GB2152861A - Apparatus for making printed circuit boards - Google Patents

Abstract

Printed circuit boards are made by imaging a liquid polymer which has been coated onto the board. The uncured polymer which remains liquid is removed after imaging so that the board can be processed by etch resist, plate resist or solder mask techniques. To position the circuit board to be processed a non- magnetic vacuum platen is used. This platen has a series of holes 63 drilled through its upper surface and a vacuum is drawn on the underside of the platen to a degree sufficient to keep the board locked in position on the platen surface. The holes are countersunk to accept magnetic balls therein capable of sealing the holes.The balls can be removed by a magnet to release the seal. <IMAGE>

Description

1

SPECIFICATION

Apparatus for making printed circuit boards This invention relates to new equipment for making printed circuit boards. More particularly, this invention relates to equipmentfor use in photoresist processeswhich utilize liquid polymer compositions curable bythe application of non-coherent collimated light. The present application is a divisional application of Application No. 8419444, itself a divisional of Application No. 8201146 (publication No. 2091493) directed to photoresist processes.

There are various ways to apply an image to the surface of a metal clad board when making printed circuit boards. These fall into thefour broad categories of wetfilm techniques, dryfilm techniques, hand screening, and machine screening. The wetfilm imaging techniques generally consist of applying a photoresist composition in a liquid stateto a circuit board blank, drying thefilm and imaging it. The dry film techniques consist of laminating a dry photoresist composition ontothe circuit board blank and imaging it. The hand screening and machine screening consist of the use of conventional screen printing techniques for applying a resist composition in a particular design onto the coppersurface of the blank board. Hand screening is used when a limited number of boards a re req u i red.

The techniques of the present invention fall into the wetfilm category, but differfrom the prior wet film techniques in thatthe film is not dried priorto exposing the film to the fight source which induces a change in the part of the film which is exposed. That is, although the various prior art techniques coated the circuit board blankwith a liquid, this film was usually dried priorto the subsequent steps. This drying of the film to harden is considered a requirement since the medium for applying a circuit design to a blank board (usually called a photo tool) is placed in contact with this coating. It has been considered necessaryto place the photo tool in contactwith the coating in orderto get good definition in the final printed circuit. Also, the present processes do not require any complex de- veloping agents ortechniques and after imaging all that is necessary is thatthe uncured liquid polymer be removed.

Broadly, it has been discovered that liquid polymeric coatings on printed circuit board blanks can be imaged while undried and yet get very good definition. This is the case even though an air gap must be maintained between the photo tool which bears the printed circuit design image and the liquid polymeric coating. This coating can range from an easilyflowable liquidto atackyviscous liquid. During imaging, lightfromthe non-coherent collimated light source passes through the transparent areas of the photo tool, strikes the liquid polymer, and curesthe liquid polymerto a solid. Where no light passes through the photo tool, the liquid polymer remains liquid, and is removed from the printed circuit board blank to expose the underlying metal. Although the GB 2 152 861 A 1 liquid polymercan be removed by physicaltechniquessuch as air knife or by wiping, it is most effective to removetheliquid polymer by means of a liquid in whichthe liquid polymer is at least partially soluble, but in which the cured solid polymerhas essentially no solubility. The primary advantage is that such a technique f ul ly exposes the metal underlyingthe liquid polymer thereby permitting a moreeffective etching of this metal in a subsequent step.

As discussed above, one of the drawbacks of imaging a liquid coating isthe necessity of an air gap between the photo tool and the liquid coating. The use of an air gap usually results in a loss of definition in the printed circuit. However, in the present processes this is not a problem. This is at least in partthe result of the use of a non-coherent collimated light beam for imaging, the processing techniques for removal of the uncured liquid polymer, the good adhesion of the cured polymerto the underlying metal, the resistance of the cured liquid polymerto etching solutions, and the ease of removal of the cured liquid polymer after etching. That is, the solution which is used to remove the uncured liquid polymerto expose the underlying metal does not attackthe cured polymer. If the cured polymer is also attacked,there will be a loss in definition in the final circuit and a possible loss of adhesion of the cured polymerto the underlying metal. Any loss of adhesion would cause excess metal to be removed during the etching step. However, when a solution containing cations derived from a strong base contacts the cured polymer, it is effective in removing the cured liquid polymerwithout attack- ing the underlying metal. The net result is that no organic containing solutions are required in processing the imaged printed circuit board. This provides for significant cost savings since orgnics are more expensive, are fire hazards, and create fumes and odorswhich under governmental regulations would haveto be removed from thework area. It is, therefor, very useful to be able to use aqueous solutions in the commercial production of the circuit boards.

The term "non-coherent collimated light" means light having a half angle of not more than 3 degrees, and preferably not more than 1.5 degrees. By contrast, laser light is coherent light. Light is collimated (formed into a beam) by lenses, parabolic reflectors or various arrangements of mirrors.

In brief summary, the present invention relates to single station and multi-station equipmentto perform etch and plate resist processesto produce printed circuit boards using liquid polymers, and to perform a process to make solder marks using liquid polymers.

Each of these processes images circuit board blanks which have a liquid polymer coating and each only requires aqueous solutions to processthe board after imaging. As a resuitthe same equipment can be used to plate resist, etch resist and solder mask. This versatility provides many cost advantages, and particularlyto lowervolume producers of the various types of printed circuit boards.

The invention of Application No. 8419444 provides a multiple chambervacuum platen comprising a base Pages 8 and 9 of the drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

2 member; a plurality of continuous walls of a uniform height extending upwardlytherefrom, said continuous wal Is defining separate regions; an upper member having a plurality of openings therethrough and sealably contacting the upper part of said walls to 70 form a plurality of sealed chambers; and means for connecting each of said chambers to a device to draw avacuumthereon.

In the etch and plate resist processes, the circuit board blank is first coated with the liquid polymer. A photo tool of the desired circuit is then placed in registration above, but not in contactwith the liquid polymer coating. The blank is then imaged whereby underthe transparent areas of the photo tool the polymer cures to a solid, whilethat underthe opaque areas remains liquid. This liquid polymer is then removed, preferably by use of solution in which it is at least partially soluble. Afterthis step the processes for making plate resists and etch resists differ. In making etch resists, the exposed metal is etched away using an aqueous acid solution and the cured liquid polymer then removed using an alkaline solution containing cations derived from a strong base. In making plate resists, tin, nickel and/or similar metals are plated on the exposed metal after liquid polymer removal, the cured liquid polymer removed using an alkaline solution containing cations derived from a strong base, andthe metal layerwhich has been exposed removed by etching.

