Method of forming conformable masks

Abstract: A dry film (21) for forming a solder mask includes a cover sheet (18), a photoimageable composition layer (20) which is curable to form a solder mask and a top coat (24) interposed between the cover sheet and the photoimageable composition layer which is selectively adherent to the photoimageable composition layer. The dry film is applied to a surface of a printed circuit board (10) with a minor portion of the area of the photoimageable composition layer tacked to the printed circuit board. The cover sheet is peeled away. With heat and vacuum, the photoimageable composition layer (20) is laminated to the irregular surface of the printed circuit board conforming the photoimageable composition layer to the contours thereof and leaving the top coat as a protective covering over the photoimageable composition layer. The photoimageable composition layer is exposed to patterned actinic radiation, developed and cured to form a hard, permanent solder mask.

Description: The present invention is directed to dry films which form masks that conform to irregular surfaces and a method of forming such conformable masks. One aspect of the present invention is directed to a dry film which includes a photoimageable layer which is developable and curable to form a permanent solder mask over a printed circuit board or the like and to a method of 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 which covers the surface of a printed circuit board or the like, encapsulating the traces of the printed circuit itself. By solder mask is meant herein a hard, permanent layer which meets the abrasion resistance tests as defined in IPC-SM-840A, Table 12, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting and Packaging Electronic Circuits). The solder mask is patterned to fully cover the circuitry, except for those portions intended to be exposed, e.g., for soldering to another component. Solder masks are typically formed from a layer of photoimageable composition which is applied to a surface of the printed circuit board. The photoimageable layer is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent or an aqueous solution which washes away either exposed or unexposed portions of the layer (depending upon whether the photoimageable material is positive acting or negative acting). The portion of the layer which remains on the surface is then cured, e.g. with heat and/or UV light to form a hard, permanent solder mask intended to protect the printed circuitry for the life of the board. One prior art method of applying the layer of photoimageable composition to the circuit board surface is to apply the material in liquid form and then either allow it to dry or partially cure the material to form a semi-stable layer. There are a number of advantages to applying a photoimageable layer to a circuit board as a dry film rather than as a liquid. In particular, dry films are free of organic solvent and therefore eliminate this hazard from the workplace and eliminate the need for apparatus to protect the immediate work environment and the more general environment from organic solvent emissions. Typically, a dry film comprises a cover sheet of support material which is somewhat flexible but which has sufficient rigidity to provide structure to a layer of photoimageable composition which overlies one surface of the cover sheet. Typically, the cover sheet is formed of polyester material, such as polyethylene terephthalate (PET), such as that sold as MELINEX R . To protect the photoimageable layer and to enable the dry film to be rolled, it is conventional that the exposed surface of the photoimageable layer be covered with a removable protective sheet e.g., a sheet of polyethylene. An example of such a dry film is sold as LAMINAR DM R by the Dynachem Division of Morton international, Inc. The application of dry films of this type is described for example in GB-A- 1 517 302. The method of use of such a prior art dry film is generally as follows. The protective sheet is removed from the photoimageable composition layer immediately prior to application of the dry film to the surface of the printed circuit board. This may be accomplished, for example, using automated apparatus which peels away and rolls up the protective sheet as the dry film is unrolled from a reel. The dry film is applied to the surface of the circuit board with the photoimageable layer in direct contact with the board surface. Using heat, vacuum and mechanical pressure, the photoimageable layer is immediately laminated to the surface of the board. The cover sheet remains overlying the photoimageable layer, protecting the photoimageable layer from exposure to oxygen and from handling damage. The cover sheet also permits an artwork (or template) to be laid directly on top of the dry film for contact printing, if contact printing is to be used (as is usually preferred from the standpoint of obtaining optimal image resolution). The dry film is exposed