Abstract:
Phototools are located for processing both sides of printed circuit board panels by an apparatus and process that utilizes a work station with two alignment fixtures each including panel center registration PCR alignment mechanism to locate phototools and panels. Processing steps utilize the alignment fixtures to locate phototools on both sides of a panel. Each of the two phototools is initially positioned on its alignment fixture by the PCR alignment mechanism and secured to its alignment fixture by means of a vacuum supplied by a vacuum chuck which is operative as the top surface of the alignment fixture. The panel is then placed on top of the first phototool and aligned to the first phototool in the first alignment fixture using the PCR alignment mechanism. This first phototool is then affixed to the first side of the panel. The panel with the attached first phototool is then placed on the second phototool in the second alignment fixture and aligned to the second phototool using the PCR alignment mechanism. The second phototool is then affixed to the second side of the panel. The phototool-panel-phototool package is then placed in a printer for exposure.

Description:
This application is a continuation of application Ser. No. 07/751,472, filed on Aug. 29, 1991 and now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an apparatus and method for accurately aligning two or more flat objects. It is particularly concerned with aligning and attaching phototools on opposite sides of a printed circuit board panel. 
     BACKGROUND OF THE INVENTION 
     Photoprocessing of printed circuit board panels for the etching and deposition of patterns on the printed circuit board panel surfaces requires accurate positioning of the patterns on the phototools relative to patterns (e.g., drilled holes, copper features) already defined on the printed circuit board panel. 
     Techniques presently used in aligning the phototools to the printed circuit board panels range from &#34;eye-bailing it&#34; to the use of complex vision feedback systems to position the phototools. Examples of more accurate extant processes are very labor intensive and often require expensive panel-phototool positioning operations or complex aligning equipment. These existing processes are often very dependent on operator expertise for their accuracy. 
     SUMMARY OF THE INVENTION 
     Phototools are accurately aligned for processing both sides of printed circuit board panels (hereinafter referred to as panels) by an apparatus and process that preferably utilize a work station with two adjacent alignment fixtures each including a panel center registration (PCR) apparatus to align phototools and panels. Processing steps utilize the alignment fixtures to align phototools on both sides of a panel. Each of the two phototools is initially aligned on its own alignment fixture by the second PCR apparatus and secured to the alignment fixture by means of a vacuum applied by a vacuum gripping or clamping surface (designated as a vacuum chuck herein) which is also operative as the top surface of the alignment fixture. The panel is then placed on the first alignment fixture and then aligned relative to the same apparatus, thereby aligning the panel relative to the first phototool. The first phototool is then affixed to the first side of the panel with double sided adhesive tape. The panel, with the attached first phototool, is then turned over and placed on the second phototool in the second alignment fixture and aligned to the second phototool using the PCR apparatus. The second phototool is then affixed to the second side of the panel with double sided adhesive tape. The phototool-panelphototool package is then placed in a printer for exposure and photoprocessing. 
     In another embodiment the process may utilize a single alignment fixture in which the phototools are serially affixed in the single alignment fixture instead of the use of the parallel fixtures for processing as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 is a pictorial view of an alignment fixture embodying the principles of the invention; 
     FIG. 2 is a pictorial view of a work station arrangement embodying the principles of the invention and utilizing the alignment fixtures of FIG. 1; 
     FIG. 3 is a fragmentary view of the features of the planar work surface of the alignment fixture of FIG. 1 for applying the vacuum grip to secure the panels to the fixture; 
     FIG. 4 is a pictorial view of apparatus to register panels and phototools to a reference point and axis; 
     FIG. 5 is a pictorial view of a panel including structural features that interact with the apparatus to register panels and phototools shown in FIG. 4; 
     FIG. 6 is a pictorial view of a phototool including structural features that interact with the apparatus to register panels and phototools shown in FIG. 4; 
     FIGS. 7 and 8 are a flow process diagram describing a method of utilizing the work station to prepare the panels and phototools and photoprocess them; 
     FIG. 9 is a pictorial view of a vacuum pump suitable for use with the alignment fixture in FIG. 1; 
     FIG. 10 is a pictorial view of a deflector mechanism suitable for use in the alignment fixture of FIG. 1; and 
     FIG. 11 is a pictorial view of the underside of the work surface of the alignment fixture of FIG. 1; and 
     FIG. 12 is a schematic showing how FIGS. 7 and 8 are joined together. 
