM and the prism is a zinc sulfide prism. 
           [0015]      FIG. 3A  and  FIG. 3B  are graphs schematically illustrating a sequential switching scheme for the RF-generators of  FIG. 2 . 
           [0016]      FIG. 4  schematically illustrates one dispersion-compensation arrangement where an AOM is used in a double-pass mode for pulse-clipping and also for dispersion-compensation of the clipped pulses. 
           [0017]      FIG. 5  schematically illustrates another dispersion-compensation arrangement where an AOM is used in a double-pass mode for pulse-clipping and also for dispersion-compensation of the clipped pulses. 
           [0018]      FIG. 6  schematically illustrates yet another dispersion-compensation arrangement where an AOM is used in a double-pass mode for pulse-clipping and also for dispersion-compensation of the clipped pulses. 
           [0019]      FIG. 7  schematically illustrates a dispersion-compensation arrangement wherein one AOM is used for pulse clipping and another AOM is used for dispersion-compensation of the clipped pulses. 
           [0020]      FIG. 8  schematically illustrates a dispersion-compensation arrangement similar to the arrangement of  FIG. 5 , but wherein plus and minus diffraction orders of the AOM produce two clipped pulses for every pulse incident on the AOM. 
           [0021]      FIG. 9  schematically illustrates a dispersion-compensation arrangement wherein the AOM is sequentially driven at two different frequencies to provide two clipped pulses diffracted at different angle from every pulse incident on the AOM, with one prism providing dispersion-compensation for one of the clipped pulses and two prisms providing dispersion-compensation for the other of the clipped pulses. 
           [0022]      FIG. 10  schematically illustrates a dispersion-compensation arrangement similar to the arrangement of  FIG. 8 , but wherein each of the two clipped is dispersion-compensated by a corresponding prism. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates in block diagram form a via-drilling apparatus  10  in accordance with the present invention. Apparatus  10  includes a sealed-off CO laser  12  emitting laser radiation at wavelengths in a range between about 4.5 μm and about 6.0 μm. The laser is operated in a pulsed manner. By way of example, the pulses may have a selected duration between about 100 microseconds (μs) and 160 is at a selected pulse-repetition frequency (PRF) between about 0.5 kilohertz (kHz) and 2.0 (kHz). Such a laser is available from Coherent Inc. of Santa Clara Calif., the assignee of the present invention. 
         [0024]    The pulses are characterized by long rise and fall times, and in fact it can be said that the pulses are either rising or falling in amplitude with no significant period of near-constant amplitude within the pulse-duration. By way of example in a pulse having a nominal duration of 140 is at a PRF of 1.0 kHz, measured rise time was 50 μs, and measured fall time was 84 μs. In order to be useful for the inventive CO laser via-drilling, these pulses must be clipped in both the rising and falling edges. An acousto-optic modulator (AOM)  52  is provided for selecting pulses from the laser output and for effecting the rising edge and falling edge clipping of the selected pulses. 
         [0025]    An AOM functions by diverting radiation from an input path using a “refractive index grating” induced in a germanium crystal by application of a high RF-voltage to the crystal. In the case of the CO laser pulses with such broad bandwidth, this introduces dispersion in a diverted (selected and clipped) pulse-beam. This dispersion is corrected in apparatus  10  by a dispersion-compensator  64 . A turning mirror  68  directs the dispersion-compensated pulse-beam to achromatic focusing optics  70 . The achromatic focusing optics focus the dispersion-compensated pulse-beam onto a PCB  74  for via drilling. 
         [0026]      FIG. 2  schematically illustrates a preferred embodiment  12 A of the apparatus of  FIG. 1 . Here, a pulse-beam diverted and clipped by AOM  52  is diverted into a fan of rays, with the shortest wavelength (λ S ) in the pulse-spectrum being diverted at a lesser angle than the longest wavelength (λ L ) in the pulse-spectrum. The shortest wavelength and longest wavelength rays as depicted in the drawing by respectively solid and dashed lines. The AOM, here, is driven by a RF amplifier  54  fed by RF generators RFG 1 , RFG 2  and RFG 3 . 
         [0027]    Pulses not selected and discarded portions of selected pulses proceed un-diverted to a beam-dump  56 . The discarded portions of the pulses include the “clipped-off” rising-edge and falling-edge portions the pulses and some portion of the diverted pulse beam resulting from less than 100% efficient diffraction by the AOM. 
