EP0198076A1 - Procede et appareil de percement a laser - Google Patents

Procede et appareil de percement a laser

Info

Publication number
EP0198076A1
EP0198076A1 EP85905950A EP85905950A EP0198076A1 EP 0198076 A1 EP0198076 A1 EP 0198076A1 EP 85905950 A EP85905950 A EP 85905950A EP 85905950 A EP85905950 A EP 85905950A EP 0198076 A1 EP0198076 A1 EP 0198076A1
Authority
EP
European Patent Office
Prior art keywords
workpiece
pulse
sequence
pulses
drilling
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP85905950A
Other languages
German (de)
English (en)
Inventor
George Anthony Zahaykevich
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Advanced Laser Systems Inc
Original Assignee
Advanced Laser Systems Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Advanced Laser Systems Inc filed Critical Advanced Laser Systems Inc
Publication of EP0198076A1 publication Critical patent/EP0198076A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/02—Constructional details
    • H01S3/025—Constructional details of solid state lasers, e.g. housings or mountings
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/062—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam
    • B23K26/0622—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/073—Shaping the laser spot
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/38—Removing material by boring or cutting
    • B23K26/382—Removing material by boring or cutting by boring
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/40—Removing material taking account of the properties of the material involved
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/08—Non-ferrous metals or alloys
    • B23K2103/12—Copper or alloys thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/16—Composite materials, e.g. fibre reinforced
    • B23K2103/166—Multilayered materials
    • B23K2103/172—Multilayered materials wherein at least one of the layers is non-metallic
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/30—Organic material
    • B23K2103/42—Plastics
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/50—Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/0011—Working of insulating substrates or insulating layers
    • H05K3/0017—Etching of the substrate by chemical or physical means

Definitions

  • This invention relates to apparatus, including lasers, for drilling holes in a variety of materials.
  • most holes in materials are drilled by mechanically removing material by means of a drill.
  • the drill cuts a spiral chip out of the material to make the hole.
  • Special drills and backing surfaces must be used to prevent drill "wander" and drilling speeds must be reduced.
  • the laser removes material by vaporizing the material or by melting the material and blowing the melted material out of the resulting hole by means of the shock wave which accompanies the laser beam arrival at the workpiece.
  • the first laser drilling apparatus utilized solid state lasers, generally ruby lasers.
  • the ruby laser has some drawbacks which made it inefficient in normal industrial drilling applications. Specifically, the ruby laser could only be practically operated in a pulsed mode. In addition, the power output of a practical ruby laser system was limited, and thus the system required multiple pulses to drill any but the thinnest materials. Consequently, the drilling rate was slow.
  • ruby systems were used to drill small apertures in such materials such as tungsten and molybdenum for . . use in electron tubes and cathode ray tubes. An example of such a system is disclosed in U. S. Patent 3,265,855.
  • gas lasers such as C0 2 lasers were developed, and, due to their ability to produce high power outputs and to operate in a continuous mode, industrial C0 2 gas lasers became very popular for drilling applications.
  • a very high-power laser was used and, although most of the energy was reflected,, the amount that was absorbed was sufficient to drill the desired hole.
  • the high-power CO- laser could drill holes in copper and gold, the necessary power caused additional problems.
  • a typical laser drilled hole differs from a mechanically drilled hole in that it has a pronounced entrance and exit taper.
  • the side walls of the laser-drilled hole are often coated with a "recast" layer of material which is formed by liquified material that resolidifies on the walls of a hole.
  • Printed circuit boards generally consist of epoxy/fiberglass insulating layers sandwiched between copper or gold metallic conducting layers.
  • the high-power C0 2 lasers did not drill the outer metallic layers efficiently and the high-power pulses used caused formation of oxides on the metallic surface and heated the outer layers of the printed circuit board causing them to become delaminated from the underlying epoxy/fiberglass.
  • the high-power pulses often melted some of the epoxy/fiberglass material causing a "smeared" coating on the hole which prevented subsequent metallic plating of the holes.
  • Nd:YAG neodynium: yttrium-aluminum-garnet
  • S. Patent 3,601,576 discloses a low energy pulse followed by a succession of higher energy pulses
  • S. Patent 3,962,558 discloses a high energy pulse followed by a succession of lower energy pulses
  • each laser output pulse actually consists of a plurality of very short time duration, high-energy pulses, rather than a long duration medium energy pulse as occurs in a laser with a non-spiky output.
  • the short time duration pulses tend drill by vaporizing material rather then heating a large area and causing melting.
  • Ruby laser systems have several inherent problems, in particular,- it has been difficult to control the output characteristics of the ruby laser because the ruby crystal is extremely temperature sensitive and is also highly sensitive to the output energy produced by the flashlamp used to excite the ruby crystal.
  • prior art ruby drilling systems could not accurately produce consistent pulses and have been limited to drilling materials in one pulse or in a series of pulses of the which generally only the peak energy of the pulse can be controlled, in such prior art systems it has been found that hole size and shape can only be poorly controlled, in addition, prior art ruby laser systems were generally capable of producing only about one pulse per second and, thus, the drilling rates were unacceptably slow.
  • a precisely-controlled laser system is operated by a computer to deliver a pre-programmed train of light pulses to a workpiece.
  • the peak power, spot size and pulse spatial shape can be preset for each pulse in the pulse train independently from the other pulses in the train by accurate control of the laser flashlamp supply voltage.
  • Different pulse trains for different materials can be stored in the computer and repeated upon demand.
  • a small "pilot hole” can first be drilled through the material by a number of pulses in a chain. The hole can subsequently be widened to the final diameter by changing the pulse shape of the remaining pulses in the chain.
  • a train of low-power pulses can be used to drill through the initial metallic layer. The low pulse power prevents delamination of the metallic layer. After the metallic layer has been pierced, higher-power pulses can be used to drill through the underlying insulating board. Finally, at the far end of the hole, low-power pulses can be used to drill through the opposite metallic layer, again to prevent delamihation.
  • the hole diameter can be changed by accurately controlling the flashlamp voltage.
