Typically, polymers are used in the manufacture of printed wiring boards (PWB), by coating the PWB with a polymer coating. For solder masks the coating pattern covers conductor wiring traces at all locations except where electrical contact is to be made with the traces. The curing traces may also be printed on a metallic layer such as copper on an insulating substrate by means of polymer printing patterns defining areas which are to be etched in one primary imaging method. In other primary imaging methods, polymer printing patterns may define zones for accumulation of conductive materials forming conductor traces. Other printed objects are similarly manufactured by use of the polymer printing patterns.
In common use are three types of polymers, namely, (1) thermal curing epoxy, (2) ultra-violet (UV)-curing polymers generally classified as liquid polymers which are hardened by radiation, and (3) dry film polymers which are changed in solubility by radiation. The epoxies and UV-curables are applied as liquid coatings for curing in place as contrasted with the dry films.
Some of the patterns formed by these polymers are formed mechanically such as by silk screening. Others are formed by photo-imaging techniques, where the photo response characteristics of certain polymers such as the UV curing liquid polymers are utilized.
Dry film solder mask photopolymers are supplied by two manufacturers, in roll form, consisting of an inner layer of photopolymer sandwiched between a carrier film of clear polyester and a polyolefin liner. They have problems of adhering to copper, and are not conveniently used for overlaying rough surfaces. Thus, for a solder mask coating on a typical PWB, wherein the metal conductors extend 0.003 to 0.004 inch above the base laminate, the dry film needs be conformed without intermediate air bubbles. The use of a roller laminator is not usually satisfactory, as air is trapped between the photopolymer and the PWB laminate, particularly between closely-spaced conductors. A vacuum chamber laminator is normally used in a plasticizing procedure to prevent air entrapment. The lamination cycle is as follows: A PWB and a section of dry film solder mask is inserted into the heated chamber; the chamber is evacuated, and when up to temperature the photopolymer is forced into contact with the PWB, effecting an air-free lamination. Lamination occurs at a temperature of the order of 200 degrees F., at which temperature the dry film solder mask photopolymer becomes tacky and adheres to the PWB surface. The solder mask pattern is attained by exposing the photopolymer to a strong UV light source through a photographic film, wherein the light hardens the exposed photopolymer. The polyester carrier film is then peeled away and the unhardened photopolymer is washed out in a solvent spray bath.
There are several shortcomings with the dry films listed below:
1. The single photopolymer layer is expensive being of the order of four times the cost of liquid photopolymers; the equipment required to laminate is overly complicated and expensive; the labor required is excessive, as each processing step of laminating, exposing, and developing is overly lengthy.
2. The laminating step forces photopolymer into circuit holes, and the like, with the result that with small holes of the order of 0.025 inch diameter, the photopolymer does not wash out, leaving the holes plugged.
3. The laminating step forces photopolymer into large tooling holes and slots, leaving a puckered, striated coating of photopolymer, for the polyester carrier film is non-conforming and wrinkles around larger holes and sharp corners.
4. The dry photopolymer is characterized by a lack of adhesion to metal conductors. For PWB having bare copper conductors with dry film solder mask thereover, the solder coating and hot-air leveling step is usually not satisfactory without copper surface pretreatment as with a black oxide coating; otherwise the solder mask separates from the conductors.
5. The dry photopolymers need be applied in thick layers of standard thicknesses, and the thicker layers are not only expensive but also inhibit high resolution photo reproduction and require higher temperatures to photodevelop.
6. The photopolymer is temperature sensitive, delaying the application sequence. After laminating at 200 degrees F., the photopolymer must cool to room temperature prior to exposure. After exposure, which raises the temperature again, the photopolymer must cool to room temperature before washing out unexposed photopolymer.
Similarly epoxy coatings have disadvantages since the initial liquid layers required need be cured in situ, generally at very high temperatures which can damage the printed wiring boards or other substrates being printed upon.
The liquid polymers provide problems in initial application since they do not tend to stay in place, and have conventionally been applied in very thin coatings, unsuitable for pinhole free coatings, and for covering the rough surfaces of printed wiring traces.
The different polymers each have advantageous features, but it is not known in the prior art how to overcome this disadvantage in producing printed patterns on substrates therewith.
Typical of prior art polymer printing techniques used in PWB are those of U.S. Pat. No. 3,629,036--C. Isaacson, Dec. 21, 1971, where a microthin adhesive layer including a photopolymer solvent is interspersed between smooth copper surfaces and a dry film photoresist layer, which is thereby glued in place before photoimaging and developing the desired pattern.
Reference is made to U.S. Pat. No. 3,824,104 in which Kloczewski teaches a method for photoimaging a liquid photopolymer, wherein the image bearing photomask is separated from the photopolymer by a distance of eight mils during exposure, leading to a stated loss of resolution.
Reference is made to U.S. Pat. No. 4,260,675 in which Sullivan describes a method for photoimaging liquid photopolymer using a glass plate photomask with raised opaque pillars in contact only with portions of the PWB which will be free of hardened solder mask.
Each of these patents and current practices in the art of PWB manufacture has characteristics which this invention seeks to improve.
One objective is to improve the adhesion of dry film to metal conductors.
A second objective is a process to laminate a dry film to a highly irregular surface such as over PWB traces without use of a vacuum laminator.
Another objective is to reduce the time required to process dry film by negating the requirement for laminating at an elevated temperature.
Another objective is to achieve a photoimaged solder mask having the electrical and environmental characteristics of dry film, but at reduced cost.
Another objective is to achieve a photoimaged PWB solder mask in which UV-curable liquids are exposed with a photomask in contact with the liquid, and in which the solder mask coating is not thinned out over the metal conductors.