Abstract:
An improved method of and apparatus that is continuously automatically operative in an in-line system is described for applying under vacuum, heat and mechanical pressure a dry film photoresist-forming layer to printed circuit boards ( 200 ) that already have been prelaminated by the loose application thereto of the dry film resist as discrete cut sheets within the confines of the surface of the boards whereby a laminate without entrapped air bubbles and closely conforming to the raised circuit traces and irregular surface contours of the printed circuit board is obtained. Featured is a conveyorized vacuum applicator ( 12 ) comprising two independent vacuum lamination chambers ( 18,20 ) in end-to-end relation. The first vacuum chamber operates at ambient temperature to draw off all of the air entrapped between the dry film resist and the surface of the printed circuit board at conditions that do not result in premature tacking of the dry film to the surface of the board. Then, in the second vacuum chamber, the photoresist-forming layer is immediately laminated to the printed circuit board under heat and mechanical pressure. The forgoing reduces or eliminates common lamination defects such as premature resist tacking and the attendant need to repair or rework the printed circuit board.

Description:
The present application is a divisional of U.S. application Ser. No. 09/648,428 filed Aug. 25, 2000, which is now U.S. Pat. No. 6,610,459. 

   BACKGROUND OF THE INVENTION 
   The present invention is directed to an automatic conveyorized vacuum applicator and method of operation thereof having utility in the application of dry film photoresist-forming materials, such as photoresists and solder masks, to surfaces of printed circuit boards or other substrates, to assure complete conformance of the dry films around raised circuit traces and irregular surface contours. The applicator and method have particular utility for conveying and for applying vacuum, heat, and mechanical pressure to printed circuit boards or other substrates that prior to such application have had dry film loosely applied to at least one of the surfaces thereof as discrete cut sheets within the confines of the substrate. 
   A primary photoresist is a hard, temporary layer of non-conductive material which covers the metal surface of a copper-clad substrate that later becomes the printed circuit board. The photoresist is patterned in such a way so as to produce a resist stencil around which the printed circuit tracks are formed. 
   More specifically, primary photoresists, typically, are formed from a layer of photoimageable composition which is applied to the surface of a copper-clad board. The photoimageable composition is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent, aqueous, or semi-aqueous solution which washes away either exposed or unexposed portions of the layer (depending on whether the photoimageable material is positive-acting or negative-acting). Thereafter, the circuit traces are formed by either electroplating or etching. In a typical plating procedure, the areas devoid of photoresist that become the circuitry are built up from the board surface by electroplating copper thereon. After protecting the electroplated copper layer, the remaining photoresist is stripped away in an organic solvent, aqueous, or semi-aqueous solution, and the newly exposed areas of metal are then selectively removed in an etching solution, leaving behind the pattern plated copper circuit lines. In a typical etching procedure, the metal in the areas devoid of photoresist is selectively removed in an etching solution, leaving behind the residual portions of the etched metal layer as the circuit traces after the primary resist is stripped away. 
   A solder mask, on the other hand, is a hard, permanent layer of non-conductive material which covers the surface of a printed circuit board or other substrate, encapsulating the traces of the printed circuitry itself. The solder mask is patterned to fully cover the circuitry, except for those portions intended to be exposed, e.g., for soldering to another component. 
   More specifically, solder masks, typically, are formed from a layer of photoimageable composition which is applied to a surface of the printed circuit board. Similar to primary imaging resists, the photoimageable layer is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent, aqueous, or semi-aqueous solution which washes away either exposed or unexposed portions of the layer (again depending upon whether the photoimageable material is positive-acting or negative-acting). The portion of the layer which remains on the surface is then cured, e.g., with heat and/or UV light, to form a hard, permanent solder mask intended to protect the printed circuitry for the life of the board. 
   One prior art method of applying a layer of primary resist or solder mask to a circuit board surface is to apply the material in liquid form, and then, either allow it to dry or partially cure the material to form a semi-stable layer. There are a number of advantages, however, to applying a photoimageable layer to a circuit board as a dry film rather than as a liquid. In particular, dry films are free of organic solvent and therefore eliminate this hazard from the workplace and eliminate the need for apparatus to protect the immediate work environment and the more general environment from organic solvent emissions. 
   Typically, such a dry film comprises a cover sheet of support material which is somewhat flexible but which has sufficient rigidity to provide structure to a layer of photoimageable composition which overlies one surface of the cover sheet. The cover sheet may be formed of polyester material, such a polyethylene terephthalate (PET). To protect the photoimageable layer and to enable the dry film to be rolled, it is conventional for the exposed surface of the photoimageable layer to be covered with a removable protective sheet, e.g., a sheet of polyethylene. 
   The method of use of such a dry film is generally as follows. The protective polyethylene sheet is removed from the photoimageable composition layer immediately prior to application of the dry film to the surface of the printed circuit board. This may be accomplished, for example, using automated apparatus which peels away and rolls up the protective sheet as the dry film is unrolled from a reel. The dry film is applied to the surface of the circuit board with the photoimageable layer in direct contact with the board surface. Then using either heat and mechanical pressure (in the case of roll laminators) or a combination of vacuum, heat, and mechanical pressure (in the case of vacuum laminators), the photoimageable layer is immediately laminated to the surface of the board. The cover sheet remains overlying the photoimageable layer, protecting the photoimageable layer from exposure to oxygen and from handling damage. The cover sheet also permits a pattern (or template) to be laid directly on top of the dry film for contact printing, if contact printing is to be used (as is usually preferred from the standpoint of obtaining optimal image resolution). The dry film is exposed to patterned actinic radiation through the PET cover sheet. At this time, the PET cover sheet is removed, permitting access to the exposed photoimageable layer by developer. Depending-upon the composition of the photoimageable layer, the photoimageable layer is developed with organic solvent, aqueous developer, or semi-aqueous developer. The photoimageable layer may either be positive-acting, in which case the exposed portions are removed by developer, or negative-acting, in which case the unexposed portions are removed by developer. Most photoimageable layers for preparing primary imaging photoresists and solder masks are negative-acting. Subsequent to development, primary resists, in particular, are subjected to either electroplating or etching, as previously described, to form the circuit traces after which the remaining photoresist is stripped away with organic solvent, aqueous stripper, or semi-aqueous stripper. Whereas, in the case of solder masks which remain on the board permanently, most photoimageable composition layers require some cure subsequent to development to render the layer hard and permanent so as to serve as a solder mask. Depending upon the composition of the photoimageable layer, curing may be effected with heat and/or UV light. 
