NO890656L - DRY MOVIE PHOTO RESIST FOR THE MAKING OF A FORMABLE MASK AND PROCEDURE TO USE A PRINTED CIRCUIT OR LIKE. - Google Patents

Description

The present invention relates to dry films that form masks where the masks can adapt to irregular surfaces and it relates to a method for forming such malleable masks. A feature of the present invention relates to a dry film including a photobleed forming layer which can be developed and cured to form a permanent solder mask over a printed circuit board or the like and a method for forming a solder mask on a printed circuit board or the like using the dry film.

A solder mask is a hard, permanent layer of non-conductive material that covers the surface of a printed circuit board or similar with encapsulation of the conductors in the printed board itself. By solder mask is meant here a hard, permanent layer that satisfies the abrasion resistance tests laid down in IPC-SM-840A, table 12, Summary of Criteria for Oualification/ Conformance (Institute for Interconnecting and Packaging Electronic Circuits). The solder mask is patterned to completely cover the circuits except for the parts that must be exposed, for example for soldering to another component. Solder masks are typically formed by a layer of a photo-imaging material applied to the surface of the printed circuit board. The photobleed-forming layer is exposed to actinic radiation which is patterned using a template. After the exposure, the photoblend-forming layer is developed in an organic solvent or an aqueous solution which washes away either the exposed or unexposed parts of the layer (depending on whether the photoblend-forming material is positive-acting or negative-acting). The part of the layer that remains on the surface is then cured, for example, with heat and/or ultraviolet light to form a hard permanent solder mask which aims to protect the printed circuits during the board's lifetime.

A previously known method for applying the layer of photobllled-forming material to the surface of the circuit board is to apply the material in liquid form and then either allow it to dry or allow the material to partially harden to form a semi-stable layer.

There are a number of advantages associated with applying a photobllled-forming layer to a printed circuit board as a dry film rather than as a liquid. In particular, dry films are free of organic solvent and thus avoid the danger of filling the workplace with solvent and eliminate the need for equipment to protect the immediate working conditions and the more general environment from the release of organic solvent. In a typical example, a dry film comprises a cover sheet of carrier material which is somewhat flexible, but which has sufficient stiffness to provide strength to a layer of photobllled forming material overlying one side surface of the cover sheet. The cover sheet may be made of polyester material such as polyethylene terephthalate (PET), for example that sold under the name MELINEX^.

In order to protect the photobleed-forming layer and to enable the dry film to be rolled up, it is usual for the exposed surface of the photobleed-forming layer to be covered with a removable protective sheet, for example a sheet of polyethylene. An example of such a dry film is sold under the name LAMINAR DM<R> by Dynachem Division of Morton Thiokol, Inc.

The procedure for using such a previously known dry film is generally as follows: The protective sheet is removed from the layer of photoblled-forming material just before the application of the dry film on the surface of the printed circuit board. This can, for example, be done using automatic equipment that pulls off and rolls up the protective sheet as the dry film is wound off a roll. The dry film is applied to the surface of the circuit board with the photo-image forming layer in direct contact with the surface of the board. Using heat, vacuum and mechanical pressure, the photoblled-forming layer is instantly laminated to the surface of the card. The cover sheet remains on the photobllled-forming layer and protects this layer from the influence of oxygen and from damage from handling. The cover sheet also makes it possible to place a stencil (or template) directly on top of the dry film for contact copying if contact copying is to be used (which is usually preferred when it comes to achieving the best possible image resolution). The dry film is exposed with patterned actinic radiation through the PET cover sheet. At this point, the supporting cover sheet of PET is removed, so that the developer has access to the exposed photofilm-forming layer. Depending on the composition of the photofilm-forming layer, the layer is developed with organic solvent, aqueous developer or semi-aqueous developer. By semi-aqueous developer is meant here a developer that has approximately 90$ or more in volume than aqueous solution, where the rest is an organic solvent such as 2-butoxy ethanol and other glycol ethers. The photobllled-forming layer can be either positively acting, and then the exposed parts are removed by the developer, or negatively acting, and then the unexposed parts are removed by the developer. Most photobllled-forming layers for making solder masks are negative-acting. Most photobllled forming material layers require some curing after development to make the layer hard and permanent so that it can serve as a solder mask. Depending on the composition of the photobllled-forming layer, curing can take place with heat and/or ultraviolet light.

Although a number of advantages are achieved by using dry films, compared to coating circuit boards with a liquid compound, there are certain problems with solder masks formed from dry films and it is these problems that the present invention deals with.

Due to the fact that the cover sheet on a normal dry film is relatively stiff, the photo-LED-forming layer cannot completely shape itself according to the irregular outlines in the printed circuit board where the circuit conductors protrude from the flat surface of the board itself. Because of this, the thickness of the photobllled-forming layer must be somewhat greater than the height of the conductors where they protrude above the surface of the card. If, for example, the conductors protrude 75>j above the surface of the card, in a typical example the photobllled-forming layer will be about IOOjj thick and when laminated to the card, about 25jj of the photobllled-forming material will overlie the upper surfaces of the managers.

An unfortunate result of the solder mask being thicker than the height of the conductors is that raised areas of the solder mask exist between the parts of the circuit from which the photoblled-forming material is to be removed during the development of the solder mask, for example the parts of the conductors to which a component is to be soldered stuck to. Elevated areas are particularly problematic when it comes to components that are surface-mounted on the printed circuit board. Typical components include a series of contact pins, each of which must be soldered to an exposed part of a conductor in the circuit board. If the component does not fit the card exactly, it will rest on the surface of the photobllled forming material, without sufficient bonding between the pins and conductors. It would then be desirable to have low parts or "valleys" instead of the elevated parts found between exposed parts of conductors.

Some common conductors are narrower at the base than at the top and in this case you get small overhangs that are directed outwards along each side of each conductor (often referred to as a "mushroom shape"). With both liquid mixtures and ordinary dry films, you have air-filled cavities under such overhangs. Air trapped in the overhangs can cause oxidation of the conductors and can lead to possible damage to the solder mask.

The present invention relates to an improved dry film for forming a solder mask, which avoids some of the problems with both liquid application and normal dry films. The invention also relates to a method for using a dry film for applying a photo-image-forming layer which can perform a solder mask or can be a primary image-forming photoresist on a surface, in particular an uneven surface. Dry film carrying a primary image-forming resist can be applied by the method according to the invention and then provides an exceptionally good adaptation for plating operations at solder points.

