Photocopying – Projection printing and copying cameras – Photographing on both sides of photo-sensitive paper
Reexamination Certificate
2001-03-16
2002-08-20
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Photographing on both sides of photo-sensitive paper
C355S023000, C355S024000, C355S040000, C355S053000, C355S067000, C430S030000, C430S311000, C430S312000, C430S314000, C358S496000, C250S492200, C250S492220, C250S548000, C356S399000, C356S400000, C356S401000
Reexamination Certificate
active
06437850
ABSTRACT:
FIELD OF INVENTION
This invention relates to a method and a device for exposing a sheet. More particularly, this invention relates to a method and device for exposing both sides of a sensitized sheet for direct imaging of printed circuit board panels, printing plates, or other sensitized sheets.
BACKGROUND
One aspect of this invention is exposing a sheet that is provided with a sensitive layer, for example a light-sensitive layer on one or both sides (i.e., on the surface or surfaces of one or both sides). A light-sensitive layer herein includes a layer sensitive to thermal (e.g., infra-red or IR) radiation, visible light, and ultra-violet (UV) radiation.
One application of the invention is the direct imaging of a single or double-sided sensitized sheet for producing a printed circuit board (PCB). Other applications will be clear from the following detailed description.
It is known that printed circuit boards may be composed of several PCB panels, each of which is provided with an electrical circuit. When there are only two layers, the board is commonly called a double-sided board, and when there are more than two layers, the board is commonly called a multi-layer board. A common way of manufacturing a multi-layer board is by fixing several panels together, each panel having a single printed circuit on one side, or a circuit on each side. “Outer” panels are those that face the outside of a multi-layer PCB, and “inner panels” are the interior panels. Typically, the inner panels have a circuit on both sides, while the outer panels have a circuit only on one, the outer side. Each inner panel resembles a thin double-sided PCB in that the panel is comprised of an insulating substrate which is clad on both sides with metallic foil, typically copper foil. A printed circuit is formed on any circuit side of an inner panel by that side's metal cladding having a light-sensitive layer laid on top of the metal. The light-sensitive layer is exposed to light (typically ultra-violet (UV) radiation) at selected locations, then processed by a photographic process that removes the layer at selected locations. An etching process is then applied to remove those parts of the layer of metal not necessary for forming the actual circuit. Once all the double-sided inner panels are produced, they are fused (pressed) together by placing an insulating binding material, typically a partially cured epoxy-resin material called prepreg, between the panels. Unexposed outer foils are placed on the outside of the double-sided inner panels, again with prepreg in between. All the layers are now laminated by applying heat and pressure that causes the prepreg to flow and bond to the surfaces of the inner panels and the outer foils. Holes are now drilled on the laminated multi-layer board, including holes for mounting electrical components inserted into the board (called “mounting holes”), and holes for making contacts from one layer to one or more other layers (feed-throughs, also called vias or conductive vias). The holes typically are plated through. Each side of the multi-layer panel now is sensitized, then exposed and processed to form the two outer printed circuits in exactly the same manner as forming circuits on the inner panels.
Note that because a multi-layer panel is exposed in the same way as an inner PCB panel, the words “PCB panel” or simply panel will mean either a complete PCB board, an inner PCB panel, or a post-lamination multi-layer panel.
One difficulty in producing multi-layered printed circuit boards is the strict requirement for accuracy in positioning the different PCB panels together to ensure that the different circuits are positioned very accurately relative to each other. In particular, the mounting holes and vias need to be very accurately placed on each layer's circuits. For a particular tolerance for the placement of a circuit, it is clear that any deviations in the specified location of the circuits on each of the layers may be additive, so that at any one location, there could be large deviations. For the case of double-sided panels, including the multi-layer panel after lamination, it is even more difficult to position the circuits accurately enough relative to each other.
A common method for producing printed circuit boards is to first produce artwork, which is an accurately scaled configuration used to produce a master pattern of a printed circuit, and is generally prepared at an enlarged scale using various width tapes and special shapes to represent conductors. The items of artwork, once reduced, for example, by a camera onto film to the correct final size, are referred to as phototools and are used as masks for exposing the sensitized layers. Because the photographic reduction is never 100 percent accurate, more accurate phototools are produced nowadays using photoplotters rather than photographic reduction.
However produced, physical phototools are susceptible to damage. In addition, whenever any amendments need to be made to any circuit, new phototools need to be produced. Furthermore phototools, sometimes in the form of photographic negatives, are difficult to store. They also may not be stable; their characteristics might change with temperature and humidity changes.
There thus are advantages to directly imaging the required circuit patterns onto PCB panels, for example PCB panels that include a light-sensitive layer on one or both sides. The same advantage also is applicable to directly imaging printing plates that include a UV, visible light, or thermally- sensitive layer. Often such sensitive sheets as used for PCBs or thermal printing plates are rigid, so that the scanning apparatus for exposing such sheets for direct imaging (e.g., directly exposing printing plates or directly exposing PCB panels) is of the flat-bed type in which the sheet is disposed on a horizontal table for exposure by the light energy (e.g., UV light or infrared) produced by the scanner. Such scanning apparatuses are typically quite bulky because of the horizontal table. Also, such direct imaging systems expose one side at a time, and there are problems accurately aligning the two sides for double-sided exposure.
Thus there is a need in the art for an improved method and apparatus for exposing a sheet, one or both sides of which are provided with a light-sensitive layer, which may be a UV sensitive layer, a visible light-sensitive layer, or a thermally sensitive layer.
In particular, there is a need in the art for a method and apparatus for reducing the probability of differences occurring between the required locations of the desired scanned regions (e.g., the images) at the respective sides of the sheet. In the case of the sheet being a PCB panel, it is desired to reduce the probability of there being unwanted differences in locations of the printed circuits at the respective sides of the PCB panel.
SUMMARY OF THE INVENTION
Described herein are a method and an apparatus for exposing a light-sensitive sheet, in which the exposing is carried out by one or more optical beams projected onto the respective light-sensitive layers, with the particularity that the scanning is carried out from two opposite sides of the sheet to be scanned. Optical herein includes in a non-limiting way thermal radiation such as infra-red, UV and visible light.
In a preferred embodiment the exposing operations at both sides are carried out simultaneously. Furthermore, in a particular embodiment, the exposing operations are carried out by scanning simultaneously at the same portions of each side of the sheet. That is, when carrying out a scanning action at a particular region at one side, the scanning of the corresponding region at the opposite side of the sheet is done at substantially (and not necessarily exactly) the same time.
In the preferred embodiment, only one energy source, for example, a UV light source, is used for scanning both sensitive layers, this arrangement having the advantage that the resulting device for carrying out the method can be relatively compact. A further advantage of using one energy s
De Loor Ronny A.
Empsten Anne-Marie F.
Ryvkin Mark
Vernackt Marc
Adams Russell
Brown Khaled
Inventek
ManiaBarco NV
Rosenfeld Dov
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