   Printed circuit boards almost invariably have uneven surfaces which present difficulties for dry film application. During solder mask application, in particular, such unevenness is usually attributed to the circuitry traces which are raised or elevated over the surface of the board of electrically non-conducting material. It is therefore desirable that any dry film solder mask applied to the board be able to conform around the upstanding circuitry traces to minimize the risk of defects, such as short circuits. On the other hand, during primary resist application, such unevenness usually arises when creating circuitry on thin outer surfaces of multi-layered circuit boards which contain embedded components that protrude and leave impressions on the outer surface. It is desirable that any photoresist applied to such a board be able to conform to such irregular surface contours to minimize the formation of defects, such as voids, disconnects, or shorts. There has also been a demand on circuit board manufactures, due to the current trend to miniaturize electronic equipment, to reduce the size of printed circuit boards while increasing their functional capabilities which presents other difficulties for dry film photoresist application. As more circuitry needs to be fit onto smaller surfaces, the circuit lines and spaces therebetween on the circuit board have continued to shrink. The creation of this fine line and closely spaced circuitry can be achieved only with difficulty and only if the primary resist fully adheres and completely conforms to the contours of the printed circuit board. Otherwise, voiding of the minute circuit traces and creation of disconnects or shorts will occur. 
   A number of improved photoimageable dry films and vacuum lamination processes have been developed to try to improve the conformance of the dry film to the irregular surface contours of a printed circuit board, as for example, as disclosed in U.S. Pat. No. 4,889,790 (Roos et al.), U.S. Pat. No. 4,992,354 (Axon et al.), and U.S. Pat. No. 5,164,284 (Briguglio et al.), The processes disclosed in these patents involve applying a photoresist-forming layer to a printed circuit board using a dry film in which an “intermediate layer” selected for its transparency, strength and flexibility is interposed between the support film or cover sheet and the photoimageable layer. The intermediate layer of the dry film is selectively more adherent to the photoimageable composition layer than to the cover sheet, allowing the cover sheet to be removed after the photoimageable layer is laminated to a printed circuit board to assist conformance, with the intermediate layer remaining on the photoimageable composition layer as a “top coat.” The top coat is of non-tacky material and can be placed in contact with other surfaces, such as artwork for contact printing. The top coat also serves as an oxygen barrier, allowing the photoimageable composition layer to remain unexposed on the printed circuit board, after cover sheet removal, for some length of time. The use of dry film having the “intermediate layer” or “top coat” make possible the processes described in these patents. 
   In each case, to form a more conforming dry film, the protective polyethylene sheet is first peeled away and the exposed surface of the photoimageable composition layer is applied to the surface of the printed circuit board. Using vacuum, heat and mechanical pressure, the dry film is laminated to the surface of the printed circuit board, partially conforming the photoimageable layer thereto. Within about 60 seconds and before substantial cooling of the printed circuit board and dry film has occurred, the cover sheet of the dry film is removed, whereupon the photoimageable composition layer and overlying top coat fully conform to the contours of the printed circuit board and substantially encapsulate the traces and surface contours before conventional processing. Because the cover sheet is removed prior to the final conforming step, better conformance, particularly when applying thin photoimageable composition layers onto boards with closely spaced traces, is achieved. Better resolution is also achievable because the top coat may be directly contacted with artwork for contact printing and because the top coat is much thinner than a cover sheet or support film and is, therefore, much less a deterrent to good resolution than a support film. 
   In U.S. Pat. No. 4,946,524 (Stumpf et al.), there is disclosed an applicator and process for applying a conforming dry film material to the surface of a printed circuit board allowing for, at the same time, the removal of the protective sheet, subsequent handling of the board with the applied film, and the draw-off of air enclosed between the film and the board. The draw-off of air enclosed between the dry film and the surface of the printed circuit board is facilitated when, before vacuum lamination, the surface of the board is covered with a loose sheet of film. To that end the applicator of U.S. Pat. No. 4,946,524 is operative to tack the dry film to a board at the leading and trailing edges with the intermediate portion of the film loosely applied thereto. The film is tacked to the board as a discrete cut sheet within the confines of the perimeter of the surface of the board. For convenience, a printed circuit board having such loose application of a dry film sheet to the surface or surfaces thereof is referred to hereinafter as being “prelaminated.” 
   In order to adapt the processes described in the preceding patents for continuous automatic operation in an in-line system, there is disclosed in U.S. Pat. No. 5,292,388 (Candore) an automatic conveyorized vacuum laminator apparatus. The apparatus of U.S. Pat. No. 5,292,388 provides an improved and efficient means for automatically conveying and applying vacuum, heat, and mechanical pressure to prelaminated printed circuit boards or substrates and overcomes the difficulties encountered with the utilization of a conventional batch vacuum laminator in an automated in-line system. The automatic conveyorized vacuum laminator is comprised of two main parts, a vacuum laminator and an input roll conveyor for feeding prelaminated circuit boards into the vacuum laminator from the preceding prelaminating equipment. The vacuum laminator, in particular, comprises a single vacuum chamber defined by heated upper and lower platens, and an endless belt conveyor disposed between the platens for movement of the printed circuit boards into and out of the vacuum chamber region. In operation, the prelaminated circuit board (i.e., having the dry film photoimageable material loosely applied to its surface) to be vacuum laminated is transferred from the input roll conveyor to the endless belt which moves the board into proper vacuum lamination position between the heated upper and lower platens. Thereafter, the lower platen is raised into sealing engagement with the upper platen in order to capture in the vacuum chamber the endless belt conveyor and the prelaminated board resting on the endless belt. Next, a vacuum is drawn in the vacuum chamber between the platens to evacuate all air between the dry film and surface of the prelaminated board, followed by application of heat and mechanical pressure to conform the dry film to the board. When the cycle is complete, the lower platen is lowered and the laminated board in conveyed away to subsequent processing equipment, while the next board to be vacuum laminated arrives for the next vacuum lamination cycle. 
