Other

M&t Chemicals, Inc.

Atochem North America, Inc.

Telemark Co., Inc.

Donald F Sullivan

A photopolymer layer is used as a radiation transparent phototool replacing a light blocking phototool in the photographic process of producing solder mask layers on printed wiring boards. Various simplification and energy reducing steps are introduced with off-contact photoprinting of patterns on liquid photopolymers having special characteristics that permit the polymerization of patterned outer skin areas partly through the thickness thereof with a controlled amount of radiation energy. This creates an in-situ surrogate phototool that permits further polymerization throughout the thickness of the layer with controlled quantities of unpatterned radiation energy. Photodiscrimination is provided by changes of photosensitivity in the layer transparent to radiation rather than by opaque imaging. Thus a first low energy photoflash through an off-contact phototool with uncollimated radiation creates on a liquid photopolymer surface two contrasting patterned areas of liquid polymer and polymerized skin. This can be achieved in an inert gas atmosphere. The second curing step in the presence of an inhibiting gas such as air with controlled radiation energy then fully cures through the thickness of the layer those skin covered portions without curing the liquid surfaced areas.

Double exposure method of photoprinting with liquid photopolymers

Improved printing with polymers is achieved by interposing a liquid layer conforming to rough substrate surfaces and possibly exhibiting different characteristics from an outer surface polymer layer. This is particularly advantageous in printed wiring board (PWB) manufacture requiring solder mask coating. Thus, a two layer composite polymer coating is provided. One inner adhesive photopolymer layer is applied to the PWB in the liquid state, displacing air from PWB surface. The outer layer of the composite therefore can be epoxy, dry film or liquid polymer. Dry film thus carried on a thin plastic sheet may be overlaminated onto a liquid inner layer already on the substrate, without the need for a vacuum laminator to eliminate bubbles or a plasticizing heat step to conform the dry film to the surface. The dry film solder mask so laminated is then exposed through a phototransparency to harden the light struck dry film solder mask and light struck inner layer photopolymer, thereby cojoining the dry film solder mask, inner layer and PWB surface. A solvent washout step removes unexposed dry film solder mask and unexposed inner photopolymer. This provides a faster process, requires less equipment, and improves adhesion to metal conductors. This embodiment gives a high resolution product with good solder mask qualities. The outer layer need not be photoimaged. Thus, pre-patterned epoxy solder masks may be printed on the carrier sheet, partially hardened and overlaminated in register onto liquid photopolymer-coated PWB, then exposed to light source through a phototransparency and through the pre-patterned solder mask, thereby permanently adhering the outer layer to PWB. The composite coating can be a combination of known solder mask materials, dry film, UV-curable liquid polymers and thermal-curing epoxy, chosen for desired characteristics including electrical performance, printing resolution and cost.

Polymer printing

A liquid photopolymer layer covering a phototool image surface with one surface exposed to air is exposed by radiation through the phototool to partially harden in a pattern defined by the image the photopolymer layer through a part of its thickness. The air exposed surface portion is thus left in liquid form for contacting the surface of a printed wiring board, preferably by scanning along the length of a phototool with a pressurized roll or blade to contact and adhere the preimaged laminate to board surface in an air bubble free lamination where subsequent radiation exposure finishes the curing and bonds the cured portion of the polymer layer to the PWB substrate. After removal of the phototool, the polymer layer is developed by washing out uncured polymer portions as defined by the image pattern.

Photoflashing a liquid polymer layer on a phototool surface exposed to

Paste-consistency photopolymer is imaged onto printed wiring boards by coating the board overall and positioning the photographic film over the board in register and off-contact. A resilient blade is pressed against the phototool at one end of the board and drawn across the surface, thus forcing the photographic film into intimate contact with the photopolymer and purging all air therebetween. Mounted on the resilient blade aft of leading edge is a shuttered tubular lamp which hardens the photopolymer where the photographic film is clear, so that in a single pass the photographic film is sequentially mated and photopolymer exposed, to produce line widths unmatched by competing dry film systems and at speeds of 0.5 feet per second.

Photoprinting process and apparatus for exposing paste consistency

High density primary wiring patterns are formed on printed wiring boards with far less than 0.005 inch spacings and wiring conductor widths, which surprisingly permit wider conductors of at least three times the wiring spacing thus less likely to have open circuit or substrate adherance defects. This is achieved by depositing on an irregular surface of a conventional 'flat' panel insulator a thick liquid photopolymer layer of paste-like consistency, such as 0.006 inch thickness, flattening it with the image bearing side of a glass plate phototransparency to produce high resolution wiring patterns comprising ridge tops defining insulating spacing between channel conductor areas therebetween by means of uncollimated actinic radiation, forming thin conductive layers 0.0014 inch thick on the channel bottoms and sidewalls to produce wider conductors, and sanding off the flat ridge tops to assure that there are no short circuits between adjacent conductors.