to patterned actinic radiation through the PET cover sheet. At this time, the PET support sheet is removed, permitting access to the exposed photoimageable layer by developer. Depending upon the composition of the photoimageable layer, the photoimageable layer is developed with organic solvent, aqueous developer, or semi-aqueous developer. By semi-aqueous developer is meant herein a developer which is about 90% or more by volume aqueous solution, balance an organic solvent, such as, 2-butoxy ethanol and other glycol ethers. The photoimageable layer may either be positive acting, in which case the exposed portions are removed by developer, or negative acting, in which case the unexposed portions are removed by developer. Most photoimageable layers for preparing solder masks are negative acting. Most photoimageable composition layers require some cure subsequent to development to render the layer hard and permanent so as to serve as a solder mask. Depending upon the composition of the photoimageable layer, curing may be effected with heat and/or UV light. Although a number of advantages accrue through the use of dry films as opposed to coating the circuit board with a liquid composition, there are some problems with solder mask-forming dry films which the present invention addresses. Because the cover sheet of a conventional dry film is relatively rigid, the photoimageable layer cannot fully conform to the irregular contours of the printed circuit board, from which circuitry traces rise from the flat surface of the board itself. Because of this, the thickness of the photoimageable layer must be slightly greater than the elevation of the traces above the surface of the board. For example, if the traces rise 75µm above the surface of the board, the photoimageable layer will typically be about 100µm thick; when laminated to the board, approximately 25µm of photoimageable composition overlies the upper surfaces of the traces. One unfortunate result of the solder mask being thicker than the elevation of the traces is that elevated regions of solder mask exist between those portions of the circuitry from which photoimageable composition is removed during the development of the solder mask, e.g. those portions of the traces to which a component is to be soldered. Elevated regions are particularly problematic with respect to components which are surface-mounted to the printed circuit board. Components typically contain a plurality of pins which are each to be soldered to an exposed portion of a trace of the circuit board. If the component does not fit closely to the board, it may rest on the surface of photoimageable composition without adequate bonding being established between the pins and the traces. It would be desirable if low regions, or "valleys", rather than elevated regions existed between exposed trace portions. Some common traces are narrower at the foot than at the top, in which case small overhangs extend outward along each side of each trace (an effect known as "mushrooming"). With both liquid compositions and conventional dry films, there inherently remains air-filled voids below such overhangs. The air trapped in such voids may produce some oxidation of the traces and may lead to eventual defects in the solder mask. The present invention is directed to the use of an improved dry film for forming a solder mask which eliminates some of the problems of both liquid application and conventional dry films. The present invention is directed to a method of using the dry film to apply a photoimageable layer, which forms a solder mask, to an uneven surface. Dry film carrying a primary imaging resist may be applied by the method of the present invention, providing excellent conformation for tab plating operations. The present invention consists in a method of making a composite comprising a non-conductive substrate having circuitry traces thereon an a layer of a photoimageable composition overlying said traces, comprising the steps of: a) providing a non-conducting substrate (10) having a flat surface with circuitry traces (16) extending upwards from said flat surface; b) laying over said surface a dry film (21) comprising a cover sheet (18) and a layer (20) of a photoimageable composition having an average thickness equal to or less than the height of said circuitry traces above said surface, said composition being soluble in a developer and capable of being rendered insoluble in the developer when exposed to actinic radiation, and being curable after sequential steps of exposure to patterned actinic radiation and development in the developer to provide a hard, permanent solder mask overlying said non-conducting substrate; c) tacking said photoimageable composition layer to said surface and d) laminating the photoimageable composition layer to the surface of said printed circuit board, using heat and vacuum and/or mechanical pressure to conform the layer to the surface and encapsulate the circuitry traces to a thickness of at