According to a feature of the invention, a dry film has been arrived at for forming a solder mask on a printed circuit board or the like. The dry film includes a cover sheet made of flexible, but largely non-moldable material which makes the dry film shape-resistant, but allows it to be wound into a roll. On one side surface of the cover sheet there is a thin layer of material which is referred to here as the top layer and which is preferably soluble in the developer for the photoblelled-forming material. The photobllled forming material forms a layer on the other side of the top layer. A removable sheet (which is optional but highly preferred) protects the layer of photobllled-forming material. The material used in the top layer selectively adheres to the layer of photobllled-forming material in relation to the adhesion of the cover sheet, so that the cover sheet can be removed from the top layer and thereby leave this behind as a protective coating for the layer of photobllled-forming material.

To form a solder mask according to an aspect of the present invention, the protective removable sheet of the dry film is peeled away and the exposed surface of the layer of photoblled forming material is adhered to a small portion of the surface of the printed circuit board. The cover sheet is removed, leaving the top layer as a protective coating on the layer of photobllled-forming material. At this point, the photoblled-forming material layer is firmly laminated to the surface of the board using heat, vacuum and mechanical pressure, which shapes the photoblled-forming material layer to the outline of the circuit board's surface and will largely encapsulate the conductors. The photobleach-forming material layer is then exposed with patterned actinic radiation through the top layer. A developer is used to remove either exposed or unexposed portions of the photobleed-forming material layer, leaving the remaining portions of the layer laminated to the circuit board. Then the parts of the photoblled-forming material layer that are back on the circuit board are cured, for example with heat and/or ultraviolet light. A similar process is appropriate for applying and developing a malleable primary image-forming photoresist layer, even if heat and curing by ultraviolet light is not used.

According to a modified embodiment of the present invention, after the photo-image-forming material layer is adhered to a smaller part of the card's surface, for example along the front and back edges, the dry film is laminated to the card with heat, vacuum and mechanical pressure. The cover sheet is then removed. At this point, the remaining layers of the dry film are again vacuum-laminated using heat, vacuum and mechanical pressure to shape the remaining layers of the dry film to the contours of the card surface.

Usually, the second lamination is carried out in a shorter time.

According to another embodiment of the present invention, after the photoblled-forming material layer is adhered to a small portion of the surface of the card, for example along the leading and trailing edges, the dry film is laminated with heat, vacuum and mechanical pressure to the card, the cover sheet is then removed. At this point, the remaining layers of the dry film are shaped to the outline of the card by mechanical pressure, exerted by heated rollers that are passed over the card's surface.

The invention is characterized by the features reproduced in the claims and will be explained in more detail in the following with reference to the drawings in which: Figure 1 shows a section through an ordinary dry film which is used to form a solder mask, during application to a printed circuit board,

figure 2A is a section through a dry film according to the invention, used to form a solder mask,

Figure 2B shows removal of the cover sheet from a dry film adhered to a printed circuit board,

Figure 3A is a section through a conductor on a printed circuit board covered by a solder mask formed with a previously known liquid photodiode forming material;

Figure 3B is a section through an alternative conductor on a printed circuit board covered with a solder mask made with a previously known liquid photodiode forming material,

Figure 4A is a section through a conductor on a printed circuit board covered with a solder mask of a prior art conventional dry film,

figure 4B shows a section through an alternative embodiment of a conductor on a printed circuit board over which a solder mask of a previously known conventional dry film is placed,

figure 5A is a section through a conductor on a printed circuit board, encapsulated with a solder mask formed by a dry film according to the present invention,

Figure 5B is a section through an alternative conductor on a printed circuit board encapsulated with a solder mask made of a dry film according to the present invention,

Figures 6-9 are photographs of perpendicular slices of a printed circuit board with a solder mask formed using a dry

film according to the present invention, where the photographs are taken at an angle of 60° in relation to the plane of the card to thereby show both the cross-section of the card and the outside of the photomask. Figure 6 is a 300x magnification of a 50p conductor covered with a 50>j solder mask. Figure 7 is a 750x magnification of a 75p conductor, covered with a 50jj solder mask. Figure 8 is a 300x magnification of a 50jj conductor with transverse overhangs and covered with a 50>j solder mask. Figure 9 is a 100x magnification of 50jj conductors covered with a 50jj solder mask.

Figure 10 is a 300x magnification of a primary photoresist applied by the method according to the invention on a circuit board with conductors,

Figure 11 is a schematic representation of the use of a roller to conform the layers of dry film from which the cover sheet has been removed to the surface of a card and

figures 13A, 13B and 13C schematically show a vacuum application device with plate and the sequence of operations.

Figure 1 shows a circuit board 10 which is covered with a previously known dry film 12. The circuit board comprises a board 14 of non-conductive material for example made of epoxy and with conductors such as 16 which make up the printed circuit. The dry film comprises a cover sheet 18 of flexible largely non-moldable material which may be a polyester such as polyethylene terephthalate, a layer 20 of photoresist-forming material which is finally processed to become a solder mask and a protective sheet 22 made of, for example, polyethylene .

In a typical example, the dry film is drawn from a roll 2 (figure 11) to the circuit board (10). Just before the application to the surface of the circuit board, the protective sheet 22 is picked away as shown for example when winding on a collection roll 4 (figure 11). As the photofilm-forming layer is applied to the surface of the circuit board 10, it is laminated to this with heat, mechanical pressure and vacuum. Although the cover sheet 18 is flexible, it is not directly formable and it therefore retains its flat extent when the dry film 12 is applied to the circuit board 10. The photoblled-forming material layer 20 is formed during the lamination at its contact surface against the circuit board where the conductors 16 protrude. However, the surface of the photoblled-forming material layer in contact with the cover sheet 18 remains flat or planar. To ensure that the photo-image forming material layer 20 completely encloses the conductors 16, the material layer must be thicker than the height of the conductors above the circuit board 14, measured from the non-conductive material. The standard requirement is that the thickness of the solder mask over all parts of the conductors must be at least 18jj. Because the majority of the conductors are raised 50 µm or more above the surface of the card 14, the photocell forming material layer must typically be 68 µm or thicker. Conventional dry films 12 for making solder masks typically have photoblelled-forming layers that are 100p thick.