Additive printed circuit boards with flat surface and indented primary

Densely packed high resolution printed wiring boards may be achieved by providing channels below grade in a substrate planar surface, thereby providing greater conductor width for closely spaced conductors. By mechanically removing the substrate surface between the channels, high virtual resolution is obtained with high quality conductors and insulation spacings of the order of about 0.003 inch, and with actual conductor widths of greater than 0.012 inch. Thus the packing density of printed wiring patterns may be increased significantly. Also the quality of conductor adherence and conductivity is improved. Thus a non-critical and inexpensive process unexpectedly provides an improved quality, more precise printed wiring pattern.

High density printing wiring

High photo resolution of over 100 lines is obtained for solder masks of thicknesses greater than 0.002 in. (0.005 cm) with liquid photopolymers of special compositions deposited from mesh screens over printed wiring board traces and photoexposed with a phototool image in contact with the photopolymer. Special interrelated properties of the liquid photopolymer interacting in an explicit process of screen depositing two adjacent liquid polymer layers include freedom from volatiles, a paste-like viscosity permitting use with mesh screens of about 60 mesh to obtain a uniform thickness layer free of bubbles and mesh marks, solder mask insulator and heat resistance characteristics, an ability to photo flash through a phototool image in contact with one layer surface and the other surface exposed to air to polymerize through a significant thickness of about one-third of the layer thickness, photo response to polymerize to a depth of about 0.02 in. (0.05 cm) when exposed to about 1 joule per square cm, absence of ingredients refracting, blocking or reflecting the actinic radiation and good adherence to copper wiring. These properties react in the process of bubble-free lamination of two paste consistency layers, a first to cover uneven wiring traces and a second to provide a flat photo-image surface, thereby to achieve thicknesses of about 0.006 in. (0.015 cm) for covering wiring traces 0.004 in. (0.001 cm) thick with assurance of a covering of 0.002 in. (0.005 cm) thickness over all wiring traces. This is achieved by photo flashing the uppermost layer through a photo-image with one surface exposed to air to polymerize a surface layer that covers the traces permitting the unpolymerized liquid paste surface to meet in bubble-free surface contact with an initial layer deposited directly over the wiring traces.

High resolution solder mask photopolymers for screen coating over

Improved printing with polymers is achieved by interposing a liquid layer conforming to rough substrate surfaces and possibly exhibiting different characteristics from an outer surface polymer layer. This is particularly advantageous in printed wiring board (PWB) manufacture requiring solder mask coating. Thus, a two layer composite polymer coating is provided. One inner adhesive photopolymer layer is applied to the PWB in the liquid state, displacing air from PWB surface. The outer layer of the composite therefore can be epoxy, dry film or liquid polymer. Dry film thus carried on a thin plastic sheet may be overlaminated onto a liquid inner layer already on the substrate, without the need for a vacuum laminator to eliminate bubbles or a plasticizing heat step to conform the dry film to the surface. The dry film solder mask so laminated is then exposed through a phototransparency to harden the light struck dry film solder mask and light struck inner layer photopolymer, thereby cojoining the dry film solder mask, inner layer and PWB surface. A solvent washout step removes unexposed dry film solder mask and unexposed inner layer photopolymer. This provides a faster process, requires less equipment, and improves adhesion to metal conductors. This embodiment gives a high resolution product with good solder mask qualities. The outer layer need not be photoimaged. Thus, pre-patterned epoxy solder masks may be printed on the carrier sheet, partially hardened and overlaminated in registor onto liquid photopolymer-coated PWB, then exposed to light source through a phototransparency and through the pre-patterned solder mask, thereby permanently adhering the outer layer to PWB. The composite coating can be a combination of known solder mask materials, dry film, UV-curable liquid polymers and thermal-curing epoxy, chosen for desired characteristics including electrical performance, printing resolution and cost.

In the manufacturing process for a printed wiring board a photopolymer insulation layer having a flat outer plateau surface is extended from the board substrate surface carrying the wiring pattern and provides access channels to a plurality of wiring pattern conductor pad areas. A patterned conductor layer disposed on the insulation layer surface including the sidewalls of the access channels thus electrically connects to the conductor pad areas. Circuit wiring pattern test current or plating currents are passed through the conductor layer pattern during the manufacturing process, and the conductor layer may thereafter be easily removed from the flat surface by sanding or the like. Permanently retained conductor layer portions, such as feasible by indenting the plateau surface, aid in expanding conductor surface areas at solder joint pads, or in increasing the density of circuit wires feasible in a given substrate board area. The conductor layer may provide indicia marking nomenclature patterns which are in dot matrix format to reduce capacitance coupling between wiring circuits on the board substrate.