Solder masks are produced by coating a printed circuit board with a liquid polymer, imaging the coated board to create cured polymer on certain parts of the circuit design, while leavingthe polymer uncured on other parts. The uncured liquid polymer is then removed using a solution in which it is at least partially soluble and the exposed metal coated with solder. The cured polymer is usually not removed, but left on the board as an insulating layer.

The single station equipmentfound very useful in practicing these processes consists of a device for holding a printed circuit board or printed circuit board blank in a set position, a retractable roller coaterfor coating one side of a board, a photo tool in registration with the circuit blank and bearing an image of a circuit and which can be retractably moved to a point above, but in close proximityto, the coated board, and a non-coherent collimated light source positioned for passing lightthrough th phototool to cure liquid polymer. Various arrangements forthese units are possible. However, the preferred arrangement isto havethe board blank held in place by means of a vacuum with a roller coater horizontally retractableto the coatthe board.The photo tool assembly is vertically retractable and is located abovethe posi- tioned circuit board blankand in the path of the non-coherent collimated lightfrom the light source. Therefore, in operation the machineoperator positionsthe board and activates the vacuum holding means. Thereafter, stepwise,the rollercoater is activated and passes overthe board, coatsthe board 125 and retracts. Following this step the photo tool assembly lowes into close relationship with the coating, and when in position the lights source is activated for a time to curethe polymer. When the light goes offthe photo tool assembly retracts GB 2 152 861 A 2 upwardly and the board moves on a conveyerto a spray bath for removal of the uncured liquid polymer. The equipment forthe subsequent steps consists of a state - of the - art etching, metal plating and solder depositing devices. The multi-station equipment consists of separate coating and exposing units, but otherwise is similarto the single station equipment.

The present processes and equipmentwill be discussed in more detail with referenceto the following drawings:

Figure 1 is a stepwise diagram of the etch resist processfor making printed circuit boards.

Figure 2 is a stepwise diagram of the plate resist process for making printed circuit boards.

Figure 3 is a stepwise diagram of the process for making solder masks.

Figure 4 is an elevational view of a circuit board blank positioned on a vacuum holding plate.

Figure 5 is an elevational view of the circuit board blank being coated by a retractable roller coater.

Figure 6 is an elevational view of the photo tool lowered into a close relationship with the coated circuit board blank.

Figure 7 is an elevational view of circuit board blank with the photo tool retracted showing certain area of cured liquid polymer.

Figure 8 is a schematic diagram illustrating the non-coherent collimated lightwhich is used.

Figure 9 is a front elevational viewof a singlestation apparatuswhich contains the components of Figures 4and7.

Figure 10 is a sideviewof the singlestation apparatus of Figure 8.

Figure 11 is a front elevational viewof the multi- station apparatusto stepwise perform the operations of the apparatus of Figures8 and 10.

Figure 12 is a planarviewof the phototool surface which is in airgap relationship with the circuit board blank during imaging.

Figure 13 is a perspective view of the platen assemblywhich holdsthe circuitboard blank in position.

Figure 14 is a sectional viewof the platen assembly of Figures 13 along line 14-14.

Figure 1 sets outthe stepsfor producing a printed circuit board using etch resist techniques. The process starts with an electroconductive clad blank. Such blanks consist of a non-metallic substrate, usually a plastic containing reinforcing fibers, having on one or both sides a metal layerwith thickness of about 0.001 to 0.003 inches (0. 025 to 0.076 mm).The substrate can be of any reasonable thickness, but is usuailly about 0.003to 0.25 inches (0.076 mm to 6.35 mm) thick. Commonly, these substrates are phenolic or epoxy plastics reinforced with fiberglass or an equivalent material. The metal cladding is usually copper.

In making an etch resist according to the present process, the metal clad blank is coated on one side with liquid polymerto a thicknes of aboutOA to 10 mils (0.0025 to 0.25 mm) or more. Preferablythe coating thickness is uniform overthe entire blank, and in a thickness of about 0.5 to 5 mils (0.0127to 0.127 mm). The coating hasto be sufficiently thick to insurethat there are no voids, but notso thickthatthe liquid polymerflows off the edge of the blank orthrough the 3 many small holes which traverse the blank. Coating can be by any convenienttechnique, such as spraying, brushing, or roller coating. However, roller coating. is preferred since it deposits a more uniform wet polymerfilm on the board, and it has unexpectedly 70 been found that in roller coating the liquid polymer tends notto flow into any of the holes through the blank even though a vacuum is being drawn on the underside of the board. This is primarily attributable to an interplay between this mode of application and the 75 surface tension of the liquid polymers. These holes are used later in connecting circuit components to the finished boards. If polymerflowed into these holes, it would have to be removed before circuit assembly.

In the next step, a photo tool which bears the circuit 80 design is placed in an air gap relationship with the liquid polymer coating. It is desirable to have the photo tool as close as possible, but not contacting the liquid polymer coating. An air gap of about 5to 500 mils (0.127to 12.7 mm) is generally used. An assembly 85 holds the photo tool in position and in registration with the polymer coated blank. In the preferred method, the photo tool is held in position by means of a metal frame and a vacuum drawn on the edge of the photo tool. When the photo tool is an emulsion on a glass plate, the glass plate is mechanically held in place. In the instances where the photo tool is an emulsion on a plasticfilm, such as a poiyesterfilm of about4to 10 mils (0.1 to 0.25mm) thick, a glass plate is above the film to assure thatthefilm is maintained in the same plane. Regardless of whether an emulsion on a glass plate ora plasticfilm is used as the photo tool, the emulsion side is what is in air gap relationship with the liquid polymer coating.

Theterm "photo tool" is broadly used to mean any image bearing medium and can be any silver or silverless emulsion. This includes diazo emulsions.

This emulsion can be supported on a polymerflim, glass, or any other convenient medium. The term "registration- is used to denote the placement of the photo tool overthe circuit board blank in a manner so thatthe desired circuit is placed on predetermined areas of the blank.