         [0028]    Dispersion-compensator  64  in apparatus  12 A is a prism  65  configured and arranged to about collimate the fan of rays from the AOM. The term “about collimated” as used here in the appended claims means that it is not necessary that the dispersion-compensated rays are exactly collimated and can be in some reduced range of directions. 
         [0029]    Prism  65  directs the about-collimated beam to achromatic focusing optics  70  which in embodiment  12 A is a concave mirror  72 . Minor  702  focuses the about-collimated beam onto PCB  74 . Those skilled in the art will recognize, without further detailed description or illustration, that a more complex focusing arrangement having more than one element may be used without departing from the spirit and scope of the present invention. Such focusing arrangements may comprise only reflective elements (mirrors), only transmissive elements (lens elements), or some combination of reflective and transmissive elements. 
         [0030]      FIG. 2A  is a graph schematically illustrating calculated dispersion-compensation as a function of wavelength for the prism arrangement of  FIG. 1 . In this case the AOM is a germanium (Ge) AOM and the prism is a zinc sulfide (ZnS) prism. Dispersion-compensation (in milliradians) is the difference between dispersion produced by the AOM and dispersion produced by the prism. It can be seen that at wavelengths between 5.0 μm and 6.0 μm, net dispersion is less than 20 microradians. 
         [0031]    Continuing with reference to  FIG. 2 , and with reference in addition to  FIG. 3A  and  FIG. 3B , in apparatus  10 A, as noted above, AOM  52  is driven by RF amplifier  54  which can be driven, in turn, by any one of the three RF-generators RFG 1 , RFG 2  and RFG 3 . Each of the RF-generators has the same frequency but has selectively variable output amplitude. The purpose of this plural-generator driving arrangement is to be able to “clip” a pulse using a plurality of time sequenced “slices”. Each of the RF-generators clips one slice. The selectively variable amplitude of the RF-generator amplitude (and a corresponding selectively variable efficiency of the induced refractive index grating in the AOM) allows the amplitude of the individual slice to be selectively varied. 
         [0032]      FIG. 3A  is a reproduction of an oscilloscope trace depicting a pulse (solid bold curve) to be sliced. The parallel, vertical, fine solid lines depict the “on” and “off” times of the RF-generators RFG 1 , RFG 2  and RFG 3 , in time-sequence. There will, of course, need to be a very small time interval (not visible in  FIG. 4A ) between switching one RF-generator “off” and switching the next RF-generator “on”. 
         [0033]      FIG. 3B  depicts how an oscilloscope trace (solid bold curve) comparable to that reproduced in  FIG. 3A  would appear with “slicing” according to the time-sequence of  FIG. 3A , but wherein the amplitude of the RFG 1 -slice is greater than the amplitude of the RFG 3 -slice, which, in turn, is greater than the amplitude of the RFG 2 -slice. In the example of  FIG. 3B  the RF generator amplitudes have been selected such that peak power in each pulse slice is about equal. 
         [0034]    It should be noted here that the RF generators can also be operated at different frequencies in which case the three pulse “slices would leave the AOM at different angles and could be used separately, by separate focusing optics for via drilling. This is discussed in more detail further hereinbelow. 
         [0035]      FIG. 4  schematically illustrates one alternative dispersion-compensating arrangement which can be used in apparatus in accordance with the present invention. In this arrangement, a portion  52 A of AOM  52  acts as dispersion-compensator  64 . Here, a pulse beam from CO laser  12  (depicted by a bold solid line) is clipped by AOM  52  in an upper portion thereof. A grid of dotted lines schematically illustrates the refractive index grid induced by application of RF power to the AOM. 
         [0036]    The clipped pulse beam is diffracted into a narrow fan of rays bounded by a longest λ L  (dashed lines) and a shortest λ S  (solid lines). Turning mirrors  102  and  104  in a retro-reflecting arrangement direct the pulse-beam back to a lower portion  52 A of the AOM with incident angles of λ L  and λ S  in the pulse-beam on portion  52 A corresponding to the diffracted angles from AOM  52 . On the second passage through the AOM dispersion is compensated and the λ L  and λ S  rays propagate parallel to each other to the achromatic focusing optics. Unclipped radiation and residuals of clipped radiation proceed in the zero-order direction to beam-dump  56 , as schematically illustrated in the drawing. 