Figure 2A shows a dry film 21 according to the present invention. This dry film 21 also includes a cover sheet 18, a layer 20 of a photoblled-forming material that can be processed to form a hard permanent solder mask, and an optional but preferred protective sheet 22. In addition, the dry film 21 includes a top layer 24 between the cover sheet 18 and the photobllled-forming material layer

20. The material for the top layer 24 is selected so that it has selective adhesion to the photoblled-forming material layer 20, compared to its adhesion to the cover sheet 18, whereby the cover sheet can be peeled away from the remaining layers 20, 24 of dry film adhered to the circuit board and leaving the top layer 24 as protection for the photo-LED forming material layer 20. The top layer 24 is made of a material selected for its tightness against oxygen penetration, so that it protects the underlying layer of photo-LED-forming material 20 from oxygen barrier below it free radical photopolymerization in the exposure step. Furthermore, the top layer is also chosen so that it is soluble in the developer for the photo-image forming material so that the top layer is removed during the development process. However, it is possible that the top layer is not soluble in the developer and in that case the top layer must be removed in a step that precedes development. The material in the top layer has also been chosen to be transparent, have strength and flexibility.

Dry film for the application of photoresist layers with a similar structure is described in US-PS 4,530,896 and in US patent application S.N. 06/931,396 from 14 November 1986 and reference is made to these publications. Patent

No. 4,530,896 is limited to the application of very thin resist coatings. Primary photoresist materials are typically applied to a metal-coated substrate to determine the pattern of the circuit conductors on a printed circuit board. In each of the aforementioned publications, it is explained that a dry film should in sequence have a cover sheet, an intermediate layer which acts as a top layer after the cover sheet is removed, a photoresist material layer and an (optional) protective sheet.

Although the sequence of layers corresponds to that of the dry film according to the invention, there are several reasons why it is not obvious to an average expert in the field that there would be advantages in a dry film for the production of solder mask, with the same layer division.

A main advantage of a dry film for applying a primary image-forming photoresist layer on a substrate is that the relatively thick cover layer must be removed before image formation, which greatly improves the achievable resolution of the image. Although similar advantages in resolution are obtained with a dry film to form a solder mask, the resolution of the image is of considerably less importance in the case of solder masks than when dealing with primary photoresist materials.

The main advantages of a dry film for solder masks with this layered structure are achieved by the application of the solder mask by the new method according to the invention. The new method according to the invention leads to the photobllled-forming material layer 20 which eventually forms the solder mask, joining carefully and closely to the contours of the surface of a circuit board which is irregular due to the elevated conductors. In contrast, primary imaging photoresist layers are usually applied to a substantially planar surface. By closely fitting the photo-image-forming material layer 20 to the circuit board's surface, a good encapsulation of the conductors 16 is obtained. Moreover, much thinner photo-image-forming material layer 20 can be used than previous dry films that have been used to form solder masks.

In the same manner as for the conventional dry film 12, first the protective sheet 22 is picked from the photoblled forming material layer 20 and the exposed surface of the photoblled forming material layer is applied over a surface of the printed circuit board as shown in Figure 2A. In contrast to what is done with normal dry film 12, the dry film 21 according to the invention does not necessarily have to be completely laminated to the card at the time when the film is applied. Instead, a light pressure is used to adhere the dry film 21 to the card. You can use either cold stapling or hot stapling. Cold stapling generally means work at room temperature, for example 20 - 25°C. Heat sealing can take place at temperatures up to around 70°C. According to a feature of the invention, the dry film 21 is not adhered to the entire surface, but only to a part which is sufficient to provide the necessary adhesion to the card, so that it becomes possible to remove the cover sheet 18 without the other layers 20 , 24 of the dry film is removed from the circuit board 10. At the time of bonding, only a small part, for example 5# or less of the entire surface area of the dry film lies above the board and there are passages between the dry film 21 and the board 10 for later removal of air by applying a vacuum. If the film is applied with a machine, for example, the leading and trailing edges can be attached to the card. If applied manually, stapling in the corners is sufficient. Mechanical pressure, such as that easily applied with a finger, is sufficient for adhesion.

As an example of how the adhesion can be carried out mechanically, reference should again be made to figure 11. Here, a circuit board 10 is guided from left to right by drive rollers 23 and the dry films 21 are made to move together with the surface of the board. Non-powered rollers 25 move back and forth and up and down in the direction shown by the double arrows 6 between vertical positions in contact with the dry film and vertical positions at a distance from the surface. When the front edge of the circuit board is flush with the roller 25, the roller 25 is brought into contact with the dry film and presses it against and adheres it to the front edge of the board. The roll 25 is at once drawn back to the card to enable the middle portion thereof to pass untouched by the rolls. The rollers 25 are again brought back into contact with the card to adhere the dry films 21 to the rear edge of the card 10.

At this point, the cover sheets 18 are removed. Because only a small portion of each area of the dry film 21 is adhered to the circuit board 10, Figure 2B shows a preferred way of removing the cover sheet 18, this being pulled back in a plane parallel to the plane of the circuit board 10 (as opposed to being lifted largely perpendicularly, which would tend to also detach the photo-image forming layer 20 from the circuit board 10).

With the cover sheet 18 removed, the photoblled forming layer 20 with the top layer 24 is now vacuum laminated at an elevated temperature to the card. A vacuum of at least 0.3 millibar is used and preferably a vacuum of at least 1 millibar. Lamination temperatures range from about 40°C to about 100°C, with a range of 55°C to about 80°C being preferred. Because the cover sheet 18 is removed, the two remaining layers (the photoblled-forming layer 20 and the top layer 24) will easily conform to the contours of the irregular surface of the circuit board 10.

In normal vacuum lamination, in addition to heat and vacuum, mechanical pressure is exerted against the dry film. In what is called the "fold-down" (figure 13) method, a belt and/or plate is used to press the dry film against the card. Commercial examples of laminators that use mechanical pressure in addition to heat and vacuum are the models 724 and 730 sold by the Dynachem Division of Morton Thiokol, Inc.