    
    
     DETAILED DESCRIPTION 
     An alignment fixture embodying the principles of the invention is shown in FIG. 1. It includes a work surface 101 on which phototools may be positioned and which includes structural features, as shown in FIG. 3, to enable it to be operative as a vacuum chuck to secure the phototools to the surface 101 as discussed below. This surface 101 is supported by a structural support housing 102. An internal plenum and vacuum drawing mechanisms are attached to the underside of surface 101; this enables the surface 101 to draw a vacuum through a crisscrossing grid of channels 106 etched into surface 101, as discussed below with reference to FIG. 3, in order to activate the vacuum hold capability of the planar surface 101. The vacuum is applied through to the surface channels 302 by a plurality of apertures 304 joining the channels of the grid to a vacuum of the internal plenum. This vacuum is directly applied to these apertures by drawing a bulk vacuum in the plenum. 
     This vacuum may be applied by a manifold arrangement such as is shown in FIG. 11. The underside of the work surface of the alignment fixture is shown pictorially in FIG. 11. Attached to the underside of work surface 1101 are the locater mechanism 1102 (shown in FIG. 4), a vacuum plenum 1103 and a vacuum pump 1104 (shown in FIG. 9) connected to the vacuum plenum 1103. The vacuum plenum 1103 connects the vacuum pump 1104 (discussed herein with reference to FIG. 9) to the apertures in the work surface (shown as apertures 304 in FIG. 3). The vacuum manifold is comprised of an internal vacuum plenum, a port to connect the vacuum port of the vacuum pump 1104 to this plenum, and apertures in the work surface (shown as apertures 304 in FIG. 3) to connect this plenum to the grooves (302) in the work surface. 
     A functional gasket 119 is provided by the perimeter of the work surface to assist operation of the vacuum in holding phototools to the work surface. This functional gasket is provided by an edge of the work surface since the grid of the work surface is not extended to the perimeter and, hence, the flat surface at the edge or perimeter operates as an air sealing gasket. In a different type working surface, however, it may be necessary to provide a discrete gasket to achieve the desired vacuum seal. 
     Located at each comer of the alignment fixture, and external to the work surface 101, are vacuum actuated deflectors 121-124 each of which is placed at corners of the alignment fixtures in order to pull down the comers of the phototools (an adhesive is attached to corners of the phototools or the panel and must remain disengaged during alignment operations), deflecting them from the plane of the surface 101 during the positioning of a panel on top of the phototool secured to the surface 101. Each deflector has a suction grip device (suction cup) located slightly below the plane of the work surface 101 to prevent adhesive on the phototool from contacting the panel during positioning of the panel. The work surface is shaped and proportioned so that a portion of the phototool overhangs the working surface and can be readily engaged by the comer deflectors without interference from the work surface. 
     The corner deflectors 121-124 are simultaneously activated by use of a controlled vacuum pump or source directly activating all of the deflectors. Vacuum pumps energized by compressed air are commercially available and are suitable for application herein. These vacuum pumps use a venturi effect to generate a vacuum from a compressed airflow. 
     A suitable corner deflector apparatus for use with the alignment fixture of FIG. 1 is shown in FIG. 10. It includes a hollow threaded shank 1011 which may be attached to the side of the alignment fixture and may connect to a vacuum hose or manifold energized by a vacuum source such as disclosed in FIG. 9. The threaded shank 1011 is also connected to a 90 degree corner fitting 1012 which in turn is connected to vertical tubing 1013 which terminates in a flexible suction drawing cup 1014 which, when applied to the phototool and energized with vacuum, applies a holding suction thereto for deflecting the corners of the phototool below the plane of the working surface. 