  • a change of the flashlamp voltage changes the effective focal length of the optical system due to an effect known as ⁇ thermal lensing" of the laser rod. using this effect, the diameter of the pilot hole can be quickly and easily widened to the final size. Holes with an entrance taper, no entrance taper or an exit taper can also be produced with the inventive system.
  • Figure 1 shows a front view of the laser drilling system including a T.V. monitor, the computer and the laser head unit.
  • Figure 2 shows a side view of the illustrative laser drilling apparatus.
  • Figure 3 is a schematic view of the main system components, including the power supply arrangement and the optical system of the laser drilling apparatus.
  • Figure 4 is a cross-sectional, cut-away diagram of the laser head.
  • Figure 5 is a cross-sectional, cut-away diagram of the laser rod support and contoured water jacket.
  • Figure 6 shows schematically the memory file layout of a laser firing sequence within the computer memory.
  • Figure 7 is a flow diagram of a sample computer program which is used to control the flashlamp power
  • Figure 8 is an electrical schematic diagram of the circuit which interfaces the computer to the flashlamp power supply.
  • Figure 9 is an electrical schematic diagram of the power supply comparator circuit which allows . closer control of the power supply voltage.
  • Figure 10 is a cross-sectional view of the workpiece drilled in accordance with Example 1 20 herein.
  • Figure 11 is a cross-sectional view of the workpiece drilled in accordance with Example 2 herein.
  • the drilling apparatus consists of a cabinet 1, which is approximately the size of. an ordinary office desk.
  • the cabinet houses a power supply 2 on its left side and a refrigerant
  • the exact computer which-can be used with the system is not important for operation of the inventive drilling system, in the preferred embodiment, a Model 11+ personal computer manufactured by Apple Computer, incorporated, Cupertino, California, is used. Similarly, personal computers manufactured by the international Business Machines Corporation, Armonk, New York, or other manufacturers can be used.
  • the laser and optical stand 4 is arranged with the laser 5 mounted horizontally on the top and stand 4 has an optical bench 6 arranged vertically down its front face.
  • the optical bench 6 contains two lenses 7 and 9 and a pinhole 8 which are arranged in series, as will be described hereinafter in more detail.
  • T.V. camera 11 which is connected, via cable 12, to monitor 10. Camera 11 is focussed on workpiece 14 and can be used to monitor workpiece 14 after each series of laser pulses.
  • the exact model of the television monitor and camera is not important to the operation of the invention as long as the camera and monitor have sufficient resolution to satisfactorily observe the progress of the drilling.
  • a Model 5000 high-resolution camera and monitor system manufactured by COHU, Inc. San Diego, CA are used.
  • 5 camera lens is used to permit the camera to be kept on at all times.
  • FIG. 2 is a side view of the laser drilling apparatus showing the cooling system control panel 20. Also shown in more detail is the optical stand
  • a laser assembly 5 consisting of a ruby rod and its cooling jacket 34 and a flashlamp and its cooling jacket ' 36. Both rod 34 and flashlamp 36 are " enclosed in a reflector 35. Two mirrors 32 and 33 are mounted on the optical stand 4
  • a conventional refrigerated cooling system is used.
  • the. system uses distilled, de-ionized water which is circulated by a circulator pump through cooling jackets surrounding the ruby rod and flashlamp. The flow rate of the water is adjusted to about four gallons per minute at 15 P.S.I.
  • the cooling water is in turn chilled by a conventional refrigerant system.
  • the cold refrigerant liquid removes heat from the water by means of a heat exchanger which is a conventional unit made from copper using a tube-in-tube design.
  • the refrigerant system is controlled electronically.
  • the electronic control utilizes a platinum sensor to activate a controller which, in turn, operates a valve to control the flow of refrigerant to the heat exchanger.
  • Use of the electronic control eliminates compressor cycling and provides a very quick response, in turn, giving good accuracy and stability.
  • Temperature regulation can also be improved using conventional computer monitoring techniques. Using well-known conventional refrigeration techniques, a temperature control accuracy on the order of +.0.1 degrees centigrade at the laser head can be reliably achieved.
  • the design of the cooling water jacket is important, in a ruby laser, cooling is normally a problem because the ruby rod has a relatively low coefficient of thermal conductivity at normal ambient temperatures, in addition, the coefficient of thermal conductivity changes with rod temperature. Thus, in high-power output situations, the rod can become unevenly heated resulting in stresses and eventual failure.
  • an efficient cooling system must uniformly cool the entire rod.
  • the cooling system is designed to keep the rod at 40 degrees centigrade +_0.1 degree Centigrade, in order to reliably hold the rod at this temperature, coolant is pumped directly over the rod by means of a glass cooling jacket.
  • the glass cooling jacket which surrounds the rod must be specially contoured to prevent uneven cooling, as will hereinafter be explained in detail in connection with Figure 4.
  • the conventional way of firing a laser pumping flashlamp is to utilize a high-voltage D.C. supply to charge a large capacitor. After the capacitor is fully charged the power supply is disconnected and, when it is desired to flash the lamp, electronic switches are closed to connect the charged capacitor across the flashlamp which, in turn, causes” the energy in the capacitor to be released into the lamp.
  • D.C. high-voltage D.C. supply
  • electronic switches are closed to connect the charged capacitor across the flashlamp which, in turn, causes” the energy in the capacitor to be released into the lamp.
  • a conventional power supply/capacitor firing arrangement can only regulate the voltage applied to the flashlamp to within 5% of the desired value on each lamp firing.
  • Prior art methods are known which can reduce this voltage variation.
  • some conventional power supplies have internal circuitry which recharges the charge storage capacitor during the holding time between the charging and the firing. These power supplies typically repetitively generate a desired firing voltage with only a 1% voltage variation.
  • An example of such an improved power supply/ capacitor system is a Model HVD-5000, 5-killijoule power supply manufactured by the Candella Corporation, Natick, MA.
  • this power supply must be specially controlled to bring the final voltage variation in the range of 0.1% to 0.05%.