   Difficulty has been encountered, however, with the operation of such a conveyorized vacuum lamination apparatus, as described in U.S. Pat. No. 5,292,388. Particularly, premature tacking of the dry film to the board surface prior to chamber evacuation has been a problem. The problem is particularly prevalent with thin boards (e.g. &lt;0.25 mm.), since they are susceptible to rapid heating. In order to assure complete conformance of the dry film around the circuit traces and substrate surface contours, it is necessary that the loose sheet of dry film prelaminated to the board allow for all air enclosed between it and surface of the printed circuit board to be evacuated before applying heat and mechanical pressure to conform the film to the board. Yet, with the above apparatus, the residual heat given off by the belt conveyor just after having completed a prior vacuum lamination cycle has a tendency to cause premature tacking of the film on the next board entering into the vacuum chamber prior to commencement of the vacuum lamination cycle. Premature adhesion prevents air from escaping from certain areas during vacuum lamination, which, in turn, prevents film conformance. In the case of solder masks, lack of film conformance results in lamination defects, such as unwanted puddling caused by premature adhesion. In the case of primary resists, lack of film conformance tends to result in voiding of entire portions of the circuit traces caused by incomplete adhesion, as well as puddling as previously described. The present invention was devised to address this problem. 
   While there has been some attempt to address this premature tacking problem, a satisfactory answer has yet to be devised. For instance, it has been proposed to process the dry films in conventional batch-oriented vacuum laminating equipment outfitted with removable copper heat shields between the upper and lower platens. The removable heat shields are manually inserted between the upper and lower platens immediately before placement of the board in the vacuum chamber. Evacuation is then commenced with the heat shields serving to insulate the resist from elevated temperatures long enough to be able to remove all of the air between the resist and the board before application of heat and mechanical pressure. However, batchwise processing is highly undesirable because it is entirely too slow for mass production of printed circuit boards and extremely labor intensive. 
   SUMMARY OF THE INVENTION 
   An object of the invention is, therefore, to provide an improved method of and apparatus for applying under vacuum, heat and mechanical pressure a dry film photoresist or solder mask to prelaminated printed circuit boards or other substrates, thereby to remove all of the air entrapped between the dry film and the surface of the printed circuit board or substrate to assure complete conformance of the dry film around the raised circuit traces and the substrate surface contours. 
   Another object of the invention is to provide an improved method of and apparatus for vacuum laminating prelaminated printed circuit boards and substrates, which method and apparatus prevent premature tacking of the loosely applied prelaminated dry film to the surface of the printed circuit board or substrate prior to evacuation of all of the air between the dry film and the board or substrate surface. 
   Still another object of the invention is to provide an improved method of and apparatus for vacuum laminating prelaminated printed circuit boards and substrates which are both operable in an in-line system and in a fully automated continuous manner. 
   In accomplishing the forgoing and other objectives of the invention, there is provided an improved method of laminating a prelaminated printed circuit board or other substrate which prevents premature tacking of the dry film photoresist-forming layer to the board comprising the following key features: (a) placing the board in a first vacuum lamination chamber of a vacuum laminator having two independent (i.e., dual) vacuum lamination chambers; (b) drawing a vacuum in the first chamber at ambient temperature for a time sufficient to evacuate substantially all of the air from between the dry film and the surface of the board or substrate and thereby place the dry film in intimate contact with the surface of the board or substrate; (c) immediately placing the board in a second independent vacuum lamination chamber of the vacuum laminator; and, (d) applying sufficient heat to the dry film on the board or substrate in a second vacuum lamination chamber to cause the dry film to flow and then sufficient mechanical pressure on the board or substrate to thereby force the heated laminate to conform closely to the surface contours of the board or substrate. 
   The aforesaid steps (a)–(d) are preferably performed in-line and in a continuous automated manner, so that the method can be adapted for use in an fully automated in-line system for manufacturing printed circuit boards. 
   Steps (b) and (d) are also preferably performed in alternating sequence to allow for at least one prelaminated board to be in each vacuum chamber at the same time which, in turn, provides for at least a two-fold increase in manufacturing productivity. 
   In accomplishing these and other objectives of the invention, there is also provided an improved dry film photoresist or solder mask vacuum lamination apparatus comprising the following key features: the provision of two independent (i.e., dual) vacuum lamination chambers in end-to-end relation, the first lamination chamber being operated at ambient temperature while a vacuum is drawn so as to reduce the air pressure within the chamber and draw off all of the air between the loosely applied prelaminated dry film and the surface of the printed circuit board or substrate, thereby to place the dry film in intimate contact with the substrate surface while at the same time preventing premature tacking or adhesion of the dry film to the substrate prior to conforming lamination, and the second lamination chamber being operated immediately after the first chamber so as to laminate the previously evacuated dry film to the printed circuit board or substrate under heat and mechanical pressure, thereby to assure complete conformance of the dry film around the raised circuit traces and the substrate surface contours. 
   The aforesaid apparatus is preferably further characterized by the capacity thereof for continuous operation and the provision of conveyor belts for conveying prelaminated printed circuit boards or substrates into and out of the first and second vacuum lamination chambers of the vacuum applicator. It is also preferable to provide such a continuously operative conveyorized vacuum applicator that is operative, in association with automated input roll conveyors for feeding prelaminated printed circuit boards or substrates onto the automated conveyor belts, in such a way as to allow at least one board or substrate to be in each vacuum chamber of the vacuum laminator, while the next board or substrate to be vacuum laminated is staged in position on an input roll conveyor ready for the next vacuum lamination cycle to begin. Upon completion of the vacuum lamination cycle in each chamber, the printed circuit board in the second vacuum chamber is automatically conveyed out of the vacuum laminator, the board in the first vacuum chamber is conveyed to the second vacuum chamber, and the staged new printed circuit board to be vacuum laminated is conveyed into the first vacuum chamber. 
   The automatic conveyorized vacuum applicator has particular utility in conveying printed circuit boards and applying heat, vacuum and mechanical pressure to printed circuit boards that have been prelaminated with photoresist or solder mask dry film in accordance with the process described in U.S. Pat. No. 4,946,524 and fabricated in accordance with processes described in U.S. Pat. Nos. 4,889,790, 4,992,354, and 5,164,284. 
   The conveyorized dry film photoresist or solder mask applicator of the invention is an important component in the total arrangement of an automatic continuous flow of material in in-line processing of dry photoresist or solder mask films requiring vacuum lamination during processing. 
   The invention provides the means to automate the vacuum application process as an in-line system, while at the same time 1) reducing common lamination defects, such as premature resist adhesion, 2) substantially eliminating the need to repair or rework finished printed circuit boards, and 3) increasing printed circuit board manufacturing productivity by at least two-fold. 
