Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2001-07-27
2003-06-17
Huff, Mark F. (Department: 1746)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S311000, C430S944000, C430S945000, C430S318000, C430S317000, C428S209000, C216S013000, C216S065000
Reexamination Certificate
active
06579660
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to digitally producing patterns of copper wiring for printed circuit boards and, more particularly, to a process and materials for efficient imaging of etch resistant layers over the copper layer of a circuit board.
Printed circuit boards (PCBs) are built of an insulating substrate—for instance, glass filled epoxy or polyimide film—over which a thin conducting layer, usually of copper, in a pattern designed for any specific application, has been produced. The patterned conducting layer (also referred to as a printed circuit), is the means of carrying electrical voltages and currents between various electrical components, such as resistors, capacitors, integrated circuits and other electronic devices. The electrical components are soldered onto the copper printed circuit ‘wires’ at a stage after the formation of the printed circuit.
According to conventional methods, the process of printing a specific circuit onto a PCB begins with a blank board, consisting of an insulating substrate and a copper layer (formed either by electrodeposition or as a rolled foil laminate. A photosensitive resist (photoresist) layer (whose function is described below) is spread over the copper—either by depositing it from solution or by providing it as a dry film and bonding it to the copper by pressure sensitive adhesive. A sheet of intermediate photographic film (known as a phototool) that contains a negative image of the circuit pattern is then superimposed on the photosensitive resist layer in close contact and this combination is exposed to a flood of ultra-violet (UV) light. Ultra-violet light transmitted through the transparent areas of the recording film (which correspond to prospective conducting portions of the circuit) causes the photosensitive layer under them to harden (i.e. polymerize). The unhardened material is then washed away with a suitable solvent, baring the underlying copper in the areas that were protected from the UV light. The board is then placed in an etch bath, where the bare copper is etched away, while portions of the copper layer that lie under the remaining resist layer are protected from the etching action. Finally, the remaining (hardened) photoresist is removed with a suitable solvent.
Since nowadays the circuit pattern (i.e. wiring configuration) is produced digitally on a computer, the aforementioned phototool (intermediate film) is usually produced by laser recording the image on the film from the digital data, using an imagesetter. The process of thus producing the phototool involves several steps, including chemical processing of the film. It is, therefore, costly, in terms of both labor and materials. It would thus be economically advantageous if the image data could be transferred directly from the computer onto the circuit board, circumventing the entire process of producing the phototool. The elimination of the intermediate film, with its chemical processing, would also have environmental advantages.
Various attempts have been made to digitally print PCBs directly. U.S. Pat. No. 5,270,368 describes a method for image-wise applying an etch-resistant material onto the copper layer, using ink-jet techniques. It discloses resist compositions suitable for jetting, which consist of a mixture of at least two acrylates plus a photoinitiator plus an organic carrier (solvent). The viscosity of the composition is from about 1 to 10 centipoise. Upon jetting the material onto the copper layer, the solvent evaporates and the resist is set; it is then hardened by exposing it to a flood of UV light for curing. The main disadvantage of the jetting method is its limited spatial resolution; the wiring of modern PCBs contains features measured in tens of microns—which is beyond the practical capabilities of ink-jet techniques because of inaccuracy of dot placement and difficulty in producing very small dots.
U.S. Pat. No. 4,717,639 describes a method for image-wise applying an etch-resistant material onto the copper layer, using electrophotographic techniques, akin to xerography. It discloses a specially formulated toner powder, which is printed onto the copper and which, following the etching stage, is removed with strong alkali. This method, as well, places limitations on the achievable resolution because of the limitation on the smallest toner particles to be used.
Rather than directly printing a resist pattern onto the copper, an alternative approach, which inherently yields much higher resolution, is to coat the copper layer with a photoresist (as in the conventional process) and to form thereon a mask, which is opaque to the UV light that subsequently floods the photoresist to harden it. Methods and processes for forming a UV-opaque mask by digital writing are known in the realm of direct digital production of printing forms for printing presses. U.S. Pat. No. 4,132,168 describes a laser ablated masking for an offset-lithographic plate. The plate consists of a conventional aluminum substrate, coated with a conventional UV sensitive layer and over that—a layer opaque to UV (which is deposited either from vacuum or from solvent). The latter is selectively removed by ablation, using a laser beam. The non-removed opaque material then acts as a mask for the UV exposure of the underlying sensitive layer. The mask is then removed and subsequently the plate is processed to remove the non-exposed sensitive layer, thus forming a lithographic image. The process disclosed in the '168 patent is not suitable for PCB production, since the photosensitive layer cannot act as an etch-resist and, moreover, generally has a thickness of the order of 1 micron or less.
U.S. Pat. No. 5,607,814 describes a masking method for producing a flexographic printing form (or plate) that uses a transfer process. The plate consists of a substrate and a thick photosensitive layer, which contains photoinitiators. A cover sheet, coated with at least one infrared (IR) sensitive layer, is placed over the plate and exposed to an IR image. The IR exposed areas transfer onto the UV sensitive layer and subsequently act as a mask while the layer is flooded by UV light. The unmasked areas of the layer polymerise (to eventually become the printing surfaces) and the remaining (unpolymerised) material, as well as the mask, is subsequently removed. The process disclosed in the '814 patent is, again, not suitable for PCB production, since the photosensitive layer is much too thick and consists of elastomeric material, which is not inherently etch resistant. Moreover, the photosensitive elastomeric layer, which serves as the printing form, is necessarily designed to be sturdy and durable, whereas the photosensitive resist layer of a PCB must be easily removable after the etching process.
U.S. Pat. No. 5,262,275 also describes a UV masking method for producing a flexographic printing plate, but here the mask is formed of a layer of infrared ablatable material. The patent discloses a barrier layer between the IR and the UV layer, to prevent migration of materials from the UV layer into the IR layer and to prevent oxygen inhibition of the UV curing process. This method is not suitable for PCB production, for the reason given above, with respect to the '168 and '814 patents, and because it uses a strong solvent, such as methylene chloride, to remove the mask, which solvent may attack the UV sensitive layer if it were as thin as required in the PCB etching process.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and a process for digitally forming a UV-opaque mask directly over an etch-resistant photosensitive layer, usable in the course of PCB production, which permit each layer to function without chemical interaction therebetween.
SUMMARY OF THE INVENTION
The present invention successfully addresses the shortcomings of the presently known configurations by providing a blank printed circuit board (PCB) with only two coatings over the metal layer, which have improved properties that enable
Chacko Davis Daborah
Creo Il Ltd.
Huff Mark F.
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