Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
2001-03-14
2004-10-12
Mehta, Bhavesh M. (Department: 2623)
Image analysis
Applications
Manufacturing or product inspection
C348S087000
Reexamination Certificate
active
06804388
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to alignment systems and more specifically to an automated method of handling and processing printed circuit board panels, printing plates, or other sensitized sheets.
BACKGROUND
It is known today that printed circuit boards may be composed of several PCB panels, each panel having two sides, one or more of which is provided with a layer forming an electrical circuit. When there is only one panel having 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 metallic foil 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 (“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.
Since 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, 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 each layer's circuit may be additive, so that at any one location, there could be large deviations. For the case of double-sided panels, including a multi-layer panel after lamination, it is even more difficult to position the circuits accurately enough relative to each other.
New technology for making PCB panels, like sequential build up (SBU) or direct ablation of the copper can be used with direct imaging technology. See below for a discussion of direct imaging. For technologies such as SBU, where each new layer is directly added to the previous stack of layers as an additive process, the relationship between the imaging process and the registration process becomes very critical so that the registration imaging processes need to interact more closely. This relationship between the imaging and registration processes becomes increasingly critical as the geometrical accuracy and PCB layout density is increased. Further, to maintain the registration accuracy through the imaging process, the registration apparatus must be accurately mated to the imaging apparatus as the need for accuracy increases.
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. Artwork 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 and distortion. 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 and can suffer degraded quality over time. Some changes include image degradation, localized or global shrinkage or expansion.
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.
Direct imaging addresses some of the production issues such as the difficulties associated with photoplotters, phototools, and the image transfer process. Direct imaging, however, does not ensure proper alignment of the PCB panel to be processed, especially with outer layers where the image must match the drilled holes pattern. In a typical direct imaging process, a first PCB panel is imaged at a nominal 100% scale. Then the imaged first PCB panel is drilled. The imaged and drilled first PCB panel is then registered for a subsequent layer to be added. The entire image of the subsequent layer is scaled as determined in the registration of the imaged and drilled first PCB panel.
Further, direct imaging, alone, does not address the handling of the PCB panels. Modern PCB panels can be large scale such as up to 24 inches in width and up to 36 inches in length (609.6 mm×914.4 mm) or even larger PCB panels are know to be used.
The manufacturing difficulties of precise alignment and handling described above are further amplified as the overall physical size of the PCB panel increases and/or as PCB circuit density increases (e.g., line widths get smaller and closer together). In many specialized applications the PCB panel can be a large scale PCB panel—as large as 24 inches in width and 36 inches in length (609.6 mm×914.4 mm) or even larger. Large-scale sizes are more difficult to handle and accurately align for processing than more typical, smaller PCB panels. The result is a very slow, complicated a
Inventek
ManiaBarco, Inc.
Mehta Bhavesh M.
Rosenfeld Dov
Strege John
LandOfFree
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