   With this description of the invention, a detailed description follows with reference being made to the accompanying figures of drawing which form part of the specification in which like parts are designated by the same reference numbers and of which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a cabinet structure in which the conveyorized dual chamber vacuum applicator of the present invention is housed; 
       FIG. 2  is a diagrammatic perspective view on a scale larger than of  FIG. 1  illustrating the conveyor system of the conveyorized vacuum applicator for sequentially feeding prelaminated printed circuit boards or substrates through the vacuum laminator; 
       FIGS. 3–5  and  10  are fragmented detail views which illustrate various features of the applicator of  FIGS. 1 and 2 ; 
       FIGS. 6–9  are cross sectional views of a vacuum laminator that advantageously may be used with the conveyorized vacuum applicator and which illustrate a platen operation sequence thereof; 
       FIGS. 11–24  are diagrammatic perspective views on a smaller scale than shown in  FIG. 2  that illustrate the function cycle of the conveyorized vacuum applicator when employed to feed printed circuit boards or substrates one at a time through the vacuum laminator; and, 
       FIG. 25  is a diagrammatic perspective view of an alternative second chamber that may be employed in the conveyorized vacuum applicator of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The conveyorized vacuum applicator according to the present invention has particular utility in the vacuum lamination of printed circuit boards and substrates of varying thickness and sizes, typically in a range from between 0.1 and 3.2 mm. and a range from between 25×38 and 60×71 cm., which boards or substrates have been “prelaminated” with a loose sheet of dry film primary photoresist or solder mask, with our without a “top coat” layer, as hereinbefore described. The specific function of the conveyorized vacuum applicator is to automatically apply a combination of vacuum, heat and mechanical pressure in such a way so as to avoid premature tacking and thus completely remove all of the air between the dry film and the surface of the board or substrate to assure positive conformance of the dry film around etched or electroplated circuit traces and irregular substrate surface contours. 
   Referring to  FIGS. 1 and 2  there is shown a support structure or frame  10  on which is mounted the conveyorized vacuum applicator, designated  12 , according to the invention. The conveyorized vacuum applicator  12  is comprised of two parts. One part comprises first and second input or feed conveyors  14  and  16 . The other part comprises first and second vacuum lamination sections  18  and  20 . Each of the first and second vacuum lamination sections  18  and  20  include a first and second ¾ belt conveyor  22  and  24  and a first and second vacuum laminator  26  and  28 , respectively. 
   As shown in  FIG. 2 , the first input conveyor  14 , first ¾ belt conveyor  22 , second input conveyor  16 , and second ¾ belt conveyor  24  extend in end-to-end relation, in that order, to define a continuous  18  path into and out of each vacuum section  18  and  20 . 
   Each of the first and second input conveyors  14  and  16  comprise a plurality of chain coupled rolls  15  and  17 , respectively, which rolls  15  and  17  extend for a substantial distance across the width of the applicator  12 . Positioned for vertical movement between the exit end  14   b  of the first input conveyor  14  and the entrance end  22   a  of the first ¾ belt conveyor  22  is an adjustable barrier  30 . The barrier  30  extends across the width of the applicator  12  and is movable upwardly by an individually associated air cylinder  32 , as shown in  FIG. 2 . Such movement is from a “down” or non-blocking position to an “up” position to block the transport to the first ¾ belt conveyor  22  of a printed circuit board being transported on the first input conveyor  14  from preceding equipment indicated at  34 . 
   As seen in  FIG. 2 , a photocell  36  is provided for sensing the approach of a printed circuit board to the exit end  14   b  of the input conveyor  14  and for initiating the actuation of the air cylinder  32  for effecting the movement of the barrier  30  between the printed circuit board non-blocking and blocking positions thereof. 
   Each of the ¾ belt conveyors  22  and  24  includes an input roll  38  and  40  and an output roll  42  and  44 , respectively, which rolls extend across the width of the applicator  12 . Wound around each pair of cooperating input and output rolls are a pair of spaced endless chains, with the spacing being such that one of each pair of chains  46  and  48  is on one side of applicator  12  and the other of each pair of chains  50  and  52  is on the other side thereof. Chains  46  and  48  mesh with individual gears  54  and  56 , respectively, provided on the end of each corresponding input roll  38  and  40 , and gears  58  and  60  provided on the end of each corresponding output roll  42  and  44 , as shown in  FIG. 2 . Similarly chains  50  and  52  mesh with gears provided on the other ends of each corresponding input rolls  38  and  40  and output rolls  42  and  44 . Thus, as shown in  FIG. 3 , chains  50  and  52  mesh with gears  62  and  64 , respectively, on the end of the output rolls  42  and  44 . 
   Positioned between each associated pair of chains  46 ,  50  and  48 ,  52  and securely attached thereto at each end by suitable grippers  66  and  68 , as shown in  FIG. 3 , are respective belts  70  and  72  that each extend about three quarters of the distance around the loop formed by the chains. Each gripper  66  and  68  includes a respective bar  66   a  and  68   a  that is securely attached at one end to chain  46  and  48  and at the other end to the chain  50  and  52 , respectively. Carried by each of the bars  66   a  and  68   b  and securely attached thereto by suitable bolts or rivets are respective bar members  66   b  and  68   b  and  66   c  and  68   c  of shorter length between which the ends of the respective belts  70  and  72  are captured and retained. Thus, as best seen in  FIG. 2 , each of the belts  70  and  72  have an associated aperture or opening  74  and  76  therein for the full width thereof, the length of which aperture  74  and  76  is about a quarter of the distance around the loop of each individually associated belt conveyor  22  and  24 . 
   Each of the belts  70  and  72  may be made of very thin fiberglass reinforced rubber or Teflon coated fiberglass. A total thickness of the belt in the range of 0.013 to 0.025 cm. is desirable to ensure that there is a complete seal when drawing a vacuum in each vacuum laminator  26  and  28 . This is for the reason that the upper run  70   a  and  72   a  of each belt  70  and  72  is captured between the upper and lower platens of each vacuum laminator  26  and  28  during the vacuum lamination process. 
   Motive power for driving the chain coupled rolls of the first input conveyor  14  and the first ¾ belt conveyor  22  associated therewith is provided by a first electrical motor  78 . Motive power for driving the chain coupled rolls of the second input conveyor  16  and the second ¾ belt conveyor  22  associated therewith is provided by a second electrical motor  80 . Motors  78  and  80  may each comprise a direct current electrical motor provided with separate drive gears  82  and  84  and  86  and  88 , respectively, for driving their respective input conveyors  14  and  16  and belt conveyors  22  and  24 . 