Afterthe photo tool is in place, a non-coherent collimated light source, which is preferably a source of 110 2000 to 5000 Angstrom radiation and which is located to pass lightthrough the photo tool, is activated. Convenient light sources include carbon arcs, mercury arcs, mercuryxenon lamps, tungsten halide lamps, and argon glow lamps. Preferablythe light source is a mercury arc or mercuryxenon light beam. The light passesthrough thetransparent areas of the phototool, strikesthe liquid polymerand cures the liquid polymerto a solid. The liquid polymerwhich is notcontacted by any light remains liquid. Atime duration of about 5to 120 seconds is usual iy sufficient to curethe liquid polymer. However, where necessary, longertime durations can be used.

Afterthe liquid polymer is solidified where con- tacted with light, the uncured liquid polymerwhich has remained liquid, is removed to expose the underlying metal. This can be accomplised using an air kniveto blowthe liquid polymerfrom the surface, bywiping or by using a liquid in which the liquid polymer is at least partially soluble. The preferred GB 2 152 861 A 3 technique isto use an aqueous aikakine solution in which the liquid polymer is at least partially soluble. Suitable solutions are alkaline solutions containing sodium, potassium, or ammonium ions such as sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium sulfite, sodium bisulfite, potassium hydroxide, potassium carbonate, potassium bicarbonate, potassium sulfite, potassium bisulfite, ammonium hydroxide, ammonium carbonate, and combinations of these materials. Alkaline detergents or even organic-aqueous mixtures can also be used so long asthey do notattackthe cured polymer.

Subsequerittothe removal of the uncured liquid polymer,the exposed metal is removed by etching. Usuallythis metal is copper andthe etching solution can be anycommonly used copper etchant, such as an acidicchloride solution. Commonly used copper etchant solutions are ferric chloride or hydrochloric acid solutionswhich may also contain some other components. Afterthe completion of this step, there remainsthe blankwith the metal layer now only present underthe cured liquid polymer.

The laststep comprises removing the cured liquid polymer. This is accomplished by contacting the cured liquid polymerwith an alkaline solution containing cations derivedfrom a strong base. Sodium or potassium containing compounds are preferred alkaline materiaisfor making such solutions. However, other reagentswhich will yield strongly basic cations can also be used.

Figure 2 sets outthe steps in producing a printed circuit board using plate resist techniques. The first foursteps in plate resist processing are essentiallythe same asthoseforetch resist processing. As a result afterthe step of removing the uncured liquid polymer, the exposed metal is in a pattern identical to that of the final circuit design. The remainder of the metal is covered bythe cured polymer.

The next step in producing a plate resist consists of plating tin, lead, nickel, or combinations of these metals onto the exposed metal. This is conveniently done by electroless or electroplating onto the exposed metal. The cured polymer is removed in the same manner as in the etch resist techniques. That is, the board iswashed with alkaline solution containing cations derived from a strong base.

Thefinal step in making the plate resist consists of etching awaythe metal, usually copper, which has been exposed upon removal of the cured polymer.

This etching isthe same as for etch resists with the plating metal protecting the underlying metal. The final product is a printed circuit board with a circuit design coated with the plated metal.

Figure 3 sets outthe steps for applying solder masks to printed circuit boards. In producing a solder mask, the starting substrate is a completed printed circuit board. Inthefirststepthe printed circuit board is coated with the liquid polymer using any of the techniques as disclosed in the Figure 1 etch resist processing. A photo tool which has opaque areas above the areas where solder isto be deposited, is illen placed in exacting registration abovethe coated printed circuit board and the light source activated. The liquid polymer cures to a solid in all areas contacted bythe light with all other areas remaining 4 liquid. The uncured liquid polymer is then removed in the same manner as etch resist processing. The exposed metal areas are contacted with solderwhich remains on the exposed metal. Wave soldering methods can be conveniently used. The solder in these selected areas is then used to conneavarious electronic components and wires.

These arethethree primary processes for utilizing the present liquid polymer processes in printed circuit board manufacturing. Figures4through 7 broadly illustratethe preferred mechanisms used in accom plishing these stepsthrough the curing of the liquid polymer. In Figure4,the printed circuit board blank 10 is positioned on platen 11, by means of guide and registration pins 12 which pass through holes 13 in the 80 circuit board blank. This platen has a series of small holes 14 drilled through the platen. Avacuum is drawn on the underside of this platen to a degree sufficieritto keep the board locked in position on the platen surface. The pins 12 are then retracted belowthe 85 board surface.

Nowthat the printed circuit board blank is locked in position, Figure 5 illustrates the coating with liquid polymer. The retractable roller coater consists of application roller 15 and doctor roller 16. A supply of liquid polymer remains atthe nip of the rollers. Each roller is at least as long as the width of the circuit board blanks. The doctor roller allows a predetermined amount of liquid polymerto be picked up by the application roller.This predetermined feed regulates thethickness of the liquid polymer 17 on the blank.

After coating, the roller coating retracts.

Figure 6 shows the liquid polymer board blankwith the photo tool in place. Here the blank 10 is fully coated with a layer 17 of liquid polymer. Air gap 18, 100 which is in the range of 5 to 500 mils (0.127 to 12.7mm), separates the liquid polymerfrom the photo tool. The photo tool assembly is comprised of negative 19 and planar maintaining glass plate 20. The negative is directly above air gap 18 and preferably the 105 emulsion side of the film is adjacentthe air gap. With the photo tool in place, alight source above the photo tool is activated. Where light strikes the liquid polymer, it cures to a solid. This is illustrated in Figure 7, where 21 designates cured solid polymer areas, while 22 designates the uncured liquid polymer areas.

The board is now readyfor removal of the uncured polymer. The board can conveniently be conveyed froom the platen by means of a series of rollers 23 which carry conveyor belts 23(a) fortransport of the board. Conveyor belts 23(a) are inset into the platen and raised when the board isto be transported.

The non-coherent collimated light which is used for these processes is defined as the light angle where the light strikes the photoresist. This angle should prefer ably be alight a half angle of not morethan 3 degrees and preferably not more than 1.5 degrees. This light will produce a photo resist which has an angle of deviation from vertical of not morethan 3 degrees and preferably not more than 1.5 degrees. A plus deviation designates a shoulder on the photoresist while a negathe deviation designates an undercut.

Also, this lightshould not have an image dimension deviation of more than 112 mil (0.01 27mm) and preferably not more than 114 mils (0.00635mm). If GB 2 152 861 A 4 the deviationfrom vertical orthe imagedimension deviation exceedsthese amounts, there will resulta lackof accuracy andlor uniformity in the dimensions ofthe resulting circuit.This can result in a change in circuit resistancewhich can affectthe overall performanceofthefinal electronicitem.