least 18 µm, characterised in that the dry film (21) comprises, between the cover sheet (18) and the photoimageable composition layer (20), a top coat (24) which is selectively adherent to the photoimageable composition relative to its adherence to the cover sheet, said top coat having a thickness of 1 to 5 µm, being soluble in a developer to the photoimageable composition layer and forming a continuous oxygen barrier layer, and in that the cover sheet is removed from the top coat after the tacking step (c) and before the lamination step (d), leaving the top coat covering and protecting the photoimageable layer as it is laminated. To form a solder mask in accordance with one embodiment of the present invention, the protective, removable sheet of the dry film is peeled away, and the exposed surface of the photoimageable composition layer is tacked to a minor portion of the surface of the printed circuit board. The cover sheet is removed, leaving the top coat as a protective covering on the layer of photoimageable composition. At this time, the layer of photoimageable composition is firmly laminated to the surface of the board with the use of heat, vacuum, and mechanical pressure, conforming the photoimageable composition layer to the contours of the board surface and substantially encapsulating the traces. The photoimageable composition layer is then exposed to patterned actinic radiation through the top coat. A developer is used to remove non-exposed portions of the photoimageable composition layer, leaving the remaining portion of the layer laminated to the circuit board. Subsequently, the portions of the photoimageable composition layer remaining on the circuit board are cured, e.g., with heat and/or UV light. In accordance with a modified aspect of the present invention, after the photoimageable composition layer is tacked to a minor portion of the surface of the board, e.g., along the leading and trailing edge, the dry film is laminated with heat, vacuum and mechanical pressure to the board. The cover sheet is then removed. At this time, the remaining layers of the dry film are again vacuum laminated with heat, vacuum and mechanical pressure to conform the remaining layers of the dry film to the contours of the board surface. Usually, the second lamination is carried out for a shorter period of time. In accordance with another variation of the present invention, after the photoimageable composition layer is tacked to a minor portion of the surface of the board, e.g., along the leading and trailing edge, the dry film is laminated with heat, vacuum and mechanical pressure to the board. The cover sheet is then removed. At this time, the remaining layers of the dry film are conformed to the contours of the board by mechanical pressure supplied by heated rollers which pass across the surface of the board. Preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein: FIGURE 1 is a cross-sectional view of a conventional dry film used for forming a solder mask, shown being applied to a printed circuit board. FIGURE 2A is a cross-sectional view of a dry film in accordance with the invention used for forming a solder mask. FIGURE 2B shows removal of the cover sheet from a dry film tacked to a printed circuit board. FIGURE 3A is a cross-sectional view of a trace on a printed circuit board covered by a solder mask formed from a prior art, liquid photoimageable composition. FIGURE 3B is a cross-sectional view of an alternative trace on a printed circuit board covered by a solder mask formed from a prior art, liquid photoimageable composition. FIGURE 4A is a cross-sectional view of a trace on a printed circuit board covered by a solder mask formed from a prior art, conventional dry film. FIGURE 4B is a cross-sectional view of an alternative trace on a printed circuit board over which a solder mask formed from a prior art, conventional dry film is applied. FIGURE 5A is a cross-sectional view of a trace on a printed circuit board encapsulated by a solder mask formed from a dry film in accordance with the present invention. FIGURE 5B is a cross-sectional view of an alternative trace on a printed circuit board encapsulated by a solder mask formed from a dry film in accordance with the present invention. FIGURES 6-9 are each photographs of a perpendicular slice of a printed circuit board having a solder mask prepared using a dry film in accordance with the present invention, the photographs being taken at an angle of 60° relative to the plane of the board so as to show both the cross-section of the board and the outer surface of the photomask. Figure 6 is a 300x magnification of a 50µm trace covered with a 50µm solder mask. Figure 7 is a 750x magnification of a 75µm trace covered with a 50µm solder mask. Figure 8 is a 300x magnification of a 50µm trace having lateral overhangs and being covered with a 50µm solder mask. Figure 9 is a 100x magnification of 50µm traces covered with a 50µm solder mask. FIGURE 10 is a 300x magnification of a primary