   As shown in  FIG. 2 , motor  78  is coupled by gear  82  and chain drive gearing  90  to the first input conveyor  14 . Selective or conjoint drive of the input conveyor  14  is provided by electromagnetic clutch  92 . Motor  80  is coupled by gear  86  and chain drive gearing  94  to the second input conveyor  16 . Selective or conjoint drive of the input conveyor  16  with the other conveyors is provided by electromagnetic clutch  96 . Energization and deenergization of clutch  92  controls the rotation of the chain coupled rolls of the first input conveyor  14 . Similarly, energization and deenergization of clutch  96  controls the rotation of the chain coupled rolls of the second input conveyor  16 . 
   Motor  78  is also coupled by gear  84  and chain drive gearing  98  and  100  to the drive shaft  102  of the output roll  42  of the first ¾ belt conveyor  22 . An electromagnetic clutch  104  positioned between chain drive gearing  100  and  102  provides for the selective control of the operation of the first ¾ belt conveyor  22 . Motor  80  is similarly coupled by gear  86  and chain drive gearing  106  and  108  to the drive shaft  110  of the output roll  44  of the second ¾ belt conveyor  24 . Similarly, an electromagnetic clutch  112  positioned between the chain drive gearing  108  and  110  provides for selective control of the operation of the second ¾ belt conveyor  24 . 
   In accordance with the invention, each of the motors  78  and  80  are a variable speed motor, being selective energizable from a source of direct current (not shown) through motor speed control potentiometers  114 ,  116  and  118  and  120 ,  122  and  124 , respectively, and corresponding selector switches  126  and  128 , as shown in  FIG. 2 , to drive the input conveyors  14  and  16  at the speed of about three (3) meters/minute (m/min), to drive the input conveyors  14  and  16  and ¾ belt conveyors  22  and  24  at a speed of about nine (9) m/min, and to drive the ¾ belt conveyors  22  and  24  only at a speed of 30 m/min, as further described hereinafter. The arrangement is such that the input conveyors  14  and  16  can be driven independently of each other and of the ¾ belt conveyors  22  and  24 . Similarly, the ¾ belt conveyors  22  and  24  can be driven independently of each of the input conveyors  14  and  16 . At no time, however, when driven at the same time, can the speeds of the conveyors  14 ,  16 ,  22  and  24  be different. 
   For the purpose of enabling the tension of the ¾ belts  70  and  72  of the first and second ¾ belt conveyors  22  and  24  to be relieved at a desired point in the vacuum process, as shown in  FIG. 2 , bearings  130  and  132  in which the shaft of the input rolls  38  and  40  of each of the ¾ belt conveyors  22  and  24  are mounted for rotation are arranged to be shifted a short distance toward and away from the corresponding vacuum laminator  26  and  28  by a respective two-position air cylinder  134  and  136 . 
   For sensing when a prelaminated printed circuit board has been moved by the belt conveyors  22  and  24  to a proper position relative to its individually associated vacuum laminator  26  and  28  for the vacuum lamination process to proceed, there are provided, as best seen in  FIGS. 2 and 4 , respective cams  138  and  140  and cooperating sensors  142  and  144 . Cams  138  and  140  are mounted on and move respectively with their corresponding endless chains  46  and  48  around the loop of each of the individually associated belt conveyors  22  and  24 . Corresponding sensors  142  and  144  are mounted in any suitable manner on the frame  10  of the applicator  12  in cooperative relation with their respective cams  138  and  140 . 
   When the printed circuit board is in the proper position relative to the intended vacuum laminator  26  and  28  for the vacuum lamination process to proceed, the aperture  74  and  76  in the belt  70  and  72  of the belt conveyors  22  and  24  is positioned immediately, that is, vertically, below the vacuum laminator, as best seen in  FIG. 2 . This allows the lower platens  146  and  148  of respective first and second vacuum laminators  26  and  28  to be lifted up through the aperture  74  and  76  in each of the belts  70  and  72  into cooperative relation with the upper platens  150  and  152  of the respective vacuum laminators  26  and  28  for effecting the vacuum lamination of a printed circuit board then resting on the surface of the upper runs  70   a  and  72   b  of the belts  70  and  72  within the confines of the first and second vacuum laminators  26  and  28 , respectively. 
   There is an initial position for each of the first and second belt conveyors  22  and  24  such that upon the transfer of a printed circuit board from either the first or second input conveyors  14  and  16 , the printed circuit board will be moved within the laminating region of the respective vacuum laminator  26  and  28  while apertures  74  and  76  are moved to a position vertically below each of the vacuum laminators  26  and  28 . For convenience, that initial position of each of the belts  70  and  72  is herein referred to as the “set-point” position of the belt conveyors  22  and  24 . 
   For sensing the set-point position of each of the belt conveyors  22  and  24 , there are provided respective cams  154  and  156  that are mounted on each of the endless chains  50  and  52  and cooperating sensors  158  and  160  that may be mounted on the frame  10  of the applicator  12  as illustrated in  FIGS. 2 and 5 . 
   In order to provide a signal anticipatory of the approach of each of the belt conveyors  22  and  24  to the set-point position thereby to enable relatively fast operation in the return of the belt conveyors  22  and  24  to the set-point position, there are also provided respective cams  162  and  164  sensors  166  and  168  for slowing down the speed of each belt conveyor  22  and  24  to the set-point position, as illustrated in  FIGS. 2 and 5 . 
   For detecting the presence of a processed printed circuit board or substrate at the exit end of the belt conveyors  22  and  24 , there are provided respective output photocells  170  and  172 , as shown in  FIG. 2 . 
   Also, as shown in  FIG. 2 , an infrared sensor  174  is provided for sensing the temperature of the processed printed circuit board or substrate as it is conveyed out of the second laminator  28 . The temperature of the processed printed circuit board or substrate, as sensed by sensor  174  and indicated or displayed by suitable means, facilitates control of the heating means in the second vacuum laminator  28  thereby to preclude overheating thereof and possible damage to the circuit board or substrate being vacuum laminated. 
   Since the sheets of dry film applied to the prelaminated printed circuit boards being vacuum laminated have high flow characteristics in the temperature range of 30° C. to 150° C., the vacuum lamination process may be carried out within this range. 
   The vacuum laminators  26  and  28  that advantageously may be used in the conveyorized vacuum laminator  12  are illustrated  FIGS. 6–9 . The vacuum laminators  26  and  28  can be provided as part of an integral dual chamber machine, as shown in  FIG. 2 , or, if desired, as separate vacuum lamination units arranged in end-to-end relation. Each of the vacuum laminators  26  and  28 , although shown as being identically constructed, are operated in different modes according to this invention, as described below, in order to perform separate functions in the vacuum lamination process which in combination assure complete conformance of the dry film to the substrate surface. 