These factors which definethis lightare illustrated in detail in Figure 8. In this figure, the incidentlight is designated as I.The phototool isdesignated as 19, thecircuit board as 10,the liquid polymercoating as 17, and the air gap between the photo tool and the liquid polymer as 18. Area 21 (a) of the photo tool is a transparent area, while area 22(a) is an opaque area. Afterexposure to light, in area 21 the liquid polymer has solidified, while in area 22 it remains a liquid. The light strikes the photo tool in a substantially perpendicular condition. However, since this is an ideal condition, and since all non-coherent lightwill have some deviation, this is defined by means of the half angle A of the light. This half angle Awill produce an angle of deviation in the photoresist resulting in either a shoulder B or an undercut B'in the photoresist. The angle of deviation in the photo resist which is the angle of the shoulder or of the undercut is the angle by which the vertical side of the photoresist deviates from absolute vertical.

The dimension deviation factor is also illustrated with referenceto Figure 8. This factor is the maximum amount of deviation of the photoresistfrom the dimension of the circuit line on the photo tool. The light, itself, orthe light striking the edge of the opaque area on the photo tool, cannot be deflected or otherwise affected to an extentwherethe dimension of the polymerized polymer 21 will differfrom the dimension of the opaque area 21 (a) on the photo tool 13 by not more than 112 mill (0.0127 mm) and preferably not morethan 114 mil (0.00635 mm). The objective is to have the width dimensions of area 21 to as much as possible be identical to that of area 21 (a). This is shown by angle C.

This light shou ld have a substantial amount of its energy in the ultraviolet and visible bands, that is, in the region of 2000 to 5000 angstroms. Also, this light should have an intensity of about 4 to 30 mille watts/cM2 atthe surface of the liquid polymer. This is not a critical feature since at lower intensites, a longer exposure time can be used to achieve the same result. A light source of this kind is attimes designated as an actinic lightsource.

The other parameterof importancewith regard to the lightsource is its placementwith regard to the objectwhich isto receivethe light energy. The collimated lightshould be in a planewhich is parallel to the plane of the objectwhich isto receivethe light energy. Thus, the collimated lightwhich is emitted perpendicularto the plane of lightwill be received perpendicularto the plane of the objectwhich is receiving the light. This insuresthatthe end circuitand the circuit design on the photo tool are identical. If this orientation is not maintained, the circuit lineswill be offsetfromthat of the photo tool.Thiswill create problernswhere a board isto carry a different circuit on each side.

Figures 9 and 10 illustrate single station equipment which incorporates all of the unit operations of Figures 4through 7. By single station is meant that each of the steps of Figures 4-7 are carried out at the same area.

The preferred arrangement of the individual units is shown in each figure. The upper part of the cabinet houses light source 27 and has a vent 28 for venting heat and fumes from the apparatus. The light source is a mercury arc or mercury zenon lamp, is stationary and, thus, can be mounted in any manner which assures rigidity. The lower section 26 of the cabinet houses the vacuum pump 29 and interconnecting tubing 30 and 31, the motor 47 which drives the conveyor system for removing imaged boards to the uncured polymer removal station, and polymer pump and storage tank 48 with deliverytubing 48(a). The middle area of the cabinet is atthe operatorworking level and containsthe particular equipmentforthe unit operations asshown in Figures4through 7.

In place of a lightsource mounted in the upper part of the cabinet25,the lightsource can be located in the lower part of the cabinetwith a series of mirrors in the 85 upper partof the cabinet. These mirrors are positioned to changethe path of the lightso that itwill pass down through the phototool. This can be accomplished by the use of two angled mirrors which will each reflect the light perpendicularly so thatthe direction of the light beam changesfrom an upward direction to a downward direction and through the photo tool.

In more detail, after a circuit board blank is placed in the machine and positioned on the retractable pins (as shown in detail in Figures 4 and 5), a vacuum is drawn 95 on the underside of platen 11 by means of vacuum pump 29 and interconnecting tubing 30. In this view, the roller coater is shown in the position it is in when applying liquid polymerto a circuit board blank. The roller coater application roller 15 and doctor roller 16 100 are mounted on bracket32. The application roller is mountedforward of the doctor roller. Each rollerturns in a compatable manneras shown in Figure 5. The area between the rollers receives liquid polymerfrom pump andtank48 by means of tubing 48(a). An electronic sensing mechanism maintains a constant level of liquid polymer in the nip of the rolls by switching the liquid polymer pump or actuating a valveto flowthe polymer backto the tank. Drive mechanism 33 operates the rollers. This drive mechanism is mounted on the moving carriage and is an electric motor withchain or gear drive to each roller. Drive mechanism 34 moves the roller coater mechanism forwardly and rearwardly on fixed screw tracks 35. This drive mechanism 34 consists of a stationary reversible electric motorwhich is con nected to each rotating screw by means of a chain drive. That is, the motor shaft and each screw has a sprocket and the chain rides over each sprocket.

Preferably a spring-loaded idler sprocket is used to tension the screw drive chain. These screw tracks can both su pportthe coater and move it forward and backward in each coating cycle. However, it is preferred to use a separate bearing way 36to support the roller coater. The roller 15 is usually in contact with the blank only on the forward orthe rearward pass overthe blank. It can, however, be in contactwith the blank on both passes. When coating in the forward direction, the application roller 15, due to the position ing of the doctor roller 16 (behind roller 15), will 130 GB 2 152 861 A 5 deposit athickerfilm. In the forward direction, this coating can rangefrom about3to 10 mils (0.076to 0.25 mm),whiie in reverse coating, the coating will be thinnerand rangefrom 0.5to5 mils (0.0127to 0.127 mm). Depending on the thickness of the coating desired, the application rolleris maintained in contact oris raised outof contactwiththe blank on the forward or reverse pass over the blank. Solenoidsare usedto raisethe rollers on the forward orrearward passas desired. The two coating time rangesform4to 20 seconds,with about 8 seconds found to beconvenient. The rotating screw mechanism which moves the roller coater, can be operated atvarious speeds since it uses a separate reversible electric motorfrom thatwhich controls the rollers. This coating speed can be changed asthe need arises.