  • This special control is carried out by a comparator circuit which monitors the output voltage and continually recharges the firing capacitors, as will be hereinafter described.
  • Figure 3 of the drawing shows a schematic view of the illustrative system including the power supply arrangement and the optical system.
  • the ruby laser rod is schematically shown as rod 34.
  • Rod 34 is excited by a xenon flashlamp 36.which is fired by the power supply consisting of high voltage power supply 2, simmer supply and dump switch 51, capacitor 50 and choke coil 52.
  • a flashlamp suitable for use with the preferred embodiment is a Model FX-227C-6 flashlamp manufactured by EG&G Incorporated, Waltham, Massachusetts.
  • Power supply 2 is a conventional design which is capable of charging capacitor 50 to a voltage of . between 0 and 2500 VDC. The voltage to which the capacitor is charged is controlled by the value of the voltage input V ref provided to supply 2 by the interface circuit 40.
  • Interface circuit 40 also provides other signals to power supply 2 which control supply 2 and allow it or prevent it from charging capacitor 50. These signals are schematically shown as charge inhibit signals on Figure 3 and will be described in more detail hereinafter.
  • Dump switch 51 fires flashlamp 36 by connecting capacitor 50 to choke coil 52 under control of a trigger signal developed by the interface circuit 40.
  • the trigger signal is also provided to the power supply 2 to prevent it from attempting to charge capacitor 50 while the flashlamp 36 is being fired.
  • Choke coil 52 delivers a shaped charge pulse to lamp 36 to fire it.
  • Power supply 2 internally contains a comparator which continuously compares the output voltage to the desired voltage (as determined by the value of V ref ) to indicate when the output voltage is equal to the desired voltage, in particular, the output of the comparator is provided as signal V d * to interface circuit 40.
  • Signal V rdy * becomes "low when the output voltage is equal to the desired voltage, and, as will hereinafter be described, this -17-
  • interface circuit 40 uses interface circuit 40 to generate a trigger signal which is provided to dump switch 50 to, in turn, flash lamp 36.
  • Power supply 2 also contains internal circuitry (discussed in detail in connection with Figure 9) which continuously recharges capacitor 50 to compensate for any voltage drift between the time when the capacitor is fully charged and the lamp firing time. This recharging circuitry also operates from the internal comparator so that the power supply output is always close to the desired value when interface circuit 40 fires the lamp. This close control helps to achieve accurate control of the laser output pulses.
  • Supply 2 receives programming commands, via interface circuit 40, from computer 13. As will be described hereinafter in detail, interface circuit 40 receives a digital command word from computer 13. The value of this word is converted into an analog voltage which is provided as the control voltage V ref to power supply 2. Interface circuit 40 also interacts with computer 13 by means' of a status word and handshake signals to insure proper coordination of the firing sequences, to introduce proper timing delays and to insure safety for those operating the system.
  • Computer 13 can be programmed to store a series of data files each of which programs supply 2 to produce a train of predetermined voltages. When these voltages are applied to lamp 36, it excites laser rod 34 to produce output pulses with predetermined shapes, intensities and energies.
  • the output beam 45 of laser rod 34 is passed through a conventional condensing lens 7 and impinges on pinhole 8.
  • Pinhole . 8 blocks the marginal light rays and creates a waist 46 in the beam.
  • the diameter of pinhole 8 depends on the . exact lens arrangement and the physical distances involved. In the illustrative embodiment, pinhole 8 has a diameter approximately 2-3 times the desired spot size on the workpiece. In the illustrative embodiment, the diameter of pinhole 8 is .020 inch.
  • the pinhole 8 acts as a . new light source. The light from this source (beam 47) is directed to an additional condensing lens 9 which focuses the beam on workpiece 14 at spot 48.
  • FIG 4 shows a detailed sectional diagram of laser head 5 of the illustrative drilling apparatus.
  • Laser head 5 is of conventional design consisting of a flash lamp 36 and ruby crystal rod 34 closed in elliptical reflector 35.
  • rod 34 is a 0.25 inch diameter by 6 inch long c-axis ruby crystal of the highest commercial quality.
  • the elliptical reflector structure 35 which encloses both rod 34 and flashlamp 36 is an brass reflector with a silvered inner surface, as is conventional. In order to insure maximum power output, both the rod and the flashlamp must be precisely located at the foci of the ellipitical reflector.
  • the elliptical pump cavity is completed by end reflectors 58 and 59 which are reflecting surfaces. Surfaces 58 and 59 are backed by end plates 61 and 63, respectively. Located inside the reflecting surfaces 58 and 59 are forming plates 56 and 57 which are used to hold the brass sheet 35 in an elliptical shape.
  • the laser cavity is formed by mirrors 32 and 33 which are connected to stands 170 and 172 by means of adjusting nuts 110 and 112 respectively.
  • the adjusting nuts allow the mirrors to be adjusted so that they are perpendicular to the laser beam.
  • the laser beam 45 exits by the right hand side of the unit and is reflected from front-silvered mirror 31 in a downward direction the workpiece (not shown).
  • the mirrors In order to prevent damage by the laser beam the mirrors must be highly reflecting on the order of 99.999% reflectivity at the operating optical frequency.
  • Both laser rod 34 and flashlamp 36 are enclosed by cooling jackets to permit water cooling of the units as previously described, in particular, rod 34 is enclosed by quartz jacket 80.
  • This jacket should be transparent to all optical frequencies of interest.
  • ends 82 and 84 and jacket 80 are contoured to ensure uniform water flow-over the ends of the rod and the holding structures.
  • the water jacket is completed by water inlet/rod holder fittings 60 and 62. In operation, cooling water enters the assembly by means of inlet pipe 66 and leaves the assembly by means of pipe 64.
  • the ends of the rod assembly are fitted with nitrogen purge fittings 66 and 68 which allow the ends of the rod to be surrounded with dry nitrogen gas, via fittings 70 and 72, respectfully.
  • Rod 34 is. fixed in rod holder fitting 100.