   Referring to  FIG. 6  (with the prime (′) symbol being used to denote previously unnumbered parts associated with the first laminator  26  and double prime (″) being used for those of the second laminator  28 ), each of the laminators  26  and  28  include an upper stationary platen  150  and  152  and a corresponding movable lower platen  146  and  148 , respectively. Associated with each of the upper platens  150  and  152  is a resilient silicon rubber blanket  176 ′,  176 ″ that forms a ceiling for the vacuum chamber region indicated at  178 ′,  178 ″ in  FIGS. 6 ,  8  and  9 . Each lower platen  146  and  148  has a well  180 ′,  180 ″ into which a prelaminated printed circuit board or substrate to be vacuum laminated is positioned on a silicon rubber insert  182 ′,  182 ″ for vacuum lamination. Sealing means  184 ′,  184 ″ in the form of an O-ring surrounding the circumference of each of the lower platens  146  and  148  is provided for hermetically sealing the well  180 ′,  180 ″ for the evacuation of air therefrom by a vacuum pump  186 ′,  186 ″ when the respective lower platen  146  and  148  is moved upward into contact with an upper platen  150  and  152 . One or more shim inserts  188 ′,  188 ″ may be provided, as shown in  FIG. 6 , to accommodate printed circuit boards of different thicknesses, that is, for adjusting the printed circuit boards to an optimum position in the well  180 ′,  180 ″ for best vacuum lamination operation. 
   Both upper and lower platens include heaters, specifically a heater  190 ′,  190 ″ in each of the upper platens  150  and  152  and a heater  192 ′,  192 ″ in each of the lower platens  146  and  148 . As described below, the platen heaters may be on or off depending on the desired mode of laminator operation. 
   Printed circuit boards that have been prelaminated, that is, have had dry film photoresist or solder mask previously loosely applied to one or both sides thereof, as described hereinbefore, are vacuum laminated in the vacuum laminators  26  and  28  in the following sequence in accordance with the present invention:
     (1) The board to be vacuum laminated is first placed in the well  180 ′ of the lower platen  146  of the first vacuum laminator  26  on top of the silicon rubber insert  182 ′. This is facilitated by relieving the tension on the first conveyor belt  70  on the surface of which the board has been conveyed to the region of the first vacuum chamber  178 ′.   (2) The lower platen  148  is then moved upward, as shown in  FIG. 8 , to seal, by means of the O-ring  184 ′, the well  180 ′ which together with the blanket  176 ′ forms the first vacuum chamber  178 ′. Note that the belt  70  on which the board being vacuum laminated rests is also captured between the upper platen  150  and the lower platen  146 .   (3) With the platen heaters  190 ′ and  192 ′ dormant, the vacuum process cycle is started by the energization of the vacuum pump  186 ′ thereby to evacuate air from the vacuum chamber  178 ′. During this stage, channels  194 ′ in the upper platen  150  of the first vacuum laminator  26  are closed, so that air is not also evacuated from the region between the upper platen  150  and the blanket  176 ′. Note that this process step operates at ambient temperature which prevents pretacking of the prelaminated film to the board.   (4) When the first vacuum cycle is complete, the vacuum in the first vacuum chamber  178 ′ is released by allowing atmospheric air to enter therein, whereby the lower platen  146  is moved downward out of contact with the upper platen  150 . Tension in the belt  70  is then restored to allow the board to be conveyed to the second vacuum laminating operation.   (5) The board is then moved immediately to the second vacuum laminator  28  and placed in the well  180 ″ of the lower platen  148  thereof on top of the silicon rubber insert  182 ″. Similarly, this is facilitated by relieving the tension on the second conveyor belt  72  on the surface of which the board has been conveyed to the region of the second vacuum chamber  178 ″.   (6) The lower platen  148  of the second vacuum laminator  28 , which in this stage is heated, is moved upward, as shown in  FIG. 8 , to seal, by means of the O-ring  184 ″, the well  180 ″ which together with the blanket  176 ″ forms in the same manner as set forth above the second vacuum chamber  178 ″. Note that the belt  72  on which the board being vacuum laminated rests is also captured between the upper platen  152 , which is this stage is heated as well, and the lower platen  148 .   (7) The second vacuum process cycle is started by the energization of the vacuum pump  186 ″ thereby to evacuate air from the vacuum chamber  178 ″ and from the region between the upper platen  152  and the blanket  176 ″.   (8) For a set period at the end of a first stage of the second vacuum process cycle, there is a second stage or “slap down” of the blanket  176 ″ in the upper platen  152 , as shown in  FIG. 9 . This is effected by opening channels  194 ″ in the upper platen  152  to allow atmospheric air or compressed air (e.g. 1 to 5 bars) to enter the space between the blanket  176 ″ and the upper platen  152 . Such slap down applies mechanical pressure on the printed circuit board to force the now heated film to conform around the raised circuit traces or substrate surface contours. While it is not necessary to pull a vacuum in the second chamber for film evacuation, with this equipment it enables slap down to effectively occur.   (9) When the second vacuum cycle is complete, the vacuum in the second vacuum chamber  178 ″ is released by allowing atmospheric air to enter therein whereby the heated lower platen  148  is moved downward out of contact with the heated upper platen  152 . Tension in the belt  70  is then restored to allow the board to be moved to the next operation.   

   It is noted that, in accord with the invention, the prelaminated boards to be vacuum laminated by the conveyorized vacuum applicator  12  will have been centered by preceding equipment in the in-line system, although, if desired, adjustable guides  196  may be provided for that purpose in association with the input conveyors  14  and  16 , as illustrated in  FIG. 10 . 
   The function cycle of the improved conveyorized vacuum applicator  12  of the present invention that prevents premature tacking or adhesion of the dry film to the board prior to film evacuation is illustrated by  FIGS. 11–24 . 
   In step  1  of the sequence, as shown in  FIG. 11 , a prelaminated circuit board  200  is shown arriving on the input conveyor  14  from preceding equipment running at a speed of 3 m/min. The movable barrier  30  is in the “up” board blocking position. Being disengaged from the chain drive gearing  84  by clutch  104 , the belt conveyor  22  remains stationary. 