The application roller is constructed of a polyene or a mixture of polyenes. These materials are stable and not significantly affected by the liquid polymer. This rol [er is of a length to coatthe largest circuit board blank which can be held on the platen and is in the range of about 2 to 6 inches (5 to 15 cm) in diameter. The doctor rol ler can be of any material not affected by the liquid polymer and is conveniently stain less steel or ch rome plated steel. The diameter of this roller is less than that of the application roller. The deg ree of clearance between the application rol ier and the platen which holds the blank is set by adjusting two micrometers which raise and lower the rollers but do not raise or lower the screw mechanism for moving the roller assembly overthe blank.

Depicted above the roller coater mechanism is the photo tool assembly37. The photo tool assembly contains the film 19 which is held in planar alignment by glass plate 20. The lower side of the film is the emulsion side of the film and carries the image of the circuit design. The photo tool assembly is retractably mounted by means of guide bracket 38 and bearing way 39, so that it can be raised to an upper part of the cabinetwhen the roller coater is in a forward position applying liquid polymer, and then lowered so as to put the photo tool in close air gap relationship with the liquid polymer coating during imaging. The photo tool slideably moves upwardly and downwardly aligned bythe bearing ways 39. Chains 40 attach to side brackets 49 of the photo tool assembly and extend upwardly over sprockets 41. These sprockets are rotated by reversible motor42 via drive shaft43. Rotating these sprockets raises and lowers the photo tool assembly about 12 inches (30 cm). These sprockets need only have a circumference equal to aboutthe distancethatthe photo tool assembly isto be raised and lowered. On the lower end of the photo tool assembly, there are adjustable pins 44which are set so as to maintain the proper air gap between the photo tool and the coated surface. At least 4 of these pins are used. These pins contactthe platen surface and supportthe photo tool assembly during imaging. Tubing 31 connects the photo tool assemblyto the vacuum pump. The vacuum which is drawn on the photo tool holds it in place at its edges. Registration pins can also be used to help align the photo tool. Although the retracting mechanism has been discussed with respect to using a chain, or a cable, a rotating screw as used forthe roller coater can be used.

6 However, it is preferred to use a chain or cable elevator-type mechanism for raising and lowering the photo tool assembly.

Oncethe photo tool assembly is in place and resting on pins 44,the lamp 27 is activated which sends a 70 beam of non-coherent collimated lightdown ontothe phototool.This light is applied forfrom 5to 120 seconds, and preferably for about 20 seconds. As a lightsource ages and gets weaker, the light applica tion time is extended. Light intensity can be measured 75 manually and the time duration adjusted or an integratorcan be used which automatically monitors the time intensity application of light energy and adjusts the light source. The light source is then extinguished and the photo tool assembly is raised upwa rdlyto its rest position. The vacuum on platen is then released, conveyor mechanism 45 raised, and the board automatically removed from the apparatus forfurther processing. The conveyor is raised by activating solenoids 46 raising bracket45 which in turn raises rollers 23 and the associated conveyor belt 23(a) in the area of the platen. Motor47 is simul taneously activated with solenoids 46. Belt47(a) from the motor drives rollers 23.

The equipment can be operated on a manual or 90 automatic basis. In manual operation, the operator would control each step. However, an automatic operation is preferred, whereby afterthe circuit board is positioned, the steps are each sequentially per- formed without any operator control. Besides provid- 95 ing for better quality control, it permits safer operation of the equipment. The controls shown in Figure 9 depicts some of those which are used for automatic operation. Itwould generally be desirable to have gauges to give a constant indication of vacuum in the platen and photo tool assemblies. Switches would include a Master On-Off switch, vacuum pump switch, and light timer device. These are all state of -the - art items and still others could be added.

Figure 10 shows a side view of the apparatus of Figure 9 from the side opposite that where the circuit board blank exits, i.e., from the left side of the apparatus of Figure 9. The various parts arethe same and bearthe same numeral designations as in Figure 8, exceptthatthis view shows the use of an optional multiple chamber platen assembly. The uppersection contains the light source 27 and vent28. The lower section 26which contains the vacuum pump 29, liquid polymer pump, and tank 48 associated polymer supplytubing 49(a), and motor47 with associated belt 47(a) for driving conveyor rollers 23. The mid-section is the operator-working area, and containsthe circuit board positioning mechansim, the rollercoater mechanism, and the photo tool assembly. Each of these has been described with reference to Figure 9, butwill be described here again with referenceto this Figure 10.

The circuit board positioning mechanism consists of platen assembly 61 which has a series of holes 63 for drawing a vacuum on the upperside of the platen.

The platen here is the multi-chamber platen assembly of Figures 13 anJ 14. That is, there arefour separate chambers inthe platen assembly. Avacuum can be drawn on any or all of these chambers.This platen is used when various sized circuit boards are to be 130 GB 2 152 861 A 6 produced. In use a vacuum is onlydrawn on the chamber or chambers overwhich there is positioned a circuit board blank.

In securing the circuit board blank, itisfirst determined asto the chambers in the platen on which a vacuum is to be drawn. Valves 52 with connectlines 53to vacuum manifold 51 arethen openedforthe chambers on which a vacuum isto be drawn.The othervaives 52 remain closed. Afterthe circuit board isfirmly in position,the rollercoateris activated so that it moves forward on screws35 and coatsthe circuit board blankwith liquid polymer. The application and doctor rolls cannot beseen in thisview.The rollercoater isthen retracted andthe phototool assembly37 towers into position. Member38 guides the phototool on bearing way39. Chains 40 are attached to side brackets 49 and raise and lowerthe photo tool assembly. The photo tool lowers until adjustable pins 44contaetthe uppersurface of platen 61. In thefully-activated position,the photo tool assembly is supported bythese adjustable pins 44. By adjusting these pins, the air gap between the photo tool and the liquid polymer can be changed. When the photo tool assembly is in place, actinic light source 27 is activated. Upon the light source being extinguished, the photo tool assembly is raised and the conveyor rollers 23 and associated belts 23(a) are activated. Motor 47 and associated belt 47(a) provide the motive force to rollers 23. Atthetimethatthe photo to] is being raised to its storage position, conveyor bracket 45 is raised by solenoids 46. Simultaneously, motor 47 and conveyor rol lers 23 and the associated belts are activated so that the circuit board can be lifted off platen 61 and conveyed from the equipmentto the next work station.