photoresist applied by the method of the present invention over a circuit board having traces. FIGURE 11 is a diagrammatic illustration of the use of roller apparatus to tack down a dry film to a board. FIGURE 12 is a diagrammatic illustration of the use of a roller to conform layers of dry film, from which the cover sheet has been removed, to the surface of a board. FIGURE 13A, 13B and 13C illustrate diagramatically a vacuum applicator and a platen operation sequence thereof. Illustrated in FIGURE 1 is an illustration of a circuit board 10 being covered with a prior art dry film 12. The circuit board comprises a board 14 of non-conductive material, e.g., formed of epoxy, and the conductive traces 16 which comprise the printed circuit. The dry film comprises a cover sheet 18 of flexible, but generally non-conforming material, typically a polyester, such as polyethylene terephthalate, a layer 20 of photoimageable material which eventually is processed to become the solder mask and a protective sheet 22 formed, for example, of polyethylene. Typically the dry film is supplied from a reel 2 (Figure 11) to the circuit board 10. Just prior to application to the surface of the circuit board, the protective sheet 22 is peeled away, as shown, e.g., by rolling onto a take-up reel 4 (Figure 11). As the photoimageable layer is applied to the surface of the circuit board 10, it is laminated thereto with heat, mechanical pressure and vacuum. Although the cover sheet 18 is flexible, it is non-conforming and therefore retains its flat configuration when the dry film 12 is applied to the circuit board 10. The photoimageable composition layer 20 during lamination conforms at its contact surface to the circuit board from which the traces 16 protrude; however, the surface of the photoimageable composition layer which is in contact with the cover sheet 18 remains flat. In order that the photoimageable composition layer 20 may fully encapsulate the traces 16, it must be thicker than the elevation of the traces above the board 14 of non-conductive material. Standard requirements are that the thickness of the solder mask over all portions of the traces be at least 18µm. Because the great majority of traces are elevated 50µm or more above the surface of the board 14, the photoimageable composition layer must usually be 68µm or thicker. Conventional dry films 12 for forming solder masks typically have photoimageable layers 100µm thick. Illustrated in FIGURE 2A is a dry film 21 in accordance with the present invention. This dry film 21 also includes a cover sheet 18, a layer 20 of photoimageable composition which is processable to form a hard, permanent solder mask and an optional, but preferred, protective sheet 22. In addition, the dry film 21 includes a top coat 24 intermediate the cover sheet 18 and the photoimageable composition layer 20. The material for the top coat 24 is selected to have selective adherence to the photoimageable composition layer 20 relative to its adherence to the cover sheet 18, whereby the cover sheet may be peeled away from the remaining layers 20, 24 of the circuit board-adhered dry film, leaving the top coat 24 protecting the photoimageable composition layer 20. The top coat 24 is formed of material which also is selected for oxygen-impermeability, whereby it protects the underlying layer of photoimageable composition 20 from oxygen inhibition during free radical photopolymerization in the exposure step. The top coat is also selected to be soluble in the developer for the photoimageable composition, whereby the top coat is removed during the development process. The top coat material is further selected for transparency, strength and flexibility. Dry film for applying photoresist layers having a similar configuration are described in US-A- 4,530,896, FR-A- 2081665 and EP-A- 0267806 US-A- 4,530,896 is limited to application of very thin resist coatings. Primary photoresists are typically applied to a metal-covered substrate in order to define the pattern of circuitry traces on a printed circuit board. In each of these references, a dry film is described having, in order, a cover sheet, an intermediate layer which acts as a top coat after removal of the cover sheet, a photoresist composition layer and an (optional) protective sheet. Although the order of layers are similar to those of the dry film of the present invention, there are several reasons why it would not be obvious to one with ordinary skill in the art that there would be advantages to a solder mask-forming dry film of similar laminate arrangement. It is a major advantage in a dry film for applying a primary imaging photoresist layer to a substrate that the relatively thick cover sheet be removed prior to imaging, thereby substantially improving the resolution of the image which may be achieved. Although similar advantages of resolution are obtained, in fact, with a solder mask-forming dry film, resolution of image is of substantially less concern with solder masks