   In step  2  of the sequence, as shown in  FIG. 12 , the board  200  is stopped at the exit end  14   b  of the input conveyor  14  by the barrier  30  and is moved into alignment therewith, that is squared up with respect thereto. As noted hereinbefore, the board  200  already has been centered on the conveyor  14 , having been centered by preceding equipment or by adjustable guides  196  associated with the input conveyor  14 . The first input conveyor  14  is stopped, as by actuation of electromagnetic clutch  92 , as soon as the board  200  is sensed at the exit end  14   b  thereof by the photocell  36 . 
   As controlled by a programmable logic controller (PLC) indicated schematically by the reference numeral  198  in  FIG. 2 , the barrier  30  is actuated downwardly, by actuation of air cylinder  32  in step  3  of the sequence, as shown in  FIG. 13 , to release the board  200 . Immediately thereafter the input conveyor  14  and the first belt conveyor  22  are both started by appropriate energization of the direct current motor  78  for operation at a speed of 9 m/min to load the board  200  very quickly onto the belt  70  on the first belt conveyor  22  and thereby into the first vacuum chamber of the first vacuum laminator  26 . 
   In step  4  of the sequence, as seen in  FIG. 14 , a cam  138  and cooperating sensor  142  provide a signal to stop the belt conveyor  70  of the first vacuum laminator  26  and the input conveyor  14  when the board  200  is in the first vacuum chamber  178 ′ at a position directly vertically above the well  180  in the lower platen  146 . The barrier  30  is moved up by actuation of air cylinder  32  and the input roll  38  of the belt conveyor  22  is shifted by the actuation of the two-position air cylinder  134  in the direction of the first vacuum chamber in order to release the tension of the belt  70 . The input conveyor  14  starts to run at a speed of 3 m/min. Being disengaged from the chain drive gearing  84  by the electromagnetic clutch  104 , the first belt conveyor  22  remains stationary. 
   As seen in  FIG. 15 , in step  5  of the sequence, the lower platen  146  of the first laminator  26  is moved vertically upward by a pneumatic ram  202 . The lower platen  146  passes upward through the aperture  74  in the belt  70 , which aperture  74  is then in vertical alignment with the lower platen  146 . Vacuum pump  186 ′ is actuated for a predetermined time in a first stage of the vacuum process at ambient temperature conditions. Accordingly, at no time during this phase is the vacuum chamber heated by the platen heaters  190 ′ and  192 ′, the heaters remaining dormant. Meanwhile, a new prelaminated board  200   a  to be vacuum laminated has arrived on the input conveyor  14  and is moved to and is stopped at the barrier  30 , which, as shown in  FIG. 15 , is in the up position. 
   Step  6  of the sequence is shown in  FIG. 16 . This is after the first stage of the vacuum process has been completed. The vacuum in the first vacuum chamber is released by actuating a valve to allow the introduction of atmospheric air into the vacuum chamber  178 ′. The lower platen  146  is then lowered by the hydraulic cylinder  202  down through the aperture  74  in the belt  70  of the first belt conveyor  22 . Meanwhile, the new board  200   a  is aligned or squared up on barrier  30  and the first input conveyor  14  is stopped. 
   In  FIG. 17 , which shows step  7  of the sequence, the input belt roll  38  is moved back toward the exit end  14   b  of input conveyor  14  by the two-position air cylinder  134  to restore the tension of the belt  70  of the first belt conveyor  22 . The new board  200   a  is waiting in aligned position at the barrier  30  on the input conveyor  14 . 
   As shown in  FIG. 18 , which shows step  8  of the, sequence, the actuation of the electromagnetic clutches  96 ,  104  and  112  is such that the belt conveyors  22  and  24  of both laminators  26  and  28  start running along with input conveyor  16 . Being disengaged from the chain drive gearing  90  by clutch  92 , the first input conveyor  14  remains stationary. The simultaneous energization of both motors  78  and  80  as controlled by the PLC  198  is then such that both belt conveyors  22  and  24  and input conveyor  16  start at a speed of 9 m/min to effect a rapid unloading and loading of the partially processed board  200  from the vacuum chamber of the first vacuum laminator  26  into the vacuum chamber of the second vacuum laminator  28 . A cam  140  and cooperating sensor  144  provide a signal to stop the belt conveyor  72  and input conveyor  16  when the board  200  is in the second vacuum chamber at a position directly above the well in the lower platen  148 . 
   In step  9  of the sequence, shown in  FIG. 19 , as soon as the partially processed board  200  is completely off the first belt  70 , as sensed by the photocell  170 , the speed of the belt conveyor  70  is increased to 30 m/min in order to move the belt  70  quickly to the set point and to load the new board  200   a  that has been waiting at the exit end  14   b  of input conveyor  14 . A few centimeters before the set point is reached the speed of the belt conveyor  22  is slowed down to 3 m/min and then the belt conveyor  22  is stopped precisely at the set point. Meanwhile, with the partially processed board  200  having been introduced in the second vacuum chamber  178 ″, the input roll  40  of the second belt conveyor  24  is shifted by the actuation of the two-position air cylinder  136  in the direction of the second vacuum chamber in order to release the tension of the belt  72 . 
   In step  10  of the sequence, as shown in  FIG. 20 , the lower platen  148  of the second vacuum laminator  28  is moved vertically upward by a pneumatic ram  204 . The platen  148  passes upward through the aperture  76  in the belt  72 , which aperture  76  is then in vertical alignment with the lower platen  148 . Vacuum pump  186 ″ is actuated for a predetermined time in a first stage of the second vacuum process, after which, for a short period of time, a slap down action, as described in connection with  FIG. 9 , is applied. During the vacuum phase the board  200  is heated by the heaters  190 ″ and  192 ″ in the upper and lower platens  152  and  148 , respectively. It should be understood that during the second vacuum process, vacuum is applied not to draw-off air between the film and the surface at the board  200 , as this has already been accomplished in the first stage of the two stage operation, but to create a good situation to apply mechanical pressure to the board  200  by slap down action. Meanwhile, the barrier  30  is actuated downwardly, by actuation of air cylinder  32  to release the new prelaminated board  200   a  awaiting at the entrance of the first vacuum laminator  26 . Immediately thereafter the input conveyor  14  and the first belt conveyor  22  are both started by appropriate engagement of electromagnetic clutches  92  and  104  and energization of the motor  78  for operation at a speed of 9 m/min to load the new board  200   a  onto the belt  70  on the first belt conveyor  22  and thereby into the first vacuum chamber. Cam  138  and cooperating sensor  142  provide a signal to stop the belt conveyor  22  after the board  200   a  has moved to the proper position in the first vacuum chamber. 