  • a seal is made around the periphery of the rod at its end by O-ring 92 which
  • the end of rod 34 is restrained only by friction of O-ring 92.
  • the rod is free to move due to thermal stresses, in particular, the shape of the rod changes during the firing of a laser pulse due to a phenomenon known as "thermal lensing".
  • thermal lensing a phenomenon known as "thermal lensing"
  • the thermal lensing effect is repeatable and can be used with advantage to change the shape and pulse width of the laser pulse as well as the effective focal length of the system in order to drill optimally certain materials.
  • Rod holder fitting 100 is, in turn, sealed to the inlet fitting 62 by means of O-ring 94.
  • the coolant water enters the structure via inlet pipe 66 and proceeds down space 83 between water jacket 80 and rod 34 to cool the rod.
  • the end 82 of jacket 80 is contoured to match the slope of rod holder 100. This contouring ensures a uniform flow velocity over the ends of the rod and ensures that no eddies are formed which would cause uneven rod heating and imprecise performance.
  • inlet fitting 62 is attached to endplate 63 by means of screws (not shown) in order to mount the the rod holder assembly in the end plate.
  • fitting 68 Attached to the end of O-ring compressor fitting 102 is purge fitting 68 which has a transparent window 69. As previously mentioned, fitting 68 allows cavity 101 to be purged with dry nitrogen via pipe 72.
  • flashlamp 36 is also surrounded by a glass water jacket 150.
  • cooling water enters jacket 150 by means of tube 128, flows around electrode 120, down jacket 150, around electrode 122 and exits via pipe 130.
  • pipe 130 is connected to inlet fitting 66 so that water flows around flashlamp 36 then through jacket 80 around rod 34 and exits via fitting 64.
  • suitable materials include antiultraviolet quartz or pyrex.
  • Power to lamp 36 is provided by electrodes 120 and 122 which are connected to the high voltage power supply (not shown in Figure 4) via leads 124 and 126, respectively.
  • the inventive laser drilling system is controlled by a computer.
  • the computer can controls the power supply voltage provided to the flashlamp in precise 5-volt steps for each shot fired and the computer is capable of firing a pre-programmed sequence of shots with any given voltage. This sequence of shots can be pre-programmed for each particular material to be drilled providing for optimum drilling speed and hole shape for each material.
  • Figure 6 of the drawing shows the organization of the control data base structure within the computer memory which stores the information that is used to control the laser system for each output pulse sequence, in particular, the database is arranged in a plurality of data "files".
  • Each of the files contains information regarding a set of power supply voltages which are used to develop a particular pre-programmed series of laser pulses or ⁇ shots".
  • a number of files can be stored in a storage device, such as a magnetic disk or tape, and recalled in any sequence to generate various output pulse patterns.
  • the information in each file is organized in an "array" or matrix which consists of N+l rows by 4 columns.
  • Each row and column location in the array consists of an address in memory at which is stored information that is used by the computer program to control the power supply to generate a predetermined voltage.
  • the first row of the array contains general information which is used by the computer to initialize the program by setting limit registers and parameters.
  • Each of the remaining array rows contains information which is used by the program to develop and sequence flashlamp firing voltages for a discrete set or "group" of output pulses.
  • Each pulse group developed by the system consists of a predetermined number of laser pulses each generated by the same predetermined flashlamp voltage.
  • information stored in the first location, 600 indicates the total number of groups in the file.
  • Location 650 at row 1, column 2 contains a number representing the total number of laser pulses o shots which will be fired for the entire file. The total shot number is used to set an internal counter which then compares the total number of shots actually fired with the total number stored in the array so that the computer can determine that all laser shots have been fired and that the file has been completely processed.
  • Location 652 in the third column contains information indicating the types of lasers which are to be controlled by the information in the array.
  • ruby laser any laser used for drilling purposes
  • the same computer system may be used to sequentially control several different types such as one or more ruby lasers, Nd:YAG lasers or C0 2 lasers.
  • Location 654 is not used.
  • Each of the remaining rows consists of information relating to a particular group of shots to be fired.
  • the information - is arranged in the same manner for each group.
  • the first location in column one contains information indicating the number of shots to be fired during processing of that particular group.
  • the information in the second column indicates the type of laser which is to be used for firing shots in that group.
  • the information in the third column (for example, location 658) contains numerical information which indicates the "raw" or unrounded flashlamp voltage which is to be used to fire each of the shots in the group.
  • the fourth location for each group contains a code word which is-a binary approximation of the raw voltage (expressed in decimal volts) to be applied to the power supply.
  • the code word is actually applied to the interface circuitry which (as will hereinafter be described) controls the flashlamp power supply. Due to rounding errors, and an inexact conversion between binary and decimal mathematics the code word represents a voltage which may not be exactly equivalent to the "raw" voltage information stored in location 658. In particular, the code word is related to the raw voltage by the following equation:
  • B is the code code word that is stored in location 660
  • A is the .raw voltage stored in location 658
  • INT () is an integer function which selects the integer portion of the variable enclosed in parentheses. The addition of 0.5 in the above equation causes the volts to be the nearest binary code to the raw voltage withing a + 2.5 volt tolerance.
  • a file such as that shown in Figure 6 may be set up by standard file manipulation routines and the information consisting of the group information may be entered, changed or deleted by means of standard data manipulation routines, since the operation and programming of such file and data routines is well-known, it will not be described in detail herein.
  • a simple program or routine can be used to send the stored data to the interface circuitry to control the flashlamp power supply which, in turn, controls the laser output.
  • a routine is shown in flowchart form in Figure 7. Referring to Figure 7, the file output routine starts with step 702. In step 704, the routine loads an internal register with the total number of groups. For a particular file, this number is
  • step 706 the count of a group counter set is equal to zero.
  • step 708 the count of the group counter is compared to the total number of groups
  • step 15 proceeds to step 710, in which the group counter is incremented.
  • step 712 the routine then loads the total number of shots for the group (with a group number equal to the count in the group counter) obtained from the first file array column
  • step 714 the routine sets a shot counter equal to ' zero.