than with primary photoresists. The main advantages of a solder mask dry film having this laminate arrangement are achieved through the application of the solder mask by the novel process of the present invention. The novel process of the present invention closely conforms the photoimageable composition layer 20, which eventually forms the solder mask, to the contours of a circuit board surface which are irregular due to the elevated traces. In contrast, a primary imaging photoresist layer is usually applied to a substantially flat surface. By conforming the photoimageable composition layer 20 closely to the circuit board surface, excellent encapsulation of the traces 16 is achieved. Also, a much thinner photoimageable composition layer 20 may be used than has been used in prior art, solder mask-forming dry films. As in the case with the conventional dry film 12, the protective sheet 22 is initially peeled from the photoimageable composition layer 20, and the exposed surface of the photoimageable composition layer is applied over a surface of the printed circuit board as seen in Figure 2A. Unlike the case with the conventional dry film 12, the dry film 21 in accordance with one aspect of the present invention may not be fully laminated to the board at the time of application. Rather, slight pressure is used to tack the dry film 21 to the board. Either cold-tacking or hot-tacking may be used. By cold-tacking is generally meant at room temperature, e.g., 20-25°C. Hot-tacking may be at temperatures up to about 70°C. In accordance with one aspect of the invention, the dry film 21 is not tacked to the entire surface, but only to a portion sufficient to provide adequate adherence to the board so as to allow removal of the cover sheet 18 without removal of the remaining layers 20, 24 of the dry film 21 from the circuit board 10. By tacking only a small portion, typically 5% or less of the entire surface area of the dry film which lies over the board, passageways are left between the dry film 21 and the board 10 for subsequent removal of air by application of vacuum. If the dry film is applied by machine, edges, e.g., the leading and trailing edge, may be tacked. If manually applied, tacking at the corners is sufficient. Mechanical pressure, such as that which may easily be provided by a human finger, is sufficient for tacking. As an example of how tacking may be performed mechanically, reference is again made to Figure 11. A circuit board 10 is carried from left to right by drive rollers 23, and the dry films 21 are caused to travel in synchronous motion along the surface thereof. Non-driven rollers 25 are reciprocal upward and downward in the direction of double arrows 6 between vertical positions in contact with the dry film and vertical positions remote from the surface. When the leading edge of the board is aligned with the rollers 25, the rollers 25 are brought into contact with the dry film, pressing the dry film onto and tacking the dry film to the leading edge of the board. The rollers 25 are immediately reciprocated away from the board to allow the middle regions of the board to pass thereby untouched by the rollers. The rollers 25 are again reciprocated into contact with the board to tack the dry films 21 to the trailing edge of the board 10. At this time, the cover sheets 18 are removed. Because only a minor portion of each dry film 21 area is tacked to the circuit board 10, Figure 2B shows a preferred mode of removal in which the cover sheet 18 is peeled back in a plane parallel to the plane of the circuit board 10 (as opposed to lifting in a generally perpendicular direction which would tend to remove the photoimageable layer 20 from the circuit board 10). With the cover sheet 18 removed, the photoimageable layer 20 with the top coat 24 thereon is now vacuum-laminated at elevated temperature to the board. A vacuum of at least 0.3 millibar is used and preferably a vacuum of at least 1 millibar. Laminating temperatures range from 40°C to 100°C, a range of 55°C to 80°C being preferred. Because the cover sheet 18 is removed, the two remaining layer (photoimageable layer 20 and top coat 24) conform easily to the contours of the irregular surface of the circuit board 10. In conventional vacuum laminating, in addition to heat and vacuum, mechanical pressure is brought to bear against the dry film. In what is known as a "slap-down" (Figure 13) process, a blanket and/or platen is used to press the dry film against the board. Commercial examples of laminators using mechanical pressure in addition to heat and vacuum are models 724 and 730 sold by Dynachem Division of Morton International, Inc. Such vacuum laminating apparatus 100 is briefly described with respect to Figures 13A, 13B and 13C. The apparatus includes an upper stationary platen 102 and a lower movable platen 104. Associated with the upper platen 102 is an upper resilient blanket 106; associated with the lower platen 104 is a lower resilient blanket 108. The lower platen 