As discussed, it is preferred to operate this equipment in an automatic mode. Therefore, after turning on the master switch an operator, after inserting the film bearing the circuit design in the photo tool assembly, will position the circuit board blank on the vacuum platen and draw a vacuum on the platen to secure the circuit board blank. Whenthe vacuum on the platen reaches a pre-set level,the liquid polymer roller coater automatically comes forward, and re- verse coats the circuit board blank. The roller coater assembly then fully retracts. When the rollercoater is in its stored position, the photo tool assembly lowers to within the pre-set airgap distancefrom the coated board. When the photo tool is in position, the fight source is automatically activated forthe pre-settime. The lightsource is then automatically extinguished and the photo tool assembly elevator mechanism raises the photo tool upwardly about 12 inches to its stored position.

Afterthe photo tool assembly has been raised a distance of about 2 inches (5cm),the conveyor mechanism in the area of the platen is raised about 0. 5 to 1 inch (12.5 to 25 mm) to contactthe underside of the imaged board and the entire conveyor is simuf- taneously switched on to move the imaged circuit board to the nextwork station. The equipment is then ready to repeatthe roller coating and imaging cycle.

Figure 11 illustrates the multi-station embodiment of the apparatus to practice the foregoing processes. While in the single station embodiment of Figures 9 7 and 10, the coating of the circuit board and its subsequent exposure is conducted at the same location; in the multi-station embodiment these steps are conducted at separate locations. The prime advantage of the multi-station embodiment over the single station embodiment is that daily production can be doubled. This is the result of being able to simultaneously coat one board with liquid polymer while another board is being exposed to the non- coherent collimated light.

In more detail, 80 is the circuit board liquid polymer coating station. The same type of retractable coater as in the single station apparatus of Figures 9 and 10 can be used. However, a coaterwherethe coating rolls are stationary and the circuit board to be coated passes through the rolls is preferred. This is the case sincethe board can be continuously moved through to the non-coherent collimated light exposure section. Suitable coaters are commercially available from, for instance, the Union Tool Company. The coaterwhich is illustrated is of the preferred type.

The coaterconsists of imputconveyor81 which feedsthe board into rolls 83 and 84. Roller83 is a supporting roller, while roller84 isthe coating roller.

Roller82 isthe nip roller. The supply of liquid polymer 90 is maintained in the nip between rollers 82 and 84. After passing through these rollers and being coated, the board passes onto conveyor 86, which isfully enclosed. This enclosure consists of a sheet metal enclosure or optionally can have one or more sides of 95 a transparent plastic. Conveyor 86 transports the coated board into exposure section 90. Immediately priortothe exposure section the board can be pre-exposedto a low level of lightto partially polymerize the coating. This would be using low level, 100 non-collimated light radiation, such as by use of fluorescent lights having an output in the UV visible range. This pre-exposure section 88(a) is shown in broken- iine outline since it is an optional feature.

Conveyor 86 delivers the board onto platen 89 which is the same as the platen in the single station apparatus. That is, a conveyor moves the board into position overthe platen and it is then retracted downwardly with the platen then supporting the board. Guide and registration pins are used to locate the board on the platen until the vacuum is drawn to secure the board onto the platen. However, rather than using guide and registration pins, various other equivalent mechanical techniques can be used.

When the board is in place, the photo tool 93 lowers into place via guideways and elevator mechanism 94. This photo tool is the same as that of the single station apparatus and is more fully described in Figure 12. When the photo tool is in place and in registration with the board, the non-coherent collimated light source 97 is activated. This light source is the same as in the single station apparatus. It can be located in the upper part 91 of this section of the apparatus, or in the lower area 92 with mirrors located in the upper section to changethe direction of the light so that after it passes upwardly it can, via multiple reflections, be passed downwardlythrough the photo tool. Regardless of the arrangement, this light source must havethe characteristics as previously setforth, and discussed in detail with reference to Figure 8. Fan and 130 GB 2 152 861 A 7 ventsystem 98 withdraws fumes and heatfroom the apparatus.

The lower region 92will also contain the various utilitiesforthe apparatus such as atransformer, vacuum pump, electric motors, valves, switches, and the like. These are arranged as space permits in this area.

After a board is exposed the photo tool retracts upwardly, the vacuum which holds the board onto the platen ceases, the platen conveyor moves upwardlyfrom its rest position in the platen and moves the exposed board onto exit conveyor 99 which removesthe board from the apparatus. Section 88(b) which is outlined by dashed lines, designates an optional post-cure option. That is, there can be a low intensity, non- collimated light source in the area which would further polymerize the liquid polymer on the board. Conveyor 99 delivers the board to the circuit board finishing line. This consists of apparatus 100 to removethe polymerwhich was not exposed to the collimated light. Typically, this is accomplished using an alkaline wash solution. The board can then be etched in section 101 to remove exposed metal, and in section 102 the liquid polymerwhich was polymerized by exposure to the collimated light is removed using a strong alkaline solution. These latter three sections which can be considered a developer line can consist of commercially available units from Cherncut Corporation. It should be noted thatthe board may not proceed through the full developer line, may be removed for some intermediate processing and then placed back in the developer line, or removed forsome otherform of processing.

Figure 12 showthe lowersurface of the photo tool assemblywhich is in close air gap relationship with the liquid polymer coating during imaging. Assembly 55, which is a metal such as aluminum, has inner area 59 wherethe photo tool is placed. Around this opening is vacuum channel 56. Openings 57 connect to tubing 31 on the upper surface. In this view, there are shown six adjusting pins 44. items 58 are magnets which are embedded in the surface. These magnets will hold a metal frame which in addition can be used to hold the photo tool (film or plate) in position. There may optionally be a set of registration pins 60 for aiigning the photo tool on the assembly. Such registration pins will pass through small holes in the photo tool. In use, the operator places the photo tool on the assembly and places thin steel strips in the area of the embedded magnets, whereby the photo tool is sandwiched between the surface and the strips. The vacuum system is then activated and the photo tool further secured in place. The pins 44 are adjusted to provide the proper air gap between the polymer coated blank and the photo tool. The photo tool assembly isthen ready for use.

Figure 13 and 14 are views of the multi-chamber vacuum platen embodiment. Figure 13 is a perspective view of the platen. In thisview,the multichamber platen hasthe same appearance as the single chamber platen 11. In the single chamber platen of Figure 4, a sealing plate fits onto shoulder 11 (a) to form the vacuum chamberwith vacuum line 30 attaching to this sealing plate. Italso has the same channels 62 where the conveyor belts are located 8 GB 2 152 861 A 8 when notin use. Andjurther, both platens have a series of holdswhich communicate from the upper surfacethroughtothe innerchamber. In thesingle chamber platen these are designated 14,while in the muiti-chambered platen theyare designated 63.