  • step 710 the routine compares the count of the shot count to the total number of
  • step 712 25 shots loaded in step 712 to determine if any shots remain to be fired.
  • step 718 the routine increments the shot counter
  • step 720 the routine checks to see if the first shot is being fired (if so, the shot counter will be equal to one), if the shot being fired is the first shot, then the routine skips to step 726 in which the code word indicating the voltage to be used for the firing is sent to an output port which is connected to the interface circuit.
  • This voltage code word is the code word stored in the memory location corresponding to column 4 for the group row being processed.
  • a DATA STROBE* (“*" following a signal indicates that it is active in its "low” ' state) signal is generated at another output port which signal (as will hereinafter be described) causes the interface circuitry to process the code word and set the charging voltage of the flashlamp power supply to the correct level.
  • the routine then loads a status word appearing at the computer's input port into an internal register and, in step 732, checks a status word to determine whether the requested shot has, in fact, been fired by the interface circuitry. If not, the routine continues polling the status word by repeating steps 730 and 732.
  • step 716 the routine turns to step 716 in which the count of the shot counter is compared to the total number of shots to see whether all shots have been fired. If shots remain to be fired, the routine repeats steps 718-732.
  • step 720 if the shot to be fired is not the first shot, then the routine proceeds to step 722 in which a status word is loaded. This status word consist of signals developed by the interface circuitry which indicate whether the firing system is charging or is in the process for firing a shot as commanded.
  • step 724 the status word is checked to make sure the system is ready to accept new voltage data.
  • step 726 in which the new voltage code information is sent to the output port.
  • the DATA STROBE* signal is sent (step 728) and the routine proceeds as previously described, if, in step 724, the status word indicates that the system is not ready to accept new data, the routine waits by repeating steps 722 and 724 until proper status is achieved. Operation continues in the previously-described manner until, in step 716, the shot counter equals the total number of shots for the group indicating all the shots for the group have been fired. In this case, the routine proceeds to step 734 in which an ENDGRO ⁇ P* signal is sent to the interface circuit which disables the power supply charging circuitry, as will hereinafter be described.
  • step 708 the count in the group counter is compared to the total number of groups, if there remain groups to be processed (as indicated by an inequality in the two numbers) a new group shot count is loaded and the shot routine, consisting of steps 710-732, is repeated.
  • step 736 an electrical schematic diagram of the interface circuitry which accepts data and control information from the computer and converts it into analog signals to control the power supply is shown in detail.
  • the 'interface circuit connects with the computer via the terminals shown at the left-hand side of Figure 8. in particular, the interface circuitry receives code word voltage signals from the computer via data bus 800. In addition, the interface circuitry receives a DATA STROBE* signal, via terminal 816, which signal is a negative-going signal indicating that the data on the data bus 800 is valid.
  • An ENDGRO ⁇ P* signal is provided from the computer " to the interface circuit on terminal 817.
  • the ENDGRO ⁇ P* signal is a negative-going pulse indicating that all of the shots in a group have been fired.
  • the interface circuitry In addition to-receiving information from the computer, the interface circuitry also sends to the computer status signals, via terminal 818, which signals are sent to the computer's input bus.
  • the interface circuit also connects to the high voltage flashlamp power supply via the terminal shown at the right-hand side of Figure 8.
  • the interface circuit provides the power supply with a voltage reference signal (V ref ) via terminal 810 and charging inhibit signals (R-INHIBIT and C-INHIBIT) via terminals 811 and 813.
  • V ref voltage reference signal
  • R-INHIBIT and C-INHIBIT charging inhibit signals
  • the latter charging control signals act in parallel to control the flashlamp power supply to prevent or allow charging. In order for the supply to begin charging, both the R-INHIBIT and the C-INHIBIT signals must be low.
  • the • supply automatically charges to a voltage specified by the V_ e f signal at terminal 810. After the power supply has charged to the commanded voltage level, it- returns a signal (V rd *), via terminal 819, which signal indicates that the proper charging voltage has been reached (the V rdy * signal becomes "low” when the proper voltage has been reached).
  • the interface board In response to the V rdy * signal, the interface board provides a TRIGGER* signal on terminal 815 to the dump r switch which signal causes charge stored in the capacitor to be passed through the flashlamp and a similtaneously charging disable signal, C-INHIBIT.
  • binary signals on data bus 800 indicating the firing voltage to be placed on the flashlamp are provided via bus 802 to digital/analog converter 804.
  • bus 802 is shown only as a heavy line, it, in fact, consists of eight separate leads.
  • Digital/analog converter 804 is a well-known device which converts a binary signal at its input terminals to an analog output signal. The output signal of converter 804 is provided to the positive input of a differential subtracting instrumentation amplifier 806.
  • Amplifier 806 is a well-known type of amplifier ' circuit made of two or more operational amplifiers.
  • the subtracting input of amplifier 806 receivesa signal 808 which is developed by precision inverter 812.
  • Precision inverter 812 inverts a precision reference voltage supply 814 to generate the negative reference signal 808 which undergoes a sign reversal and is summed as a fixed voltage or offset turn whith the output of converter 804 inside amplifier 806 to generate output 810.
  • the converter 806 processes the converter output signal to adjust its gain and offset so that the converter output signal will be at the proper level for application of the power supply which accepts a 0-6 volt input signal with a transfer function that yields an output of 0-2500 volts.
  • the output of amplifier 806 is provided ' to terminal 810 and from there to the flashlamp power supply, in accordance with the invention, it is important that amplifier 806 and reference inverter 812 are precision devices compensated for both temperature drift and noise so that the output voltage on terminal 810 will be consistent for the same data input.
  • the gain of amplifier 806 is adjusted so that the output voltage on terminal 810 varies between 1.57 volts to 5.91 volts. With the particular power supply used in the preferred embodiment, such a voltage range causes the output charging voltage to range between 700 and 2500 volts, As previously mentioned, after the computer places a voltage control word on data bus 800, it places a negative-going data strobe signal on terminal 816.