   Step  11  of the sequence is shown in  FIG. 21  This is after the final stage of the vacuum lamination process has been completed. The vacuum in the second vacuum chamber  178 ″ is released by actuating a value to allow the introduction of atmospheric air into the vacuum chamber. The lower platen  148  is then lowered by the hydraulic cylinder down through the aperture in the belt  76  of the second belt conveyor  24 . While at the same time, the barrier  30  is moved up by actuation of the air cylinder  32  and the input roll of the first belt conveyor  22  is shifted by actuation of the air cylinder  134  in the direction of the vacuum chamber in order to release tension of the belt  70 . The input conveyor  14  then starts to run at a speed of 3 m/min, to receive a new prelaminated board while the belt  70  remains stationary. 
   As seen in  FIG. 22 , in step  12  of the sequence, the input roll  40  is moved back toward the exit end  16   b  of input conveyor  16  by the two-position air cylinder  136  to restore the tension of the belt  72  of the second belt conveyor  24 . While at the same time, the lower platen  146  of the first laminator is moved vertically upward through the aperture  74  in the belt  70 . Ambient evacuation occurs in the same manner as set forth in step  5 . Meanwhile, a new prelaminated board  200   b  is arriving on the first input conveyor  14 . 
   In step  13 , as shown in  FIG. 23 , the actuation of the electromagnetic clutches  96  and  112  is such that the second belt conveyor  24  only starts. The energization of the motor  80  as controlled by the PLC is then such that the second belt conveyor  24  starts at a speed of 9 m/min to effect a rapid unloading of the processed board  200 . Meanwhile, the ambient vacuum process in the first vacuum chamber is now complete. The vacuum in the first chamber  178 ′ is therefore released by actuating a value to allow introduction of atmospheric air into the vacuum chamber. The lower platen  146  is then lowered by the hydraulic cylinder down through the aperture  74  in the belt  70 . The new board  200   b  is aligned or squared upon the barrier  30  and the input conveyor  14  is stopped. 
   In step  14  of the sequence, shown in  FIG. 24 , as soon as the processed board is completely off the second belt  72 , as sensed by photocell  172 , the speed of the second belt conveyor  24  is increased to 30 m/min in order to move the belt  72  quickly to the set point to accept the new board  200   a . The temperature of the processed board  200  is read by infrared sensor  174  as it leaves the second vacuum laminator  28  as well. While at the same time, the input roll  38  of the first belt conveyor  22  is moved back toward the exit end  146  of the first input conveyor  14  by the two position air cylinder  134  to restore the tension of the belt  70  of the first belt conveyor  22 . The next new board  200   b  awaits in aligned position at the barrier  30  on the input conveyor  14 . The cycle then repeats from step  8  illustrated in  FIG. 18 . 
   The sensing switches comprising cams  138 ,  140 ,  154 ,  156 ,  162  and  164  and cooperating sensors  142 ,  144 ,  158 ,  160 ,  166  and  168  respectively, may each be of the type known in the art as proximity switches, a non-contacting switch. More specifically, the cam may comprise a metallic object with the sensor, in each case, comprising an electronic device which is fixed in position and is responsive to the movement nearby of the metallic cam and is operative to generate an electrical signal in response to movement and hence sensing of the metallic object. 
   The programmable logic controller  198  utilized to control the sequential operation of the conveyorized vacuum applicator  12  may be a microprocessor controller of a type available commercially from Saia, Mitsubishi or others. The controller  198  responds to the various signals produced by the photocells  36 ,  170  and  172  and by the proximity switch sensors  142 ,  144 ,  158 ,  160 ,  166  and  168  to initiate, in concert with preprogrammed control data the several ensuing control functions including timing of the vacuum process laminating stages. These control functions include the actuation in the proper sequence of the air cylinders  32 ,  134  and  136 , the pneumatic rams  202  and  204 , and the electromagnetic clutches  92 ,  96 ,  104  and  112  and the selector switches  126  and  128  for the motor speed control potentiometers  114 ,  116  and  118  and  120 ,  122  and  124  respectively. For convenience of illustration, in  FIG. 2  the control paths between the PLC  198  and the several control devices just mentioned have been shown in dotted lines. It will be understood that, although not shown, the dotted lines include, where necessary and appropriate, as well known to those skilled in the art, conversion devices such as electrically operated pneumatic valves to control the various air cylinders and the pneumatic ram, and electrical relay means to control the motor speed control selector switches. The electrical circuit connections to the several input terminals (not shown) of the PLC  198  from the photocells and from the sensors have not been shown in order to avoid complication of the drawing since such circuitry is well known and understood by those skilled in the art. 
   Referring to  FIG. 25 , in an alternative embodiment, the second vacuum laminator  28  may contain a dual belt system to further insulate the evacuated board from the heated upper and lower platens. This dual belt system is more fully described in copending Italian application filed on the same day herewith by the same Applicant under the same title, the disclosure of which application, by reference, is incorporated herein. The essential feature of the dual belt vacuum laminator shown in  FIG. 25  is the provision of two independent (i.e., dual) belt conveyor systems, specially a lower belt conveyor  206  and an upper belt conveyor  208 . The lower belt conveyor  206  is positioned for movement of the prelaminated board into and out of the vacuum chamber of the second laminator  28  for application of heat and mechanical pressure. The lower belt conveyor comprises an endless belt with two distinct sections  210  and  212  upon which the board can be placed spaced apart by two apertures  214  and  216 . The two sections are so positioned such that, when one section of the lower belt is moved with the board into the vacuum chamber region, the other section is moved out of said region for cooling and vice versa. The upper belt conveyor  208  is spaced above the lower belt conveyor in the vacuum chamber region and also comprises an endless belt  218  with at least two distinct sections that alternate into and out of the vacuum chamber region such that, when one section of the upper belt is moved into the vacuum chamber region, at least one other section is moved out of said region for cooling and vice versa. In operation, as one section (i.e., a cool section) of the lower belt moves with the board to be vacuum laminated into the vacuum chamber region, one section (i.e., a cool section) of the upper belt is also indexed into the vacuum chamber region, as the other belt sections are moved out of the vacuum chamber region for ambient cooling. This enables the board being vacuum laminated to be disposed initially only between cool sections of the upper and lower belts, which act as heat shields to prevent the dry film from heating up too fast and prematurely adhering to the board, when exposed to residual heat given off by the heat platens which are still hot from a previous vacuum lamination cycle.