Abstract:
Apparatus that supports and aligns a pair of glass image masters facilitates relative movements of the masters between separated positions to accommodate insertion of a circuit board, and proximate position in precision alignment of the images carried thereby on opposite sides of the circuit board. Various configurations of reference blocks and engaging followers mounted on one and the other masters assure precision alignment of the masters in proximate position, and resilient bias force exerted in relative orientation between the masters assures precision aligning engagement of followers and reference blocks. Precision positional alignment of one image master relative to the other image master is accomplished by a differential threaded assembly between relatively moveable members interposed between an image master and a follower that engages a reference block.

Description:
This application claims benefit of Provisional Application No. 60/124,267 filed Mar. 11, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to photographic image masters and more particularly to apparatus for maintaining close-tolerance registration of such image masters on opposite sides of an image-receiving object during illumination and exposure of the image-receiving object to the image masters. 
     BACKGROUND OF THE INVENTION 
     Photographic techniques are commonly used in the fabrication of printed circuits and other chemically-milled components to form images on the surfaces of rigid or flexible substrates such as circuit boards. Certain known circuit board configurations require formation of images on opposite sides of a common circuit board in close registration at critical locations such as at through holes, apertures, peripheral borders, and the like. For such circuit board configurations, it is common to expose both light-sensitive surfaces of the circuit board through respective image masters in order to improve productivity through subsequent photographic processes that develop and etch the final circuit patterns in close registration on the opposite sides of the circuit board. 
     Conventional fixtures for supporting image masters commonly rely upon composite materials that are less dimensionally stable than glass-based image masters, and thus such conventional fixtures suffer loss of registration accuracy attributable to wear, thermal variations, and the like. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, supporting fixtures for pairs of glass image masters rely upon the dimensional and thermal stability of glass to preserve registration accuracy over time and use. The fixtures include gas springs to counterbalance the weight of the upper glass image master and to resiliently bias the upper glass image master into proper registration with the lower glass image master. Spring actuators also bias registration of the pair of glass image masters toward one side. Reference edges of the glass image masters thus remain in dimensionally accurate registration through repeated production operations that require relative separations and closures of the pair of glass image masters as light-sensitive circuit boards are inserted, exposed to light through the image masters, and removed from the assembly for further photographic processing of the exposed images. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an assembly of glass image masters according to one embodiment of the present invention; 
     FIG. 2 is a side view of a follower engaging a reference block in the assembly of FIG. 1; 
     FIG. 3 is a side view of the follower traveling along the reference block in the embodiment of FIG. 2; 
     FIG. 4 is a side view of a force block in the assembly of FIG. 1 including a resilient layer intermediate the surface layer and block; 
     FIG. 5 is a side view of a follower engaged with the force block of FIG. 4 to produce a resilient lateral force; 
     FIG. 6 is a side view of another embodiment of a force block using a resilient biasing spring; 
     FIG. 7 is a side view of the force block of FIG. 6 engaged with a follower to produce a lateral force; 
     FIG. 8 is a side view of another embodiment of a force block including a resilient elastomeric spring to produce lateral force against a follower in engagement therewith; 
     FIG. 9 is a plan view of an assembly according to another embodiment of the present invention in which the reference points for gas springs are reversed; 
     FIG. 10 is a side view of a hinge block in which a follower permits a range of vertical elevational adjustments to a stop position; 
     FIG. 11 is a plan view of an assembly according to another embodiment of the present invention with the gas springs in alternate position; and 