Figure 14is a sectionthrough the multi-chamber platen. The side walls 64alsoform partof the outer chamber67. Innerwalls 65form thewallsof chambers68,69, and 70. Openings are passed through the lowersurface and areconnectedto a vacuum manifold. The vacuum manifold and inter connecting tubing isshown in Figure 10.As discus sed above,this multi-chamber platen assemblyis more efficient when various sized circuit boards are to be produced. Thatisjora small board, a vacuum would be drawn onlyon chamber70, whileforthe largest boards a vacuum would bedrawn on all of the 75 chambers in orderto better hold the circuit board blank in place.

In place of the mu Iti-chamber vacuum platen there can be used a platen where the holes in the upper position can each be selectively opened or closed.

One technique is to use holes having an offset configuration. The hole is offset and has a ball closure. When th platen is aluminum or a similar material, magnetic balls (attracted to a magnet) can be placed selectively in the holes to blockthe holes when a vacuum is drawn. Then to remove the balls, it is only necessary to move a magnet overthe platen.

Accordingly the present invention provides a vacuum platen, of a non-magnetic material, capable of selectively having a vacuum drawn thereon comprising an upper surface having holes extending therethrough to an enclosure and means to show a vacuum on said enclosure; said holes being counter sunk so asto accept magnetic balls therein capable of sealing said holeswhen a vacuum is drawn within said enclosure.

If the platen is of a magnetic material, magneticor othershotcan be used with removal accomplished by an overpressure of airwithin the platen. This would blowthe balls out of the holes.

Any liquid polymer composition which is curable to a solid using non-coherent collimated light and strippable in liquid systems can be used in the present processes. Suitable liquid polymer composi tions which can be used arethose setforth in U.S.

Patents 3,660,088 and 3,753,720. However, a prefer red polymer is a urethane having terminal unsatura tion on one end of the polymer molecule and a terminal carboxyl group on the other end of the polymer molecule. The terminal unsaturation is curable byfree radical polymerization using non coherent collimated light, while the terminal carboxyl group provides for strippability of the cured polymer using concentrated alkaline solutions. A polymer having these characteristics and useful in the present 115 processes is one having the following formula:

on 20 0 H a 0 wherein R, is a i alkyl group of from 1 to 6 carbon atoms, R2 is the organic moeity of a diisocyanate and isalicyclic, aryl, alkyl orarylalkyl group, and (PE)x is a polyester or polyether extending chain unit where X is an integerof from 2to 50.

This polymer can be used alone or in conjunction 65 with a viscosity modifier such as a carboxytermin- ated hydroxyethyl methacrylate having the formula:

CP3 0 14 a By adding varying amounts of the carboxyterminated hyd roxyethyl e methacryl ate the viscosity of the liquid polymer composition can be changed. Other viscosity modifers can also be used. Also, useful in the liquid polymer composition is a photosensitizer such as, but not limited to, benzophenone, acetophenone, acenaphthen eq u i none, omethyolybenzophenone, dibenzosuberone, anthraquinone, hexaphenone, or 2,2 - dimethyoxy - 2 - phenyl aceto phenone. These substances enhance free radical generation and, thus speed the curing process. Other photosensitizers can be used.

These arethe prime additives in the preferred liquid polymer composition. However, these compositions can also contain cross- linking agents containing terminals unsaturated andlorterminal thiol groups. These and other possible additives include an epoxy acrylate, a multifunctional acrylate such as hexane- diol diacrylate, a multifunctional thiol such as pentaerythritol - tetrakis - (0 mercaptopropionate) and a phenolic stabilizer such as hydroquinone - mono methyl ether ortrihydroxy benzene. These additives stabilize the composition and also provide means for bridging the polymerized carboxy terminated urethane to itself and also to the carboxy terminated hydroxyethyl methacrylate. Bridging provides fora harder cured polymer, and one which generally has higherthermal stability. For instance, when solder masks are to be made a bridging agent is used so that the polymer could betterwithstand the 400'or 500'17. (204 or 260'C) temperature of tin-lead soldersprays.

This disclosure has been directed primarilyto copper clad circuit board since they are presentlythe predominent printed circuit board presently commercially made. However, the processes are operable with other metal claddings and, fu rther, would be operable with non-metal lic conductive coatings. Therefore, wherever metal cladding is used, it is understood to include equivalent claddings.

This disclosure is directed to the use of noncoherent collimated light. A light having the specifications setforth herein has been developed forthis present use bythe Optical Radiation Company. A laser light beam is a coherent collimated light beam and cannot be used in place of the non-coherent collimated lightwas to be used, the laser beam would, via a computer system, have to be set to scan the design of the desired circuit. Such a laser system would be much more expensive than the noncoherent collimated light and photo tool arrangements discussed in detail in this application, and would require a longer duration in the exposure section leading to a loss of production. Also, a laser system would not meet the image deviation factor of not more than 0.5 mil (0.01 27mm).

The following examples disclose the invention in more detail.

9 GB 2 152 861 A 9 EXAMPLE 1

This example sets forth a process for making one of the preferred liquid polymer composition.

A resin kettle isfitted with a stirrer, thermometer, drying tube and an addition funnel. 185 g of toluene diisocyanate), 0.63 g of hydroquinone monomethyi ether (MEHQ); 5.8 g of triphenyl-phosphite and 0.15 g of dibutyl-tin dilaurate are added to this kettle. Upon heating the mixture to 30'-35'C., there is added dropwise 189 g of hydroxypropyl acrylate, allowing the mixtureto exotherm to 65'C. Thetemperature is maintained between 60'-65'Cfor 1-112 hours atwhich time substantially all of the hydroxy groups are consumed, and the NCO content reaches 1.77 0.1 meg/g as determined bytitration with dibutylamine. Thereafter, 785 g of melted poly (diethyleneglycol adipate) with a molecularweight of 1000 is added along with 0.15 g of dibutyl-tin dilaurate catalyst. The reaction mixture is held at65'-70-Cfor approximately 6 hours until the isocyanate groups are consumed.