Circuit wiring disposed on solder mask coating

Photoprinting process and apparatus for exposing paste-consistency

Improved resolution in a low cost non-critical photopolymer printing process is achieved by a non-volatile liquid image quality photopolymer layer disposed with air free surfaces sandwiched between two carrier surfaces, one being transparent and carrying a transparency image in contact with the liquid photopolymer to be exposed by radiation to cure the layer and produce the image pattern. In a typical example, paste-consistency photopolymer is imaged onto printed wiring boards by coating the board overall and positioning the photographic film over the board in register and off-contact. A resilient blade is pressed against the phototool at one end of the board and drawn across the surface, thus forcing the photographic film into intimate contact with the photopolymer and purging all air therebetween. Mounted on the resilient blade aft of leading edge is a shuttered tubular lamp which hardens the photopolymer where the photographic film is clear, so that in a single pass the photographic film is sequentially mated and photopolymer exposed, to produce line widths unmatched by competing dry film systems and at speeds of 0.5 feet per second. Film thicknesses from 0.006 mm to 0.05 mm can give resolutions for line widths and spacings as fine as 0.076 mm.

Printed wiring boards have a first liquid photopolymer layer placed on the wiring board conductive layer over which a second photopolymer layer is laminated and both layers are photo cured in place to bond the two layers together and adhere them to the board. The liquid photopolymer layer covers wiring pattern ridges or other uneven surfaces in air free surface contact. The second layer is carried by a thin transparent plastic film for lamination onto the first photopolymer layer by a squeegee, and the plastic film is peeled off for reuse in further cycles. The carrying film is preferably pivotable in a clamshell arrangement to permit the photopolymer layer to be placed on the film and then to be laminated in two pivot positions. To prevent starvation of the liquid polymer from wiring ridges when the second layer is laminated thereon, the second layer is carried on a contoured surface with a complementary pattern to the wiring ridge pattern.

Transferring polymer from thin plastic films to photodevelop insulation

High resolution, high packing density printed circuit wiring with high conductivity wiring is achieved by putting down thick (0.006 in., 0.015 cm) liquid photopolymer insulator layers on an inexpensive substrate and photodeveloping conductor pattern indentations in the layer for filling with conductive materials. Thus, 0.003 in. (0.008 cm) line to line spacings can be achieved with high conductivity conductors 0.006 in. (0.015 cm) thick. A flush top layer of polymer is readily cleaned of contaminants by light surface sanding to assure designed insulation spacings of polymer between conductors without interfering smears. The finished circuits are not subject to damage in handling from surface scratches since the conductors are indented.

High resolution printed circuits formed in photopolymer pattern

High resolution photoimaging with liquid photopolymers of pastelike consistency overlying irregular surface topography such as provided by conductors extending from the surface of a printed wiring board substrate is obtained without air entrapment or starvation of the superimposed liquid polymer. Thus, for example, solder mask coatings of high resolution may be prepared. This is achieved in two photosteps, the first of which is a partial polymerization obtained by radiating with the polymer surface exposed to air, thereby polymerizing the layer portion unexposed to air adjacent the substrate surface and leaving the air exposed surface in the liquid state. This step uses an image with a greater dimension of opaque pattern area than that used in the subsequent step, which obtains high resolution by surface contact of the image with the polymer. The radiation source in each case is non-collimated. The resulting product because of the partial polymerization covering the surface produces no starvation over the rough surface because of pressing the second image into surface contact. The liquid polymer layer covers rough surfaces without entrapment of air. By radiating when the surface is exposed to air, noise causing poor resolution is eliminated, such as from surface interface reflections and pinholes in opaque surfaces, etc. A layer of liquid photopolymer is disposed upon its permanent substrate cured through only part of its thickness adjacent the substrate to leave a liquid tacky outer surface.

High resolution liquid photopolymer coating patterns over irregular

A process is described for improving the quality of images which have been formed by screen printing a liquid photo curable photopolymer. Images formed by screen printing, especially heavy coatings such as solder masks on printed wiring boards, inherently have indistinct boundaries, and may have feathered edges and photopolymer smears. These indistinct boundaries are removed and the resolution improved by a process which utilizes the oxygen inhibition effect characteristic of selected photopolymers. Thus, a screen printed image can be 0.001 inches thick at the center of a line, tapering off to 0.0001 inch thick smears, and these smears may be eliminated by irradiating the entire image with a moderate amount light energy, which cures the thick image portion and leaves a liquid boundary layer on the order of 0.0001 inch thick due to the oxygen effect. When washed with a mild solvent the liquid layer is removed, including the smears, leaving the thick image portion undisturbed. In a preferred embodiment a phototransparency is interposed over and out of contact with the images and then irradiated with non-collimated UV light so as to harden the thicker image sections while indistinct boundary areas are shielded by opaque phototransparency areas. Light undercutting due to the use of non-collimated light does not polymerize the smears because of the oxygen-inhibition effect and the thinness of the smears, and thus only the undersirable smears will be inhibited and left in the liquid state for removal to improve the resolution.

Growing community of inventors

King of Prussia, PA, United States of America

Donald F Sullivan