104 has a well 109 for carrying a dry film-covered circuit board 10. Sealing means in the form of an 0-ring 112 are provided on the lower platen 104 for hermetically sealing the well 109 when the lower platen is brought into contact with the upper platen 102 and its associated blanket 106. Also shown in Figures 13A-13C is a shim 108. The shim is also formed of resilient material and is used to accommodate boards of different thickness. Several shims may be used, including shims interposed between the upper platen 102 and upper blanket 106. Typically the lower platen 104 is reciprocal both horizontally and vertically. With the lower platen 104 out of horizontal alignment with the upper platen 102, the dry film-covered circuit board 10 is placed in the well 109 on top of the lower blanket 108. Then the lower platen 104 is moved vertically into alignment with the upper platen 102, the condition shown in Figure 13A. Next, the lower platen is moved upward until contact is effected between the 0-ring 112 and the upper platen 102 and its associated blanket 106 (Figure 13B). Not shown are means to apply a vacuum in the region of the well 109 which contains the circuit board 10. Also, not shown are means to provide a vacuum between the upper platen 102 and the upper blanket 106. After the well region 109 has been sealed, vacuum is applied both to the well region 109 and between the upper platen 102 and upper blanket 106. During this time, the circuit board 10 merely rests on the lower blanket 108. The vacuum in the well region 109 serves to remove air from between the dry film layers 20, 24 and the surfaces of the circuit board and to draw the layers of the dry film to the surfaces of the circuit board. For a short period at the end of the cycle, the vacuum is released from between the upper platen 102 and the upper blanket 106. The vacuum in the well region 109 causes the upper resilient blanket 106 to "slap down" against the board 10, pressing the board and dry film layers between the upper and lower blankets 106, 108 as seen in Figure 13C. Finally, the vacuum is released from the well region 109 and the lower platen 104 is removed vertically and then horizontally from the upper platen 102 to allow removal of the laminated board. During slap-down, mechanical pressure is conveyed to the dry film by the blankets formed of resilient material. The resilient material of the blankets is a type of rubber, e.g., silicon rubber, or other type of resilient polymeric material. The surface of each blanket which contacts the dry film is conventionally texturized to prevent a vacuum bond from forming between the blanket and the cover sheet. In prior art processes, such a bond would cause the cover sheet and possibly underlying layers to tear away from the board when the platen and blanket are withdrawn. Texturization of the blankets may be in the form of surface roughness of the polymeric material itself or it may be provided by a rough, rubberized fabric at the dry film contact surface. In the process of the present invention wherein soft remaining layers 20, 24 of the dry film 21 are laminated to the board after removal of the cover sheet, conventional blankets with rough contact surfaces have been found to be disadvantageous if that they leave a negative image imprinted on the remaining layers of dry film. To avoid this, new semi-matte blankets have been devised which have a very fine texture. Again, the texture prevents a vacuum bond from being formed between the blanket surface and the remaining layers of the dry film; however, the fine surface texture creates no significant imprint on the remaining, soft layers 20, 24 of the dry film. Preferred blanket material is silicon rubber; preferably reinforced with fabric, such as fiberglass. During vacuum lamination, the remaining layers 20, 24 conform very closely to the circuit board and provide several advantages which can best be appreciated with respect to FIGURES 3A, 3B, 4A, 4B, 5A, and 5B and the photographs which are FIGURES 6-9. As it turns out, the thin layer which comprises the top coat 24 remains fully intact during the conforming process, a result which would not necessarily have been predictable in advance. FIGURE 3A illustrates a solder mask which was initially applied as a liquid photoimageable composition. Being a liquid, the layer generally conforms to the uneven contours of the circuit board. On the top of each trace 16, the layer is of generally uniform thickness; however, in the regions 26 of the knees of the traces, i.e., the lateral edges, the layer greatly thins out, typically being only about 50% as thick as the portion of the layer which covers the flat upper surface of the trace. Frequently, the layer 20 in the regions 26 of the knees is even thinner relative to the portions of the layer on the flat upper surface of the trace. Illustrated in FIGURE 3B is a solder mask 20 covering a "mushroomed" trace 16' having overhangs 28 on each side. Not only is the solder mask thin in