     FIG. 12 is an exploded view of an adjustable hinge on force block. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is shown a plan view of an assembly of upper and lower glass image masters  1 , 2  disposed to be selectively separated to facilitate insertion of a circuit board therebetween. The image masters  1 ,  2  must be realigned in substantially plane parallel orientation about opposite sides of a circuit board during the closure of the image master  1 ,  2  about a circuit board. The lower glass image master  1  serves as a foundation for all components, and the upper glass image master  2  is disposed to be elevated or otherwise separated from the image master  1  to accommodate placement of a circuit board between the image masters  1 , 2 . Specifically, in the illustrated embodiment of FIG. 1, hinges  7 , as more fully described later herein, are mounted on the glass image master  1  near substantially opposite comers of the upper glass image master  2 . A small amount of ‘play’ between moving components of each hinge  7  is introduced to facilitate relative movement of the image masters  1 , 2  with respect to other registration-controlling components mounted on the lower glass image master  1 . Specifically, conventional gas-filled composite spring and damper elements  6  are mounted to the lower glass image master  1  and to the upper glass image master  2  to support the upper master  2  in an open or elevated position and to inhibit violent closure toward registered alignment with the lower master  2 . In addition, such gas springs  6  provide a resilient bias force substantially in the plane of the master  2  and toward forward or front reference blocks  4  that are also mounted on the lower master  1 . A rotatable follower  3  such as the outer race of a ball bearing component, or the like, is attached to the forward or front edge of the upper master  2  to engage the reference block  4 , as described later herein. Registration of the upper and lower image masters  1 , 2  along such fore-aft coordinate  8  is thus determined, not by the hinges  7 , but by the reference blocks  4  and followers  3  under the resilient bias force provided by gas springs  6  while the upper master  2  is in the closed or aligned operational position. 
     In addition, a similar reference block  4   a  and follower  3   a  are mounted, respectively, on the lower image master  1 , and on one side edge of the upper master  2  near the forward edge thereof. Force block  5 , according to an embodiment as shown in FIGS. 4-8, and an associated follower  3  are mounted, respectively, on the lower image master  1 , and on an opposite side edge of the upper image master  2  near the forward edge thereof in substantial alignment with the reference block  4   a  and follower  3   a  on the one side thereof. The force block  5  exerts a resilient bias force upon the upper image master  2 , as described later herein, along a side-side coordinate  9  to force follower  3   a  into engagement with reference block  4   a . In this way, registration of the upper and lower image masters  1 , 2  along such side-side coordinate  9  is thus determined, not by the hinges  7 , but by the reference block  4   a  and follower  3   a  under the resilient bias force provided by force block  5  and associated follower  3  while the upper master  2  is in the closed or aligned operational position. 
     Referring now to the side views of FIGS. 2 and 3, there are shown reference block  4  and follower  3  in the assembly of FIG.  1 . The reference block  4  is shown attached to lower glass image master  1  and the rotatable follower  3  is shown attached to an edge of upper glass image master  2 . The hinges  7  allow sufficient ‘play’ in the position of the upper master  2  to enable follower  3  attached to the upper master  2  to overlap the vertical reference surface of block  4 , typically by about 100-300 microns. A chamfer at the top of the vertical reference edge of block  4  and  4   a  promotes significant leverage against the resilient bias force exerted fore and aft by the gas springs  6  (or side to side by the force block  5 ) to provide tactile feedback that assures the operator of proper registration upon closure of the upper master  2  into operational position on the lower master  1 . 
     Referring now to FIGS. 4 and 5, there are shown side views of one embodiment of force block  5  that includes a layer of resilient elastomeric material  11  interposed between the block  5  and an outer metallic layer  10 . As shown in FIG. 5, a follower  3  attached to a side edge of the upper master  2  overlaps the vertical face of the metallic layer  10 , typically by about 100-300 microns. A chamfer at the top of the vertical face of layer  10  promotes significant leverage against the resilient bias force exerted side to side by the resilient layer  11 . This provides tactile feedback to an operator of the distinctive override and proper positioning of the upper master  2  in closed position upon the lower master  1 . 
     Referring now to FIGS. 6,  7  and  8 , there are shown side views of other embodiments of force block  5  for exerting side to side lateral force upon the upper master  2  against the reference block  4   a . Specifically, a spring  12  or a resilient elastomeric component  10  may be disposed within the force block to exert resilient bias force against a metallic outer layer  11  and a follower  3  engaged therewith. Various spring constants or compliance parameters may be selected for spring  12  or elastomeric component  10  to provide adequate lateral force on upper master  2  in the direction along axis  9  to assure accurate positioning against reference block  4 , and typically to provide such force in the range from about 5 kg to about 20 kg. Similar range of force in the fore-to-aft direction along axes  8  may be provided by each of the gas springs  6 . 