  • This latter signal initiates a chain of events which allows the power supply to begin charging.
  • the "low” DATA STROBE* signal on terminal 816 is supplied to the set inputs of DATA flip/flop 824 and RESET flip/flop 826. These inputs are normally held “high” by means of resistor 822 which is connected to voltage supply 820 and the negative-going DATA STROBE* signal sets both flip/flops.
  • DATA flipflop 824 When DATA flipflop 824 is set, it produces a "low” signal at its Q* output which "low” signal is supplied to the reset input of INHIBIT flip/flop 828, Flip/flop 828 is thereupon reset causing a "low” signal to appear at its Q output, which "low” signal, in turn, is applied to buffer/driver 830. In response to a "low” signal at its input, buffer/driver 830 applies a “low” signal to the coil 831 of relay K4. The other lead of coil 831 is connected to positive voltage source 832. Relay K4 thereupon operates. Operated relay K4 opens contact 834 and closes contact 836.
  • Relay K4 remains in this state until the last ENDGRO ⁇ P* signal of the computer control sequence executes and the TIMEOUT signal ends, as described hereinafter.
  • contact 834 opens, resistor 840 places a "low” signal on the R-INHIBIT terminal, 811. This signal allows the power supply to begin charging.
  • contact 836 closes, it connects voltage source 832 to light-emitting diode (LED) 838, causing it to glow. The glow of LED 836 indicates that the power supply is conditioned to charge.
  • LED light-emitting diode
  • OR gate 842 having "low” signals at both its inputs, produces a "low” signal at its output which "low” signal is applied to the C-INHIBIT terminal, 813.
  • the flashlamp power supply With “low” signals on both the R-INHIBIT terminal 811 and the C-INHIBIT terminal 813, the flashlamp power supply begins charging the storage capacitor to a voltage specified by the V ref signal on terminal 810.
  • the power supply contains an internal comparator so that, when its output voltage reaches the proper voltage, a "low” signal is applied by the comparator as the V rd * signal on terminal 819.
  • the power supply circuitry is arranged so that if the power supply discharges between the time that the voltage reaches its proper level and the time when a trigger signal is generated, internal circuitry recharges the storage capacitor to the proper level. During such a recharging operation the V_ d * becomes “high”. After the recharging operation is complete, the V . * signal again becomes "low".
  • a "low” v . * signal is provided to signal conditioning circuit 846 which provides voltage level-shifting to shift the V rd * signal to the proper voltage levels required by the integrated circuitry in the interface board and also filters out noise.
  • Circuit 846 may illustratively consist of transistor level-shifting circuits followed by a single-shot multivibrator. The design of such circuits is well-known and will not be described in detail herein.
  • TIMEOUT single-shot multivibrator 844 is a well-known circuit device which may consist of a 555 integrated circuit of one-half of a 556 integrated circuit connected to appropriate capacitors and resistors, when device 844 receives a "low” signal at its trigger input, it produces a "high” TIMEOUT signal at its output lead, which "high” signal is adjustable to last between approximately 25 milliseconds to 2.5 seconds.
  • the "high" signal provides a timed interval during which firing of the flashlamp takes place.
  • the TIMEOUT signal is used to disable charging of the power supply during the firing interval and to prevent the setting of the INHIBIT flip/flop 828 (discussed above) until the computer control sequence has time to initiate a new action such as an additional shot.
  • the "high" signal at the output of multivibrator 844 is applied to the lower input of OR gate 842 causing it, in turn, to apply a "high” signal to the C-INHIBIT output terminal, ' 813.
  • a "high” signal on terminal 813 prevents the flashlamp power supply from charging.
  • the "high" output of the TIMEOUT single-shot multivibrator 844 is also applied to the TIMEOUT status lead which is provided to the computer, indicating that the interface circuit is in a TIMEOUT period.
  • Trigger pulse generator 868 is also a single-shot multivibrator which differs from single-shot multivibrators 844 and 848 in that it is triggered by a rising-edge appearing at its trigger input. Consequently, when the "high" TRIGGER signal generated by TRIGGER multivibrator 848 appears at the input of multivibrator 868, it generates an output pulse. This pulse has a duration of about 12 microseconds and is provided as a TRIGGER signal, via level shifter 847 and terminal 815, to the simer supply and dump switch and the high voltage supply. Level shifter 847 converts to 0-35 volts pulse produced by multivibrator 868 to " a 0-8 volts signal utilized by the power supply.
  • the TRIGGER signal causes the dump switch to "dump" the charge stored in the storage capacitor into the flashlamp causing the flashlamp to fire.
  • the trigger signals also inhibits the supply from charging during flashlamps firing as will be hereinafter described.
  • the "high" output of the TRIGGER single-shot multivibrator 848 is also used to generate a signal to the computer to indicate that a shot has been fired. Specifically, the "high" output of the TRIGGER single-shot multivibrator is applied to the trigger input of ACKNOWLEDGE single-shot multivibrator 870. This multivibrator is similar to trigger pulse generator 848 in that it responds to a negative-going signal at its input.
  • the ACKNOWLEDGE single-shot multivibrator produces a "high” DATA ACKNOWLEDGE* signal of a duration of approximately 2 milliseconds at its output.
  • this "high” signal becomes “low,” it pulls the left-hand lead of capacitor 872 low (the left-hand lead of capacitor 872 is normally held “high” by resistor 874 and voltage source 876).
  • capacitor 872 acts as a voltage differentiator and, accordingly, places a negative-going pulse on lead 878. This negative-going pulse is, in turn, applied to the reset input of DATA flip/flop 824, resetting it.
  • Flip/flop 824 when reset, generates signals that indicate to the computer that the laser has been fired, in particular, a "low" signal on the Q output of DATA flip/flop 824 is provided to the computer via the DATA RDY status terminal.
  • the computer is also informed that firing of the laser has been started because the pulse generated by the DATA ACKNOWLEDGE multivibrator 872 is connected to the DATA ACKNOWLEDGE terminal of status terminals 818.