the regions 26 of the knees, but a void, air-filled space 30 is left below each overhang 28. FIGURE 4A illustrates a solder mask formed from a photoimageable composition layer 20 of a conventional dry film 12. The outer surface of the solder mask has the flat configuration of the cover sheet 18 which was removed subsequent to lamination, while the laminated surface conforms generally to the irregular contours of the circuit board 10. The solder mask which is shown in FIGURE 4A is satisfactory from the standpoint of adequately encapsulating the traces 16. It is to be noted, however, that the thickness of the photoimageable composition layer 20 and solder mask formed therefrom is, of necessity, greater than the elevation of the trace, requiring a relatively large amount of photoimageable composition be used to form the layer. This thickness represents a limitation to the resolution of photoimaging which can be achieved. FIGURE 4B illustrates a solder mask resulting from a photoimageable layer 20 of a conventional dry film and covering "mushroomed" traces 16' having lateral overhangs 28. Although thickness of the solder mask above the trace 16' is adequate, air-filled voids 30 exist below the overhangs 28. FIGURES 5A and 5B illustrate solder masks formed from photoimageable composition layers 20 of dry films 21 produced in accordance with the present invention. During the lamination, which follows cover sheet removal, the remaining layers 20, 24 of the dry film 21 very substantially conform to the irregular surface contours of the circuit board. In each case (5A and 5B), the thickness of the photoimageable layer 20 which was used to form the photomask was equal to or less than the elevation of the trace 16 or 16'; however, the surface of the circuit board is fully covered by the solder mask with the traces 16 or 16' fully encapsulated and encapsulated to an adequate thickness. It is to be noted that the photomask portions in the regions 26 of the knees of the traces are substantially as thick as photomask portions over the upper flat surfaces of the traces. When using a dry film in accordance with the present invention, the resulting solder mask has thickness in the regions 26 of the knees of the traces which is at least about 70% of the thickness of the solder mask over the upper flat surface of the traces (over more than about 90% of the linear length of the traces). It is also to be noted in FIGURE 5B where a trace 16' has overhangs 28, conformance of the layer 20 during vacuum/heat lamination is sufficient to substantially remove air-filled voids from below the overhangs 28. Even though substantial deformation of the photoimageable composition layer 20 is required to achieve this conformance, the top coat 24 maintains its integrity during lamination. Subsequent to lamination, the solder mask is formed by processing the layer of photoimageable composition in a conventional manner. Artwork is laid over the remaining layers 20, 24 of the dry film; the top coat 24 prevents the artwork from sticking. The photoimageable composition layer 20 is exposed to patterned actinic radiation through the protective top coat 24. Although resolution is generally not as critical when developing solder masks as it is in developing photoresist layers, the thinness of the top coat 24, typically in the range of about 2-3µm (as opposed to a cover sheet, typically in the range of about 25µm, through which photoimageable composition layers 20 of conventional solder mask-forming dry films 12 must be exposed) contributes to improved resolution. Further contributing to enhanced resolution is the relative thinness of the photoimageable composition layer 20 itself. As noted above, with a conventional solder mask-forming dry film 12, which is intended to be exposed through the cover sheet 18, the photoimageable composition layer typically must cover traces 16 with elevation of 50µm or more, and the photoimageable composition layers are typically about 100µm thick. Because the photoimageable composition layer 20 of the dry film of the present invention is conformable to the surface contours of the circuit board 10, a thickness of 50µm or less is sufficient to cover traces elevated 50µm and even up to about 75µm above the flat surface of the board 14. Of course, a further advantage is substantially reduced levels of expensive photoimageable composition used to form the layer 20. An additional advantage of a thin, conforming photoimageable layer is that the solder mask which forms in the regions between the traces 16 is generally at or below the level of the traces. With conventional solder masks, slight elevated regions exist between the traces. Such elevated regions may result in the component, which is being soldered, sitting high on the solder mask and preventing pins of the component from being adequately soldered to exposed trace portions. Alternatively, the elevated regions may, during reflow, result in some of the pins of a soldered component breaking