A vacuum laminating apparatus 100 of this type is briefly described with reference to figures 13A, 13B and 13C. The apparatus includes an upper stationary plate 102 and a lower movable plate 104. An upper yielding band 106 is attached to the upper plate 102, while a lower yielding band 108 is arranged for the lower plate 104. The lower plate 104 has a well 109 which carries a circuit board 10 with dry film. Sealing devices in the form of an 0-ring 112 are found on the lower plate 104 for hermetically closing the well 109 when the lower plate is brought into contact with the upper plate 102 and the associated band 106. Figures 13A, 13C also show an intermediate layer piece 108. The intermediate layer piece is also made of flexible material and is used for adaptation to circuit boards of different thicknesses. Several intermediate layer pieces can be used including the intermediate layer piece between the upper plate 102 and the upper band 106. In a typical case, the lower plate 104 becomes movable back and forth both horizontally and vertically. With the lower plate 104 out of horizontal alignment with the upper plate 102, the circuit board 10 coated with dry film is placed in the well 109 on top of the lower band 108. As the lower plate 104 is moved vertically flush with the upper plate 102 , the condition shown in Figure 13A is obtained. The lower plate is then moved upwards until contact is made between the O-ring 112 and the upper plate 102 and the associated band 106 (figure 13B). No devices are shown for applying a vacuum in the area of the well 109, where the circuit board 10 is located. There are also no devices shown to create a vacuum between the upper plate 102 and the upper band 106. After the well area 109 has been sealed, a vacuum is applied both on the well area 109 and between the upper plate 102 and the upper band 106. At this point, the circuit board 10 only against the lower band 108. The vacuum in the well area 109 serves to remove air from the cavities between the dry film layers 20, 24 and the surface of the circuit board and draws the layers of dry film towards the surfaces of this board. For a short period, at the end of each work cycle, the vacuum is relieved from the area between the upper plate 102 and the upper band 106. The vacuum in the well area 109 brings the upper yielding band 106 to "smack down" against the card 10 and presses this and the dry film layers between the upper and lower bands 106, 108 as shown in Figure 13C. Finally, the vacuum is relieved from the well area 109 and the lower plate 104 is removed vertically and then horizontally from the upper plate 102, to enable the laminated card to be removed.

During the clamping, mechanical pressure is transferred to the dry film by the belts, which are made of compliant material. The compliant material in the bands is a type of rubber, for example silicone rubber or another type of compliant polymeric material. The surface of each tape contacting the dense film is textured in a conventional manner to prevent a vacuum bond from forming between the tape and the cover sheet. In previously known methods, such a bond would cause the cover sheet and any underlying layer to be torn away from the card when the surface and tape are pulled back. The texturization of the bands can be in the form of surface roughness in the polymeric material itself or the texturization can occur with the help of a coarse rubberized textile on the surface that is in contact with the dry film. In the method according to the invention, where the other layers 20, 24 of the dry film 21 are laminated to the card after the cover sheet has been removed, it has been found that ordinary tapes with rough contact surfaces are not advantageous if they leave a negative imprint on the remaining layers of the dry film. To avoid this, new semi-matt tapes have been developed and they have a very fine texture. Again, the texture will prevent a vacuum bond from forming between the tape's surface and the other layers of the dry film. The fine surface texture will also leave no imprint on the remaining soft layers 20, 24 of the dry film. Preferred material in the band is silicone rubber, preferably reinforced with textile, for example fiberglass.

During the vacuum lamination, the remaining layers 20, 24 are formed close to the circuit board, which leads to several advantages which will be best understood with reference to Figures 3A, 3B, 4A, 4B, 5A and 5B, as well as the photographs which are Figures 6-9. As can be seen, the thin layer that makes up the top layer 24 is completely intact during the formation process, a result that could not necessarily be predicted.

Figure 3A shows a solder mask that was originally applied as a liquid photodiode forming material. Since it is a liquid, the layer is mostly formed over the uneven outline of the circuit board. On top of each conductor 16, the layer has a substantially uniform thickness, while the 1 areas 26 at bends of the conductors, i.e. the transverse edges, are thinned out to a high degree and the typical is a thickness of approximately 50$ of the thickness of the layer covering the flat upper side of the conductors. Often the layer 20 in the areas 26 of the bends is even thinner compared to the parts of the layer which are located on the flat upper side of the conductors.