  • the computer places a negative-going ENDGRO ⁇ P* signal on terminal 817. The function of this pulse is to cause the interface circuit to place a "high" signal on the R-iNHIBIT terminal to prevent the power supply from ' charging for personnel safety reasons.
  • OR gate 862 will receive “low” signals at both its inputs. Accordingly, OR gate 862 places a “low” signal on its output which is applied the voltage differentiator consisting of capacitor 864 and resistor 866. This differentiator causes a negative-going pulse to be provided to the set input of INHIBIT flip/flop 828.
  • INHIBIT flip/flop 828 In response to a negative-going signal at its input, INHIBIT flip/flop 828 becomes set, and places a "high” signal on its output Q. This "high” signal is, in turn, provided, via buffer driver 830, to the coil 831 of relay K4. since relay coil 831 now has a “high”' signal in both of its leads, it releases. When released, relay K4 opens contact 836 and closes contact 834.
  • OR gate 860 indicates when there is activity during the firing cycle, that is, when the power supply is waiting to be triggered or when the power supply has been triggered to fire the flashlamp. since the output of gate 860 holds the output of gate 862 "high" during such activity, the INHIBIT flip/flop 828 cannot be set.
  • OR gate 860 monitors the status of the TIMEOUT multivibrator and the TRIGGER multivibrator.
  • the TIMEOUT signal as previously described, is "high” when a v , * signal has been received from the power supply indicating that the capacitor has been charged to the commended voltage.
  • the output of the TRIGGER single-shot multivibrator is "high” when the triggering of the power supply is in progress.
  • Figure 9 of the drawing is a schematic diagram of a portion of the electrical circuitry of the high-voltage power supply.
  • This circuitry includes a comparator circuit which allows the power supply to recharge the energy storage capacitors during the time interval between the initial charging of the capacitor and the firing of the flashlamp.
  • this recharging capability allows the power supply to maintain a constant charge voltage on the energy storage capacitors to within approximately 1% of the programmed value. This close voltage control is necessary in order to allow repeatable results when firing a sequence of pulses in accordance with the invention.
  • the reference voltage, V - (which, as previously described, is the control voltage that is developed by the interface circuity and determines the value of the high-voltage output) is provided to the high-voltage power supply via terminal 900.
  • the V ref signal is supplied to the positive input of comparator 910, via resistors 902 and 906 and capacitor 904, which together form a filter to eliminate voltage spikes on the input.
  • Comparator 910 controls the high-voltage charging circuitry, to maintain a constant charge on the energy storage capacitors and receives on its negative input 912 a voltage sample signal which is related to the high-voltage output in the following manner.
  • the high-voltage output on terminal 950 is provided to a voltage follower circuit via resistor 940.
  • Resistor 940 is a voltage sampling resistor which typically has a large impedance (on the order of 15-20 megohms) to reduce the high-voltage output developed by the power supply to a manageable level.
  • the high-voltage sample signal is provided to a filter consisting of resistors 932 and 936 and capacitor 934.
  • the output of the filter is, in turn, applied to a.differentiating voltage follower which is comprised of operational amplifier 920, resistors 916, 922 and 928 and diodes 924 and 926.
  • a varistor 930 is connected across the input to the voltage follower to prevent voltage spikes from damaging the circuitry.
  • the output of the voltage follower which is representative of the high-voltage output of the power supply, is provided to the negative input of comparator 910.
  • the output of comparator 910, on lead 915 is, in turn, provided to an oscillator circuit, 970. Oscillator circuit 970 is enabled when a "high" signal appears on lead 915.
  • oscillator 970 when the voltage reference V_ ef has a greater magnitude than the voltage sample signal, oscillator 970 is enabled. Oscillator 970, in turn, drives an SCR switching bridge 960 in a conventional manner. Switching bridge 960, in turn, drives the primary • winding of transformer 954 which is a high-voltage step-up transformer. The secondary winding of transformer 954 develops a high-voltage output which is rectified by a conventional diode bridge 952 and applied to the high-voltage output terminal 950.
  • oscillator 970 operates to charge the energy storage capacitor (not shown) until the voltage sample signal developed by voltage follower 920 equals the voltage reference signal (V ref ) applied to terminal 900. At this point, the signal on lead 915 becomes “low” and oscillator 970 is disabled, if the voltage on the energy storage capacitor drops before the flashlamp is fired, the output sample voltage will decrease in proportion to the decrease in the high voltage output. This decrease will cause its magnitude to be below the voltage reference signal V ref and comparator 910 will again be enabled to produce a "high" signal output on lead 915 which, in turn, will start oscillator 970. Accordingly, oscillator wiil begin recharging the energy storage capacitor.
  • Oscillator 970 also can be controlled means of the TRIGGER signal appearing on input 990.
  • This TRIGGER signal is the same TRIGGER signal which is used to trigger the dump switch and fire the flashlamp, as previously described.
  • the TRIGGER signal is used to disable oscillator 970 for a short time period after the flashlamp has been triggered so that the oscillator will not attempt to charge the energy storage capacitor while the flashlamp is being fired.
  • a "high" TRIGGER signal applied to terminal 990 is provided to the base of transistor 982, via a voltage divider consisting of resistors 984 and 986.
  • the "high” signal turns on transistor 982 which, thereupon discharges capacitor 976.
  • turned-on transistor 982 grounds lead 972, via diode 974.
  • the "low” signal on lead 972 inhibits oscillator 970 and prevents charging of the energy storage capacitor.
  • the signal on terminal 990 becomes “low” which, in turn, turns off transistor - 982.
  • lead 972 is held “low” by discharged capacitor 976.
  • Capacitor 976 charges via resistor 980 and voltage source 978 and, after predetermined interval, lead 972 becomes “high” and oscillator 970 is re-enabled.
  • a precisely-controlled sequence of pulses can be applied to any workpiece by appropriately building data files.
  • a predetermined sequence of pulses having different peak energy, pulse width and pulse shape can be generated which sequence is optimal for drilling a particular type of material.