     Alignment of the phototools and panels is done by synchronized motion of four pins 111-114, which are grouped into the pairs 111-112 and 113-114. The pins of each pair are located near each other and astride a longitudinal axis (shown by dotted line 103) of the surface 101 at opposing sides of the alignment fixture. These pins 111-114 are operative as engaging elements that engage structural features (e.g., tooling holes or apertures) provided in the panels and phototools (as shown in FIGS. 5 and 6) in order to align them with respect to a reference point 104 and axis 103 of the alignment fixture. In particular the pins 111-114 engage holes in the panel or phototool and apply a synchronized opposing displacement of the pin pair 111-112 and 113-114, respectively, to position a point and axis of the panel or phototool substantially congruent with a point and axis of the alignment fixture. While alignment using a synchronized pin pair is shown, other alignment methods and apparatus may be used without departing from the spirit and scope of the invention. 
     The alignment mechanism, shown in pictorial form in FIG. 4, includes the alignment pins 411-414 which engage the apertures or holes in the panel and phototool to align it on the alignment fixture. The alignment pin pairs 411-412 and 413-414 are synchronously moved in opposing directions, as shown by the arrows of FIG. 4. A detailed description of the structure and operation of this particular alignment mechanism is disclosed in U.S. Pat. No. 4,793,052 whose teaching is incorporated herein. 
     A detailed fragmentary pictorial view of the surface 101 (FIG. 1) is shown in the FIG. 3. A grid of channels 302 crisscrosses the surface 301. A plurality of holes 304 is located coincident with several intersections of these channels in the working surface. Through these holes the grid of channels 302 is supplied with the vacuum created by a vacuum drawn in the plenum of the alignment fixture. With the phototool on the surface the vacuum in the grid of channels firmly secures the phototool to the surface. This vacuum chuck preserves the location of the phototool after its reference frame has been aligned coincident with the reference frame established by the locater mechanism. The surface 301 may be comprised of metal such as copper, or a photo definable plastic. The channels of the surface may be created by conventional machining techniques or by photo patterned processes, 
     A typical panel 501 to be processed is shown in FIG. 5 and is substantially rectangular in outline. Apertures or drilled holes 511-514 are provided to interact with the pins 111-114 of the locating mechanism of FIG. 1. The apertures are grouped into the paired apertures 511-512 and 513-514. The apertures of each pair are located astride the longitudinal axis 505 of the panel. The phototools 601 shown in FIG. 6 also include like apertures 611-614 grouped in pairs 611-612 and 613-614 astride a longitudinal axis 605 of the phototool. The length and width dimensions of the phototools typically exceed the corresponding dimensions of the work surface. While a longitudinal reference axis is shown in panels and phototools of the illustrative embodiment, it is to be understood that the invention may be practiced with other axes of the phototools and the panel selected. 
     A piece of double sided adhesive tape 606 is attached to the phototool surface and located at each comer of the phototool. Preferably the exposed adhesive is a low strength adhesive compared to the adhesive adhering to the phototool. This is to permit phototools to be readily removed from the panels after photoprocessing and permit extended reuse of the adhesive. The adhesive tape preferably remains on the phototool for multiple panel processing. While this tape is shown attached to the phototool it is to be understood that it could also be initially applied to the comers of the opposing panel surfaces shown in FIG. 5. In other alternative embodiments a single sided adhesive tape may be used which is attached to the phototool over an aperture permitting it to also adhere to the panel through the aperture. 
     The apertures in the panel and phototool are differentially located to allow independent alignment of the panel after the phototool has been secured in place on the working surface of the alignment fixture. For engagement pins actuating in an outward direction, the apertures in the panel are positioned just slightly closer to each other along the longitudinal axis 505 than the apertures in the phototool are to each other along the longitudinal axis 605. That is, the distance X1 between apertures of the panel is smaller than the distance X2 between apertures of the phototool. It is readily apparent that with this arrangement the apertures of the phototool 611-614 do not interfere with the alignment apparatus in positioning the panel. For engagement pins actuating inward, the engagement pins of the panel are positioned further apart along the longitudinal axis than those of the phototool. With a positioned phototool in place, the panel may be aligned without the alignment pins experiencing interference from the holes in the already positioned phototools. 