Figure 3B shows a solder mask 20 which covers a mushroom-shaped conductor 16' with an overhang 28 on each side. Not only is the solder mask thin in the areas 26 at the bends, but an empty air-filled space 30 is left under each overhang 28. Figure 4A shows a solder mask made from a photocell-forming material layer 20 of plain dry film 12. The outside of the solder mask is flat in shape of the cover sheet 18 which was removed after the lamination, while the laminated surface is largely shaped according to the irregular outlines of the circuit board 10. The solder mask shown in figure 4A is satisfactory in terms of encapsulating the conductors 16. However, it should be pointed out that the thickness of the photodiode-forming material layer 20 and the solder mask that is formed from this are necessarily greater than the height of the conductors, which requires relatively large amounts of photodiode-forming material to design the layer. This thickness represents a limitation in terms of the resolution that can be achieved in the photo image. Figure 4B shows a solder mask which is the result of a photocell-forming layer 20 of ordinary dry film and covers mushroom-shaped conductors 16' with transverse overhangs 28. Although the thickness of the solder mask above the conductor 16' is sufficient, air-filled voids 30 are present under the overhangs 28 Figures 5 and 5B show solder masks made from photo-LED-forming material layers 20 in dry films 21 made according to the present invention. During the lamination that follows the removal of the cover sheet, the other layers 20, 24 in the dry film 21 will largely be shaped according to the irregular surface outlines in the circuit board. In each case (5A and 5B), the thickness of the photofilm-forming layer 20 used to form the photomask was equal to or less than the height of the conductor 16 or 16'. However, the surface of the circuit board was completely covered by the solder mask with the conductors 16 or 16' completely encapsulated and encapsulated with sufficient thickness. It should be noted that the photomask parts in the areas 26 at the bends on the conductors are generally as thick as the photomask parts above the upper surface of the conductors. When a dry film 1 according to the present invention is used, the resulting solder mask has thicknesses in the regions 26 of the bends of the conductors, which are at least about 70% of the thickness of the solder mask over the upper flat surface of the conductors (above more than 90% of the conductors length). It should also be noted in Figure 5B, where a conductor 16<*>has an overhang 28, that the layer 20 conforms so tightly during vacuum/heat lamination that it largely removes air-filled voids from the underside of the overhangs 28. Although significant deformation of the photobllled-forming material layer 20 is necessary to bring about this adaptation, the top layer 24 preserves its integrity during lamination. After the lamination, the solder mask is formed by processing the layer of photoblled-forming material in the usual way. A template is placed over the other layers 20, 24 of the dry film and the top layer 24 prevents the template from sticking. The photofilm-forming material layer 20 is exposed to patterned actinic radiation through the protective top layer 24. Although resolution is generally not as critical in the development of solder masks as it is in the development of photoresist layers, the thinness of the top layer 24 which typically lies between 2 - 3 jj (in contrast to the cover sheet which in typical cases is around 25 jj , through which the photobllled-forming material layers 20 on ordinary solder mask-forming dry films 12 must be exposed) contribute to better resolution. Furthermore, it contributes to improved resolution that the thickness of the photobllled-forming material layer 20 itself is smaller than before. As pointed out above, in a typical solder mask forming dry film 20 to be exposed through cover sheet 18, the image forming material layer must cover conductors 16 with a height of 50 µm or more and the photoblled forming material layers are approximately 20 µm thick. Because the photodiode-forming material layer 20 in the dry film of the invention can be shaped according to the surface contours of the circuit board 10, a thickness of 50 µm or less will be sufficient to cover conductors with a height of 50 µm and even up to about 75 jj over the flat surface of the card 14. Naturally, a further advantage is the significantly reduced amounts of expensive photodiode-forming material used in the layer 20. Another advantage of a thin photodiode-forming layer which is well moldable is that the solder mask that is formed in the areas between the conductors 16, mainly at or below the height of the conductors. With normal solder masks, there are raised areas between the conductors. Such elevated areas can result in the component to be soldered sitting higher on the solder mask and preventing the stiffeners on the component from being sufficiently well soldered to the exposed parts of the conductors. Alternatively, the higher areas during the backflow can result in some of the pins on a vertical component breaking contact and causing the component to rise up on one end, which is known as the "tombstone phenomenon". Insufficiently soldered contact and the "tombstone phenomenon" are avoided when you have "valleys" instead of high areas between exposed parts of the conductors. ;After exposure to actinic radiation, the layers 20 and 24 are developed, the developer removing the top layer 24 and the desired portions of the photofilm-forming layer 20. ;In addition, a dry film solder mask according to this invention has the same "tent formation" capability through holes like normal primary image flickers or solder mask flickers. This is a very important part of the process and cannot be reproduced with simple processes with liquid materials. ;In general, a further cure will follow the development to cure the solder mask. In typical cases, heat and/or ultraviolet light is used for this curing. The present invention is not primarily directed at the materials from which the various sheets and layers are formed, and reference should again be made to US-PS 4,530,896. The cover sheet 18 is usually made of a polyester such as PET and is generally at least 20 p thick or thicker, up to 75 n. ;PET, if the material used for the carrier sheet is preferably surface treated to give the necessary hydrophilic properties or wetting angle so that it does not attach strongly to the top team. The wetting angle or contact angle for the material in the cover sheet is preferably at least 60° and preferably 75° or less. A suitable PET cover sheet is sold under the trade name Melinex-393 by ICI. This is a PET material that has been surface treated on one side to improve its hydrophilic properties, i.e. increase its wetting angle. Other suitable cover sheet materials include, but are not limited to, polyamides, polyolefins, vinyl polymers, and cellulose esters. Any photo-image forming compound suitable for forming solder masks is suitable for carrying out the invention. The protective sheet 22 is selected so that it can be removed from the layer 20 of photographic image forming material and is, in a typical example, a polyethylene sheet between about 15 µm and about 35 µm thick. The top layer 24 is formed from a material which provides the necessary selective adhesion. The top layer is 1 a typical example from about lp to about 12j thick and especially between 2y> and about 5jj. When using a PET cover sheet and a photo-film forming material which can be developed in an aqueous or alkaline aqueous developer, a suitable top layer consists essentially of (a) between 10 weight percent and 100 weight percent of either polyvinyl alcohol hydrolyzed to at least about 75% or a copolymer formed by copolymerizing between about 95 and about 99 molar percent of polyvinyl acetate and between about 1 and about 5 molar percent of alkyl ester of alkyl acid and hydrolyzed to at least about 75%, (b) up to about 90 percent by weight of hydroxyethyl cellulose, and (c) up to about 10 weight percent of a plasticizer. Hydroxyethyl cellulose is preferably used in an amount of at least 25% by weight and even more advantageously in an amount of at least 50% by weight. The copolymers described above are sold under the trade name VINOL-118 M by Air Products, and 4 weight percent solutions of these copolymers have viscosities ranging from about 5 to about 65 centipoises at 20°C. PVC homopolymers with similar viscosity are also usable. Commercially available PVA materials are also sold by Air Products. A plasticizer, if used, tends to lower the bonding temperature of the top layer material and bonding temperatures below about 100°C have been achieved using such plasticizers. A top layer material with a lower adhesive temperature is easier to bond to the photo-imaging layer. The plasticizers are preferably compounds with a low molecular weight, i.e. below about 100 with two or more hydroxyl groups such as glycerin, ethylene glycol and propylene glycol. Although the plasticizers lower the adhesive temperature in a desired way, they will also increase the penetration of oxygen into the top layer. For that reason, amounts of plasticizers of more than 10 percent by weight should preferably be avoided. ;Other suitable materials for the top layer include, but are not limited to, the group consisting of polyvinyl ether-maleic anhydride copolymers, water-soluble cellulose ethers, water-soluble salts of carboxyalkyl cellulose, water-soluble salts of carboxyalkyl starch, polyvinyl alcohol, polyvinyl pyrrolidone, various polyacrylamides, various water-soluble polyamides, water-soluble salts of polyacrylic acid, gelatin, ethylene oxide polymers, various starches and the like. The method for forming the dry film takes place according to known methods. The top layer and the photofilm-forming material layers are applied to the cover sheet in successive layers by methods such as rolling on or by spraying solutions or dispersions of the materials. In a variant of the method of carrying out the invention described above, a dry film can be laminated both before and after it is removed from the cover sheet. In cases where the photobllled-forming layer has approximately the same thickness or is thinner than the conductors (or other irregularities in the card), one can expect that the conductors (or other irregularities in the card) can penetrate the photobllled-forming layer. However, it has been found in such cases that ordinary lamination of such film using heat, vacuum and mechanical clamping in the usual way results in adhesion of the photoblled-forming layer to the upper sides of the conductors (or other elevations) without the conductors (or elevation -nings) penetrate the photobllled-forming layer. The adhesion of the dry film over a larger area of the card facilitates bonding to it when the cover sheet is removed. A second vacuum lamination using heat, vacuum and mechanical compression after the cover sheet is removed causes the layer of photofilm-forming material (and the top layer) to conform to the contours of the uneven surface of the card. In a typical procedure, the lamination after the cover sheet is removed will take about half the time it takes to laminate before the cover sheet is removed, i.e. 30 sec. before the second lamination, compared to 60 sec. for the first laminations. Additional adhesion which takes place during the first lamination may prove to be particularly advantageous when automatic equipment is used for removing the cover sheet. As a further variant of the method according to the invention, instead of using a lamination process with heat/vacuum, after removing the protective sheet 22 and the cover sheet 18, a roller can be used which exerts mechanical pressure to shape the photodiode-forming layer 20 (and the top layer 24) to the contours of the card surface to achieve the same purpose. 12 schematically shows the use of the roller to shape the photo-image-forming layer 20 and the top layer 24 to the outline of the card. In the schematic drawing, the card 20 and the layers 20, 24 of dry film are moved from left to right under a non-driven roller 25 with the roller 1 pressing against the top layer 24. As in the methods described above, the dry film is first adhered . The dry film is then preferably vacuum-laminated to the board 10. In the case where the dry film has a thin photo-film forming layer 20, relative to the height of the conductors, the dry film layers 20, 24 sit for the most part on the top surface of the conductors 16 as shown on the left side of fig. 12. After passing the roller 45, the photofilm forming material 20 and the top layer 24 will be conformed to the outline of the card 10 with the conductors 16 as shown on the right side of fig. 12. The layers 20, 24 are pressed down over the entire surface of the card 10, as the card passes under the roller. Alternatively, a roll could be drawn across the surface of a stationary card. The roller 45 used for this purpose should exert a pressure of between 1 bar and approximately 6 bar. The roll 45 is heated depending on the particular application to between 40 and 110°C and mostly in the range of 60 to 80°C. The surface of the forming roll 45 has a certain compliance, preferably in the form of a polymeric material with a Shore A hardness of between 60 and 80. The surface of the roll is smooth. The relative speed of the roller 45 and the card 10 will vary depending on the application, generally between about 1 meter and about 10 meters per minute. second where 3 meters per second is typical. The invention will now be described in more detail in the form of specific examples. ;EXAMPLE 1;A dry film was prepared in accordance with the invention, with a cover sheet which is a 50p thick sheet of PET, a 2.5j top layer which is 10% by weight of the hydrolyzed polyvinyl acetate/acrylic acid copolymer described above and 90 weight percent hydroxyethyl cellulose, a photofilm-forming material in the form of a layer 50 µm thick and a polyethylene protective sheet 25 µm thick. The photobllled-forming material has the following formula: ;Constituents:; ; EXAMPLE 2; A dry film was prepared in accordance with Example 1, using LAMINAR<R>DM to form the layer of photofilm forming material. ;EXAMPLE 3;Plates with copper conductors and plates with tin/lead conductors were cleaned in the usual way. The dry film example 2 was applied to the cards both manually and with an automatic system. Manually, the protective polyethylene sheet was removed and the four corners of the film were stapled by pressing down with the fingers. Using a band to get a grip on the cover sheet, this was removed by pulling the PET cover sheet in a plane parallel to the card surface. In the automated version, a Dynachem model 360 laminator was used. The Dynachem equipment uses a "no contact" method that automatically removes the protective polyethylene sheet, adheres the leading edge of the dry film and then adheres the trailing edge of this film. This is done on both sides of the cards. The cover sheet is then removed. ;The lamination took place at a plate temperature of 70°C with a cycle time of 60 seconds and a vacuum of 2 millibars. Mechanical clamping took place for 6 seconds at a vacuum of 0.3 millibar. The cards were held for 20-30 minutes. ;Stencils were placed over the cards. Using Colight DMVL-824, the cards were exposed for 25 seconds at 100 mJ/cm<2>measured with Dynachem's integrating radiometer model 500, yielding Stouffer steps of 8 to 9. The cards were held for an additional 20-30 minutes. The cards were developed in 1.0 wt% Na 2 CO 3 *H 2 O which gave a 2 x breaking point. The cards were dried using a Chemcut 547 circuit card dryer. Curing took place in a Dynachem UVEX curing unit, an IL-390 unit and a high temperature oven. The hardening steps were as follows:

U. V. HERDING

1. UVEX-3 lamp (200 watts/inch)

2. 3.5 Joules/cm<2>(IL-390) side A.

3. Cooling to room temperature

4. 3.5 Joules/cm<2>(IL-390) side B

5. Cooling to room temperature

THERMAL CURING

1. Oven baking at 150°C for 60 minutes.

EXAMPLE 4

Figure 6 shows a solder mask formed by a 50p thick layer encapsulating a 50jj thick conductor. Figure 7 shows a solder mask formed by a 50jj thick layer encapsulating a conductor that is 75jj high. Good coverage is seen in the areas near the knees. Figure 8 shows a solder mask formed by a 50 µm thick layer encapsulating a 50 µm high conductor with an overhang. As you can see, the solder mask material fills the areas under the overhangs. Figure 9 shows a solder mask formed by a 50 µm thick layer encapsulating a 50 µm high conductor. You can see the adaptation of the solder mask to the outlines in the circuit board.

Although the invention particularly applies to and is mainly described as applied to a dry film bearing a layer of photoblend-forming material that can be cured into a solder mask, the invention can generally be applied to a dry film with any image-forming agent, including primary photoresist that is applied to an irregular surface, especially a surface where the conductors protrude from an otherwise flat surface. It may also be desirable to use the method according to the invention to apply a layer of primary photoresist on a copper surface with irregularities caused by "barrel glass". There are cases, for example, where it is desirable to lay a photoresist layer over a designed circuit that has already been formed and then add or subtract existing copper patterns or "tab plate gold fingers".

EXAMPLE 5

A dry film was prepared in accordance with Example 1 using Laminar<R>AX which constituted a photobleed forming material layer. Resist was laminated over raised lugs as in Example 3 and exposed with an image on solid copper 7-9 in a Stouffer<R>21 step sensitivity sensor. A covered conductor is seen in Figure 10. The exposed plate was then developed in a Chemcut 547 colander developer containing 91 liters of water, 3% by weight potassium carbonate and 1 to 5% Butyl Cellosolv<R> which released about 3/4 of each coupling ear. The developer temperature was kept between 27-33°C. After thorough rinsing, a 0.30 mil nickel layer was applied to the card followed by 30 min. gilding in a Technic RC-80 bath. The resist material was then stripped off in a 3-5% NaOH at 54-60°C. The gold-plated ears were then examined for wicking in the areas around which the resist material had adapted. No trace of gold or nickel was found anywhere over the gilded areas. A control sample that was not vacuum laminated in this way showed a wicking effect of approximately 25% at the ears.

The good adaptation obtained with the laminated dry film according to the present invention can also be achieved with a dry film comprising a layer of photo-forming material, supported by a cover sheet of deformable material, for example thermoformable vacuum film. Such a film can be applied in the same way as normal film, as heat/vacuum lamination is carried out either during or after placement on the circuit board. In this case, the cover sheet must also be able to be adapted accordingly. However, one can rely on problems when it comes to handling the more flexible sheets.