  • This ability to produce such a pulse sequence has enabled the discovery of certain general rules for drilling materials which rules were previously unnoticed because prior art systems could not consistently generate pulses with the same characteristics.
  • the resulting hole possesses virtually no recast layer on its inside wall.
  • this effect is due to the laser beam spatial power distribution. More specifically, when a hole is drilled with a single shot, a recast layer develops because melted material from the sides and bottom of the hole is splattered on the sides of the hole by the shock wave which accompanies the arrival of the laser beam. When a small pilot hole is drilled and then reamed to the final diameter, the melted material is effectively vaporized and does not deposit on the sides of the hole. This vaporization takes place because of uneven power distribution in the laser beam.
  • the center of the beam has much higher power than the outer edges of the beam, consequently, when such a beam is used to ream a existing hole, the edges of the beam melt the material on the sides of the hole. However, instead of being splattered against the sides of the hole, the melted material is ejected into .the "hot" center of the beam where it is vaporized and carried out of the hole as vapor.
  • Each of the above phases can be drilled with a pulse sequence or group of a predetermined number of pulses which are identical in peak energy, pulse width, and pulse shape. However, between pulse groups the pulse characteristics must be changed.
  • One material is a copper-laminate board consisting of an insulating material with metallic layers on both sides.
  • the other material is a fourteen layer copper-laminate board.
  • An illustrative sequence of pulses with associated flashlamp voltages is described for each example.
  • the workpiece material of this example is an FR-4 printed circuit board copper-laminate material.
  • This material consists of fiberglass/epoxy resin insulating material covered on both sides with one-ounce copper foil. Overall thickness of the material is ' .010 inches, hole size to be drilled was .006 inches in diameter.
  • the greatest problem in drilling this thin laminate material was delamination of the metal layers on entrance and exit of the laser beam. It was possible to drill satisfactory circular holes with one laser pulse but the entrance and exit of the holes was badly marred. To avoid this distortion* the entry phase pulse group sequence generated by the computer controlled laser used very low-power pulses. ⁇ he_ exit phase of the hole also had to be drilled with a low-power pulse sequence, again to prevent delamination. The middle phase of the hole could be drilled with a pulse group sequence with relatively high peak power to reduce drilling time as much as possible.
  • the material was drilled with a ruby laser constructed and controlled in accordance with the invention.
  • the energy storage capacitors were 375 microfarads and a flashlamp pulse length of 400 microseconds was used.
  • the laser threshold was approximately 970 volts and a pulse length of 400 microseconds was used.
  • the following pulse group sequence was used (in this sequence each pulse is represented by a number which is the voltage applied to the flashlamp to generate the pulse):
  • the entry pulse sequence creates a small hole about .0005 - .001 inches in diameter. After this hole has been drilled through the work piece, the hole was widened using pulses with increasing power levels up to 1,200 volts for the final reaming pulses, AS mentioned above, generally each pulse group will accomplish a particular objective, but because the test material is so thin in the example, the entry, middle and exit pulse groups are accomplished in pulse group one. pulse groups 2-10 are used to widen the hole to proper diameter.
  • Pulse group 11 is used for the final reaming, in accordance with the invention, it was found that if a small hole was initially drilled through the material, the application of successive pulses with higher peak power did not delaminate the surface and the hole could successfully be reamed to the proper final diameter.
  • the workpiece material was FR-4 epoxy/fiberglass dielectric printed circuit board laminate consisting of fourteen alternating dielectric and metallic layers.
  • One-ounce copper foil was used on all metallic layers as shown in Figure 10.
  • Entry piercing of the top layer of the copper was achieved in two to three low-power pulses.
  • a laser threshold 970 volts it was found that 2-3 three 980 volt pulses drilled through the top metallic layer and well into the first dielectric layer beneath. Drilling of the middle portion of the hole in the case of a thick multilayer material becomes complicated.
  • a successful pulse sequence for drilling the test material is as follows:
  • the exit portion of the pulse sequence is one group of twenty pulses. It is very important that the pulse that pierces the dielectric just before the last layer of copper as well as the last copper layer be at a low energy (in this example the last pulse is shot at 1050 flashlamp volts with a laser threshold of 970 volts). If the last pulse is not of low energy delamination of the last copper layer will begin to occur. At the final depth of the hole, drilling through the last two layers of dielectric and copper, respectively, takes about 6-8 laser pulses. The remaining 12-14 pulses in the exit phase group provide some leeway in case the drilling program proceeded a little faster or a little slower than expected, ideally, the last copper layer should be pierced by pulse numbers 12-14 of the exit phase pulse group. If the last layer is not pierced in the exit phase pulse group, but is pierced instead in either of the higher power pulse groups occuring before or after the exit phase group, delamination of the final metallic layer can occur.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Mechanical Engineering (AREA)
  • Electromagnetism (AREA)
  • Laser Beam Processing (AREA)

Abstract

Un procédé et un appareil sont utilisés pour percer avec une grande précision des trous dans des matériaux divers. L'appareil utilise un laser à rubis pour produire une série d'impulsions de lumière focalisées par un système optique sur une pièce à usiner. L'énergie de crête, la durée et la forme de chaque impulsion de la série peuvent être commandées indépendamment des autres impulsions de la série, de même que le diamètre et la forme du spot de chaque impulsion au niveau de la pièce à usiner. Selon le procédé, un ordinateur opère le laser pour percer un petit trou pilote dans la pièce à usiner en utilisant un groupe d'impulsions variées. Le trou pilote est ensuite élargi et alésé par le laser jusqu'à sa taille définitive. Sur des pièces à usiner à plusieurs couches, des impulsions multiples ayant une largeur d'impulsion, une énergie et une forme spatiale différentes sont utilisées à chaque étape du percement pour éviter d'endommager la pièce à usiner tout en conservant des vitesses raisonnables de percement.
EP85905950A 1984-10-16 1985-10-04 Procede et appareil de percement a laser Withdrawn EP0198076A1 (fr)

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IL76728A0 (en) 1986-02-28
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