     A preferred work station arrangement, as shown in FIG. 2, includes two alignment fixtures 210 and 220 positioned side-by-side on a work support table 201. A vacuum activator 202 is provided to energize the various vacuum operated devices of the alignment fixtures. The vacuum activator may be a discrete vacuum source 202 operative to draw a vacuum in the plenum of the alignment fixture. This activator may in the alternative comprise a source of compressed air connected by tubing to activate vacuum pumps. These vacuum pumps are preferably included in the housing of the alignment fixture and are activated by compressed air to generate a vacuum. These vacuum pumps which are included in the housing of the alignment fixture are connected through a manifold 1103 (shown in FIG. 11 ) to the apertures in the work surface. Additional vacuum distribution plumbing from vacuum pumps are directly connected to energize the corner deflectors of the alignment fixture. These vacuum pumps utilize a venturi effect which derives a vacuum from the applied compressed air flow. Devices to generate such a vacuum from compressed air are readily available commercially. A typical device, produced by PIAB Vacuum Products, is shown in FIG. 9. Compressed air is applied to the orifice 901 and exhausted at orifice 902. A vacuum suction is produced at the suction inlet 903 which in the present apparatus is connected to the suction manifold. A second, independently controlled vacuum device is connected by appropriate tubing or plumbing to the corner deflectors. Alternatively, a single vacuum source could be used with separately controllable valves leading to the working surface and to the corner deflectors. 
     A control mechanism 203, which is indexed manually, is provided to control which individual vacuum mechanisms of the alignment fixtures 210 and 220 are rendered operative, and also to control the operation of the alignment mechanisms. The control mechanism 203 may be embodied in a relay or semiconductor logic controlling a valving manifold; these valves control application of vacuum to the apertures and comer deflectors, and preferably also engage and disengage the alignment mechanisms (which are preferably actuated by pneumatic cylinders). This control mechanism is indexed by the operator, enabling sequenced application of a vacuum to the work surface, corner deflectors and the alignment mechanisms. The work station arrangement is used in combination with a photoprocessor 250 positioned nearby which illuminates the phototool-panel-phototool package after it is aligned and has been inserted into a slot 251 by the operator. The photoprocessors 250 are available commercially and need not be described in detail. 
     The preferred flow process in processing the panels is shown in the diagramed flow process shown in FIGS. 7 and 8. The process begins with the instructions of block 701 which have the operator load the two phototools onto tile first and second alignment fixtures. The operator engages the alignment mechanisms of both fixtures to align the phototools, in block 705, and operates the surface vacuum system of both fixtures to secure the phototools in place on the working surface as per block 709. The vacuum activated corner deflectors are activated to deflect the comers of the phototools below the working surface plane of the alignment fixture as per block 711. 
     Double sided adhesive is applied to the corners of the two phototools if not previously attached thereto, as directed by block 715. The alignment mechanisms of both stations are disengaged as per block 717. The panel is loaded and aligned on one of the alignment fixture surfaces as per block 719. The corner deflectors of this fixture is released and the corners of the phototool and panel are pinched together by the operator as per the instructions of block 721. The alignment mechanism is released and the surface vacuum is terminated (block 723) allowing the phototool/panel package to be removed from the alignment fixture. The package is flipped over and loaded on the other alignment fixture and aligned with the second phototool as per instructions in block 729. The comer deflectors are deactivated by removing the vacuum and the comers of the second phototool and panel are pinched together as directed by block 731. The alignment mechanism is disengaged and the vacuum chuck released as per block 733. The resulting phototool-panel-phototool package is removed and placed in a printer, as per block 739, for photoprocessing exposure. 
     In some applications it may be preferable to operate with only one alignment fixture. In this arrangement the first phototool is aligned and held on the work surface by vacuum. The panel is placed on the first phototool and aligned and secured. The second phototool is then placed on top of the panel and aligned. The phototool-panel-phototool sandwich is then pinched together and secured by the double sided adhesive. The alignment mechanism is released and the taped phototool-panel-phototool sandwich is removed from the alignment fixture for photo imaging. For engagement pins actuating in an outward direction, the tooling holes in the upper phototool would be positioned slightly closer to each other along the longitudinal axis 605 than the tooling holes in the panel are to each other along axis 505. 
     After photoimaging, the phototools, in both the double and single fixture arrangement, are stripped from the panel. The phototool(s) is (are) placed back on the alignment fixture(s) and aligned on the work surface(s). The phototool(s) is (are) then secured to the work surface(s) by applied vacuum and the corners containing the adhesive appliances are deflected by the comer deflectors. The panel in placed on (one of) the alignment fixture(s) and one of the processes described above is continued.