         [0037]    The path length from the AOM back to the AOM is made sufficiently long that the entire laser pulse can traverse the AOM, before the AOM is reactivated, by application of RF power, for providing the dispersion-compensation. This temporally separates the pulse-clipping and dispersion-compensating functions of the AOM. 
         [0038]      FIG. 5  schematically illustrates another alternative dispersion-compensating arrangement which can be used in apparatus in accordance with the present invention. This arrangement is similar to the arrangement of  FIG. 4  with an exception that only one turning-mirror  108  directs the clipped pulse-beam back to portion  52 A of AOM  52 . As in the arrangement of  FIG. 4  the diffracted from, and re-incident angles on the AOM of λ L  and λ S  rays are the same The dispersion-compensated pulse-beam is directed by turning-minor  110  to the achromatic focusing optics. 
         [0039]      FIG. 6  schematically illustrates yet another alternative dispersion-compensating arrangement which can be used in apparatus in accordance with the present invention. This arrangement is similar to the arrangement of  FIG. 4  with an exception that the beam path from the AOM back to the AOM is extended by an f-2f-f relay telescope  112 , formed by concave minors  114  (where f is the focal length of the concave mirrors). The beam at a distance f from minor  114  is imaged at a distance f from mirror  116 . This reduces spreading of the beam. 
         [0040]      FIG. 7  schematically illustrates still another alternative dispersion-compensating arrangement which can be used in apparatus in accordance with the present invention. This arrangement is similar to the arrangement of  FIG. 2  with an exception that a separate AOM  52 B is substituted for the dispersion-compensation prism of the arrangement of  FIG. 2 . As AOM  52  and AOM  52 B can be operated independently, there is no need for an extended optical path between the AOMs and beam spreading can be accordingly limited. 
         [0041]    In all of the above-described dispersion-compensating arrangements only one diffraction-order of AOM  52  is used for pulse-clipping.  FIG. 8  schematically illustrates an arrangement in which the +1 order and −1 order are used to provide, simultaneously, two clipped pulses from each pulse from CO laser  12  incident on AOM  52 . Turning minors  120  and  122  return the two clipped pulses to the AOM for dispersion-compensation, as in the dispersion-compensation arrangement of  FIG. 5 . The dispersion-compensated clipped pulses are directed to separate achromatic focusing optics  70 A and  70 B (not explicitly shown) by turning-minors  124  and  126 , respectively. 
         [0042]    Another dispersion-compensation arrangement used in conjunction with providing two clipped pulses from one incident pulse is depicted in  FIG. 9 . Here AOM  52  is driven sequentially by two different RF frequencies f 1  and f 2 . In the drawing of  FIG. 9  the clipped pulse beams are shown by a single bold line for convenience of illustration. These beams include the diffracted ray-fans described above and depicted in other drawings. In the drawing of  FIG. 9 , f 2  is assumed to be greater than f 1  such that the f 2  clipped-pulse rays are diffracted at a greater angle than the F1 clipped-pulse rays. 
         [0043]    In the arrangement of  FIG. 9 , dispersion-compensation is provided by prism  65  and  67 . Dispersion introduced by f 2  is assumed to be greater than can be compensated by a single prism. The f 2  rays traverse both prisms to provide the compensation. Dispersion introduced by f 1  is sufficiently small that only prism  65  is need to provide dispersion-compensation. The f 2  and f 1  clipped pulses are sent to separate focusing optics as described above with reference to  FIG. 8 . 
         [0044]      FIG. 10  schematically illustrates a dispersion-compensation similar to the arrangement of  FIG. 8  wherein AOM  52  provides simultaneously, two clipped pulses for a single incident pulse, by using the +1 diffracted order and the −1 diffracted order. The diffracted beams are depicted by single bold lines, for convenience of illustration, as in the arrangement of  FIG. 9 . In the arrangement of  FIG. 10 , the diffracted beams are separately dispersion-compensated by prisms  65 A and  65 B, which direct the dispersion-compensated beams to achromatic focusing optics  70 A and  70 B (not explicitly shown), respectively. 
         [0045]    The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather the invention is limited only by the claims appended hereto.