In a variant of the method according to the invention, after the cover sheet has been removed and before the vacuum/heat lamination, a flexible sheet, for example of Tedlar or Teflon, is laid over the top layer. During the vacuum lamination, the flexible sheet will follow the top layer, but can be removed and is removed before or after exposure to enable development. One reason for the application of a flexible sheet is that the permitted time period between lamination and exposure of the photofilm-forming layer is then extended. Flexible sheet material can be a polymeric material or a thin paper sheet.

Although the method according to the invention is described mainly with reference to a dry film comprising a cover sheet, a top layer on the cover sheet and a layer of photobllled-forming material on the top layer, the method according to the invention can be practiced using an ordinary dry film comprising a cover sheet and a photoblled forming material layer for example either a primary image forming photoresist or a solder mask forming photoblled forming compound which is in direct contact with the cover sheet. Such a conventional dry film may optionally have protective sheets covering the photoblled-forming material layer. The method can be carried out with an ordinary dry film on the condition that the layer of photo-LED-forming material adheres to a surface with sufficient adhesion in relation to the adhesion between the photo-LED-forming material and the cover sheet so that the cover sheet can be removed from the photo-LED-forming material layer and leaving this layer bound to the surface. After the cover sheet is removed, the photobleed-forming material layer can be covered with a sheet of flexible material such as Tedlar or Teflon (polytetrafluoroethylene). Such flexible materials facilitate contact coupling and prevent the photoblled-forming material layer from sticking to parts of the vacuum laminator. As an alternative, the belts 1 the vacuum laminator can consist of flexible materials. The photoblled-forming material layer is laminated to the surface with heat and vacuum forming the photoblled-forming material layer to the surface. The photoimage-forming material layer is exposed with patterned actinic radiation, for example by off-contact copying such as with a Tamarack Exposure source. If a flexible sheet is used, it is removed either before or after the exposure. The photobllled forming layer is then developed. If the photobllled-forming layer is a material that forms a solder mask, it is cured.

EXAMPLE 6

The following is a specification for a fine textured yielding tape according to the present invention. Product structure: Silicone rubber (methylvinyl)

balanced coated on fiberglass textile with a matt finish on a

page.

Product colour: Red.

The product's distinctive features: a. Nice matt finish on one side,

smooth on the other.

b. The dimension of stability.

c. High breaking strength.

d. High temperature resistance and release properties for silicone.

PHYSICAL PROPERTIES

Although the invention has been described with reference to certain preferred embodiments, modifications which are obvious to those skilled in the art will not deviate from the scope of the invention.

Claims (11)