     Referring now to the plan view of FIG. 9, there is shown an alternative embodiment of the registration device of the present invention in which the gas springs  6  are mounted between lower and upper image masters  1 , 2  to exert resilient positioning force in the opposite fore-and-aft direction  8 , against hinge blocks  13  that may also serve as reference blocks. Specifically, as shown in the side view of FIG.10, there is shown a hinge block  13  similar to reference block  4 ,  4   a  attached to the lower image master  1 , but with a cornice or overhang  14  protruding from the upper region of the reference surface against which follower  3  presses. In this embodiment, the follower  3  attached to the upper image master  2  is free to elevate vertically over a limited range to accommodate thickness variations of a circuit board interposed between the lower and upper image masters  1 , 2 . However, the upper image master  2  is inhibited from exceeding the vertical position beyond contact of the follower  3  with the cornice  14  in the upper region of the vertical reference without operator exertion of substantial overriding force against the resilient bias of gas springs  6 . 
     Referring now to FIG. 11, there is shown a plan view of another embodiment of the present invention in which the reference blocks  4  and associated followers  3  may be positioned near forward regions of the side edges of the upper image master  2  to provide precision positioning thereof by pulling rather than pushing the follower  3  against the vertical reference surface of the reference block  4 . Conventional adjustment mechanisms may be provided on reference blocks  4 ,  4   a  and followers  3 , 3   a  in all embodiments to facilitate alignment of images on the upper and lower image masters while in proximate relative positions. The resilient forces along axes  8 ,  9  may be oriented substantially within the plane of the upper image master  2 , or in the alternative, in a direction slightly downward toward a reference block to provide a small amount of toggle and tactile feedback. 
     Referring now to FIG. 12, there is shown an exploded top view of an adjustable hinge block or force block according to one embodiment to provide minute adjustments of positional orientation of the upper master  2  relative to the lower master  1 . Specifically, a hinge  7  or a force block may include two separable members  15 ,  16  for attachment to the upper master  2  and to a hinge block  7  or roller via pivot axle  17 . The members  15 ,  16  are supported for relative sliding movement on pins  18  that may be fixed to one member  15  (as by press fit, set screw, threaded attachment or the like) and are slidable in the other member  16 , substantially in the plane of a master  1 ,  2 . A recess  19  in one or both of the members  15 ,  16  about the pins  18  houses a coil spring  20  that resiliently pre-loads the relatively slidable members apart. 
     The relative slide position of the members  15 ,  16  is determined by the central mechanism including a pair of threaded bolts or members  21 ,  22  of dissimilar pitch or thread count, and the associated mating threaded nuts  23 ,  24 . The threaded nuts  23 ,  24  are attached via central adjustment wheel  25  to rotate together on the respective bolts  21 ,  22 . The bolts  21 ,  22  may be threadedly attached to the respective members  16 ,  15  to provide course adjustment of the relative slide positions of the member  15 ,  16 . Fine positioning of the members  15 ,  16  is accomplished by rotating the central wheel  25  in one or other rotational directions. Because of the dissimilar thread pitch or thread count of the members  21 ,  22  (e.g., 8-32; 4-40), differential rates of advance and retreat of a nut  23 ,  24  upon the respective threaded member  21 ,  22  provide differential jack screw motion (e.g., 160 turns of wheel  25  per inch of movement) of members  15 ,  16 . In this way, very fine adjustments of registration of the upper and lower masters  1 ,  2  that are attached to the members  15 ,  16  may be accomplished. 
     Therefore, the present invention provides repeatable, precision positioning of a pair of glass image masters that are disposed to separate and receive a circuit board therebetween, and that can then be accurately re-aligned on opposite sides of such circuit board for photographic exposure processing of the master images onto the opposite sides of the circuit board.