385 g of hydroxyethyl methacrylate and 0.42 g of hydroquinone are then added to this mixture. When the reaction mixture has become uniform and the temperature has dropped to below 65'C, 248 g of solid maleic anhydride and 8 g of dibutyl-tin dilaurate are added. Slowly (over about an hour) the reaction mixture is heated up to 55'C to dissolve the maleic anhydride. The heating is continued and held at 7Y- 80'C for approximately 6 hours until the maleic anhydride is completely reacted, as indicated by the absence of 1845 and 1975 cm - 1 peaks in and M spectrograph. The final product thus obtained is a viscous liquid having viscosity of 5700 cps at 25'C and acid group contents of 1.4 meg/a The prepolymer hasthe following composition:

0 CH 0 0 0 3.

H 2c-C-C-O-CH 2-CO-C 0 WC-04CH CH OCH CH -0- C-C H C-3+ R "(5 2 2 2 2 4 8_ 5 CH 3 C H H ? -C-C-C-C-OH CH3 0 A. H ? alon7 with H 2 C-C-C-O-CP 2 CH 2 0 C-C-C-C-OH EXAMPLE 2

Thefollowing composition is preparedfromthe resin product of Example 1 and is used in etch resist 75 processing to make a printed circuit board:

Component The resin product of Example 1 Epoxy acrylate (Epocry] 370) Irgacure5511 Benzophenone Hydroquinone Mono-methyl ether Pyrogallol Leveling agent (Modaflow) Chromophtal Blue (Ciba-Geigy) Pentaerythritol tetra-kis (P-Mereaptopropionate) This composition has a Brookfield viscosity of 3,450 centipoises at 24C.

A copper clad-epoxy f iberg [ass printed circuit board blank was cleaned by wet scrubbing with pu mice using a Scotch Brite pad to remove corrosion and foreig n material. The blank was then coated with the above composition to about 2.5 mil (0.0635mm) for 60 seconds through a photographic negative with a 25 mil (0.635mm) ai r ga p to the liquid coating on the board using a medium pressure mercury vapor lamp at a distance where the radiation intensity is about 25 mil-wattSlCM2. The exposed board is then spray washed using a 5% sodium carbonate solution at room tem peratu re for 10 seconds followed by water rinsing and air drying leaving the cured photo-resist as an image with high fidelity and resolution.

The washed board with the circuit pattern is then subject to hydrochloric acid-cu pric chloride etching for 60 seconds at 520C to dissolve the copper in the uncoated areas. The cured polymer resist remains intact and provides good protection of the copper wt. % 79.93 6.93 0.92 80 2.76 0.046 0.023 2.30 0.64 6.46 againstthe etchant. The cured polymer is then stripped using a 5% solution of sodium hydroxide at 55'C. About30 seconds is required forstripping. The copper remaining on the board is found to have excellent detail and integrity. The circuit board lines are well shaped with edges and have sidewalls which are essentially perpendicularto the substrate. On continuity and resistance testing, it is found that there are no short circuits and the resistance of similar circuit is essentially constant. Also, none of the liquid polymer entered any of the holesthrough the board. EXAMPLE 3 This example illustrates making plate resists using the liquid polymer of Example 2.Acircuit board blank (coppercladding of 1 ounce/sq. ft. or305g. 1m 2) isthen coated with the liquid polymer composition used in Example 2 by reverse coating. The coating thickness was nominally 1.9 mils (0.048mm). An Optical Radiation Company collimated mercury-xenon light was used as the energy source. The photo tool spacing pins were setto produce an air gap of about 30 mils (0.76mm; the photo tool was opaque areas in the same design as the desired circuit). The liquid polymerwas exposed to the mercury-zenon source for 30 seconds. The board wasthen transported to a Cherncut spray washerwhere itwas contacted with sodium carbonate at a PH of 10.5 for 10 seconds.

The exposed copper is then electroplated with tin-iead and the cured polymer is removed in a spray bath using a 5% sodium hydroxide solution at1300-1400F. (540-60'C). Stripping is complete in 30 seconds andthe board iswater rinsed. The exposed coppercladding is then removed by etching with a 3N hydrochloric acid- cupric chloride solution at 52'C. Etching is complete in 60 seconds and the board is then water rinsed. The circuit had high resolution and no short circuits.

EXAMPLE4 This example illustrates making solder masks using the liquid polymer of Example 2.

A finished copper printed circuit board was secured onto a platen and a roller coater by reverse roller coating a continuous layer of the liquid polymer composition of Example 2 onto the circuit board. The coating was about 1.5 mils (0.038mm) thick on the circuit, and about 4to 5 mils (0.1 to 0.1 3mm) thick in the none circuit bearing areas. A photo tool which is transparent, exceptforthe circuit areas where solder is to be deposited, is placed in exact registration with the circuit board. The air gap used was 20 mils (0.5mm). An Optical Radiation Company mercuryxenon collimated light source was used to curethe polymer. The light was extinguished after 60 seconds and the board spray- washed with aqueous sodium carbonate solution at 10.5 pH. Spraywashing was continued for 30 seconds. The board was removed and dried. The circuit board was then placed in a wave soldering apparatus where solderwas applied to the exposed copper area. There was no damage to the cured polymer due to contaetwith molten solder. The cured polymerwas left on the circuit board as a protective layer.

GB 2 152 861 A 10 EXAMPLE 5

Theprocedureof Example 2 was repeated but using asa phototool a circuit design where the circuit lines are to be 6 mils (0.15mm) in width andwith6mil (0.15mm)spacing between circuit lines. The photo tool was a polyesterfilm having a series of 6 mil (0.1 5mm) wide opaque areas spaced apart by 6 mil (0.1 5mm) wide transparent areas. An Optical Radiation Company collimated mercury-zenon light source was used for imaging. The other steps were the same as in Example 2. The resulting circuit board had well defined copper circuit lines which had side walls perpendicularto the board. None of the circuit lines had a short circuit. The circuit lineswere nominally6 mils (0.1 5mm) wide with the spacings also nominally 6 mils (0.1 5mm) wide.

With regard to the preferred class of polymers which can be used herein, we refer to ourApplication No. 82101144 Publication No. 2091278 filed on even date herewith and entitled -Polymer Composition

Claims (2)

Having Terminal Alkene And Terminal Carboxyl Groups". CLAIMS

1. A vacuum platen, of a non-magnetic material, capable of selectively having a vacuum drawn thereon comprising an upper surface having holes extending therethrough to an enclosure and means to show a vacuum on said enclosure; said holes being countersunk so as to accept magnetic bal Is therein capable of sealing said holes when a vacuum is drawn within said enclosure.

2. A vacuum platen according to claim land substantially as herein described with reference to Figure 1 la ofthe drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, 8818935, 8185, 18996. Published at the Patent Office, 25 Southampton Buildings, London WC2A lAY, from which copies may be obtained.