1. Method for forming a solder mask on the surface of a printed circuit board or the like, characterized in that it comprises: (A) providing a dry film comprising (1) a cover sheet, (2) a top layer on the cover sheet and (3) a layer of photobllled-forming material on the top layer, which photobllld-forming material is curable after exposure and development to a hard permanent solder mask overlying the printed circuit board, which top layer is selectively adherent to the photobllld-forming material compared to its adhesion to the cover sheet; (B) adhering said photoblled-forming material layer to the surface of the printed circuit board; (C) removing the cover sheet from the top layer so that the photoblled-forming material layer remains adhered to the surface of the circuit board while the top layer covers and protects it; (D) laminating the photoblled-forming material layer to the surface of the printed circuit board using heat and vacuum to carefully adhere the photoblled-forming material layer and the top layer to the surface of the printed circuit board; (E) exposure of the photobleed-forming material layer to patterned actinic radiation; (F) developing the photobleed-forming material to remove either exposed or unexposed portions from the printed circuit board so that either unexposed or exposed portions remain on the printed circuit board, and (G) curing the remaining portions of the photoblled-forming material layer to form a hard permanent solder mask protecting the printed circuit board. 2. Printed circuit board comprising a flat non-conductive or semi-conductive board, conductors raised above the surface of the board, which conductors have flat upper sides and transverse knees, and a solder mask covering the main part of the circuit board including the conductors, characterized in that the solder mask has an average thickness of substantially equal to or less than the height of the conductors, where the thickness of the solder mask in the area of the transverse knees is at least 70% of the thickness of the solder mask over the top surfaces of the conductors over approximately 90% of the linear extent of the conductors. 3. Method for producing a photobleed-forming layer on the surface of a carrier, characterized in that it comprises: (A) production of a dry film with (1) a cover sheet, (2) a top layer on the cover sheet, and (3) a layer of photobllled-forming material on the top layer, which top layer is selectively adherent to the photobllld-forming material relative to the adhesion to the cover sheet; (B) adhesion of the photoblled-forming material layer to the surface of the support; (C) removing the cover sheet from the top layer so that the photo-LED forming material layer is reattached to the surface of the carrier with the top layer covering and protecting the photo-LED forming material layer, (D) laminating the photo-LED-forming material layer to the surface of the carrier using heat and vacuum to form the photo-LED-forming material layer and the top layer to the surface of the carrier; (E) exposure of the photobleed-forming material layer to patterned actinic radiation, and (F) developing the photobleed-forming material to remove either exposed or unexposed portions from the printed circuit board, leaving either unexposed or exposed portions on the surface of the carrier. 4. Method for producing a solder mask on the surface of a printed circuit board or the like, characterized by (A) producing a dry film comprising a cover sheet and a layer of photo-LED-forming material on the cover sheet, which photo-LED-forming material is curable after exposure and development to a hard permanent solder mask overlying the printed circuit board; (B) adhering the photoblled-forming material layer to the surface of the printed circuit board; (C) removing the cover sheet from the photoblled-forming material layer so that this layer is reattached to the surface of the circuit board; (D) laminating the photo-LED forming material layer to the surface of the printed circuit board using heat and vacuum to form the photo-LED forming material layer to the surface of the board; (E) exposure of the photobleed-forming material layer to patterned actinic radiation; (F) developing the photobllled-forming material to remove either exposed or unexposed portions from the printed circuit board and to leave unexposed or exposed portions thereon, and (G) curing the remaining portions of the photoblled-forming material layer into a hard permanent solder mask that protects the printed circuit board. 5. Method for producing a photobleed-forming layer on the surface of a carrier, characterized by: (A) forming a dry film comprising a cover sheet and a layer of photobllled forming material on the cover sheet; (B) bonding the photobleed-forming material layer to the surface of the support; (C) removing the cover sheet from the photoblled-forming material layer so that this layer is reattached to the surface of the support; (D) laminating the photo-LED-forming material layer to the surface of the carrier using heat and vacuum to form the photo-LED-forming material layer to the surface of the carrier; (E) exposure of the photobleed-forming material layer to patterned actinic radiation, and (F) developing the photobllled-forming material to remove either exposed or unexposed portions from the printed circuit board, while leaving either unexposed or exposed portions on the surface of the carrier. 6. Method for producing a solder mask on the surface of a printed circuit board or the like, characterized by: (A) producing a dry film consisting of (1) a cover sheet, (2) a top layer on this sheet, and (3) a layer of photobllled-forming material on the top layer, which photobllld-forming material is curable after exposure and development to a hard permanent solder mask overlying the printed circuit board, which top layer is selectively adherent to the photobllld-forming material relative to its adhesion to the cover sheet, (B) adhering the photoblled-forming material layer to the surface of the printed circuit board; (C) laminating the photoblled-forming layer to the surface of the printed circuit board using heat, vacuum and mechanical pressure; (D) removing the cover sheet from the top layer so that the photoblled-forming material layer is reattached to the surface of the circuit board where the top layer covers and protects it; (E) laminating the photodiode-forming material layer to the surface of the printed circuit board using heat, vacuum and mechanical pressure to form the photodiode-forming material layer and the top layer to the surface of the board; (F) exposure of the photobleed-forming material layer to patterned actinic radiation; (G) developing the photobllled-forming material to remove either exposed or unexposed portions from the printed circuit board so that either unexposed or exposed portions remain thereon; and (H) curing the remaining portions of the photoblled-forming material layer into a hard permanent solder mask that protects the printed circuit board. 7. Method for producing a photobleed-forming layer on the surface of a carrier, characterized by: (A) production of a dry film extensively (1) a cover sheet, (2) a top layer over the cover sheet and (3) a layer of photobllled-forming material on the top layer, which top layer is selectively adhered to the photobllld-forming material layer relative to its adhesion to the cover sheet, (B) adhering the photobllld-forming material layer to the surface of the support; (C) laminating the photoblled-forming material layer on the surface of the support using heat, vacuum and mechanical pressure; (D) removing the cover sheet from the top layer so that the photoblled forming material layer is reattached to the surface of the carrier with the top layer covering and protecting the photoblled forming material layer, (E) laminating the photoblled-forming material layer to the surface of the carrier using heat, vacuum and mechanical pressure to form the photoblled-forming material layer and top layer to the surface of the carrier; (F) exposure of the photobllled-forming material layer to patterned actinic radiation and (G) developing the photoblled-forming material layer to remove either exposed or unexposed portions from the surface of the support, so that either unexposed or exposed portions remain on said surface. 8. Method for forming a photobleed-forming layer on the surface of a carrier, characterized by: (A) forming a dry film comprising a cover sheet and a layer of photobllled forming material on the cover sheet; (B) adhering the photoblled-forming material layer to the surface of the support; (C) removing the cover sheet from the image-forming material layer so that it is reattached to the surface of the support; (D) applying pressure to the photobllled-forming material by passing a heated roller over it to thereby cause the photobllld-forming layer to conform to the surface of the support; (E) exposure of the photobllled-forming material layer to patterned actinic radiation and (F) developing the photobleed-forming material to remove either exposed or unexposed portions from the surface of the support so that either unexposed or exposed portions remain on said surface. 9. Procedure for forming a solder mask on the surface of a printed circuit board or similar, characterized by: (A) production of a dry film extensively (1) a cover sheet and (2) a layer of photobllled-forming material on the cover sheet, which material can be cured after exposure and development to form a hard permanent solder mask overlying the printed circuit board; (B) adhering the photoblled-forming material layer to the surface of the printed circuit board; (C) removing the cover sheet from the photoblled-forming material layer so that it is reattached to the surface of the circuit board; (D) applying pressure to the photodiode-forming material layer by means of a heated roller passed over it to cause the photodiode-forming material layer to conform to the surface of the printed circuit board; (E) exposure of the photobleed-forming material layer to patterned actinic radiation; (F) developing the photobllled-forming material to remove either exposed or unexposed portions from the printed circuit board, leaving either unexposed or exposed portions of the printed circuit board; and (G) curing the remaining portions of the photo-image-forming material layer thereby forming a hard, permanent solder mask protecting the printed circuit board. 10. Method for producing a photobleed-forming layer on the surface of a carrier, characterized by: (A) production of a dry film, incl (1) a cover sheet, (2) a top layer on the cover sheet and (3) a layer of photobllled-forming material on the top layer, which top layer is selectively adherent to the photobllld-forming material relative to its adhesion to the cover sheet; (B) bonding the photobleed-forming material layer to the surface of the support; (C) laminating the photoblled-forming material layer to the surface of the support using heat, vacuum and mechanical pressure; (D) removing the cover sheet from the top layer so that the photoblled forming material layer is reattached to the surface of the carrier with the top layer covering and protecting the photoblled forming material layer, (E) applying pressure to the photoblled-forming material layer by means of a heated roller passed over it so that the photobllled-forming material layer is caused to conform to the surface of the support; (F) exposure of the photobllled-forming material layer to patterned actinic radiation and (G) developing the photoblled-forming material layer to remove either exposed or unexposed portions from the surface of the carrier so that either unexposed or exposed portions remain thereon. 11. Method for forming a solder mask on the surface of a printed circuit board or similar, characterized by: (A) production of a dry film extensively (1) a cover sheet, (2) a top layer on the cover sheet and (3) a layer of photodiode-forming material on the top layer which material can be cured after exposure and development to form a hard permanent solder mask overlying the printed circuit board, which top layer is selectively adherent to the photodiode-forming material as compared to its adhesion to the cover sheet, (B) adhering the photoblled-forming material to the surface of the printed circuit board; (C) laminating the photoblled-forming layer to the surface of the printed circuit board using heat, vacuum and mechanical pressure; (D) removing the cover sheet from the top layer, so that the photoblled-forming material layer is reattached to the surface of the circuit board with the top layer covering and protecting it, (E) applying pressure to the photoblled-forming material layer by means of a heated roller passed over it causing the photoblld-forming material layer to conform to the surface of the printed circuit board; (F) exposure of the photobleed-forming material layer to patterned actinic radiation; (G) developing the photobllled-forming material to remove either exposed or unexposed portions from the printed circuit board so that either unexposed or exposed portions remain thereon; and (H) curing the remaining portions of the photo-imaging material layer to form a hard permanent solder mask protecting the circuit board.