contact, causing the component to stand up at one end, an effect known as "tombstoning". Inadequate soldered contact and "tombstoning" are avoided when "valleys" instead of elevated regions exist between exposed portions of the traces. After exposure to actinic radiation, the layers 20 and 24 are developed, the developer removing the top coat 24 and appropriate portions of the photoimaged layer 20. In addition, the dry film solder mask of this invention has the same ability to "tent" via holes as conventional primary imaging or solder masks films. This is a very important part of the process and cannot be duplicated with simple liquid processes. An additional cure follows development in order to harden the solder mask. Typically, heat and/or UV light are used to effect this cure. The present invention is not primarily directed to materials from which the various sheets and layers are formed; and reference is again made to US-A- 4,530,896 and EP-A- 0267806. The cover sheet 18 is generally formed of a polyester, such as PET and is generally at least about 25µm, or thicker, and up to about 75µm thick. PET, if used for the support sheet, is preferably surface-treated to provide adequate hydrophilicity or wetting angle so that it does not adhere strongly to the top coat. Preferably, the wetting or contact angle of the cover sheet material is at least about 60° and is preferably 75° or less. One suitable PET cover sheet is sold under the trade designation Melinex-393 by ICI. This is a PET that is surface-treated on one side to enhance its hydrophilicity, 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 photoimageable composition suitable for forming solder masks is suitable for practice of the invention. The protective sheet 22 is selected to be removable from the photoimageable composition layer 20, and is typically a polyethylene sheet, between 15µm and 35µm thick. The top coat 24 is formed of a material which provides the requisite selective adherence. The top coat is from 1 to 5µm thick, preferably at least 2µm. When using a PET cover sheet and a photoimageable material which is developable in aqueous or alkaline aqueous developer, a suitable top coat consists essentially of (a) between 10 weight percent and 100 weight percent of either polyvinyl alcohol hydrolyzed to at least 75% or a copolymer formed by the copolymerization of between 95 and 99 molar percent of polyvinyl acetate and between 1 and about 5 molar percent of alkyl ester of acrylic acid and being hydrolyzed to at least 75 percent; (b) up to 90 weight percent of hydroxyethyl cellulose; and (c) up to 10 weight percent of plasticizer. Preferably, hydroxyethyl cellulose is used at a level of at least 25 wt. percent and more preferably at a level of at least 50 wt. percent. The above described copolymers are sold under the trade designation VINOL-118 M by Air Products, and 4 weight percent solutions of these copolymers and have viscosities ranging from 5 to 65 MPas at 20°C. PVC homopolymers of similar viscosity are useful as well. Commercially suitable PVA's are also sold by Air Products. A plasticizer, if used, tends to lower the tack temperature of the top coat composition, and tack temperatures below 100°C have been achieved using such plasticizers. A top coat composition having a lower tack temperature is easier to adhere to the photoimageable layer. Preferred plasticizers are low molecular weight compounds, e.g., below 200, having two or more hydroxyl groups, such as glycerine, ethylene glycol and propylene glycol. While plasticizers desirably lower the tack temperature, they also increase oxygen permeability of the top coat; accordingly amounts of plasticizers in excess of 10% by weight are desirably avoided. Other suitable materials for top coat use include, but are not limited to, the group consisting of polyvinyl ether-maleic anhydride copolymers, water-soluble cellulose ethers, water-soluble salts of carboxyalkylcellulose, 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 of forming the dry film is according to known procedures. The top coat and photoimageable composition layers are successively layered on the cover sheet by methods, such as roller coating or by spraying solutions or dispersions of the materials. In a variation of the method of practice of the invention described above, a dry film may be laminated both before and after removal of the cover sheet. In the case where the photoimageable layer is about equal in thickness or of lesser thickness than the traces (or other unevenness of the board), one might expect the traces (or other unevenness of the board) to penetrate the photoimageable layer. However, it is found in such cases, that a conventional lamination of such a dry film using heat, vacuum and mechanical slap-down in a conventional manner results in tacking of the photoimageable layer to the upper surfaces of the traces (or other elevations) without penetration of the traces (or elevations) through the photoimageable layer. The tacking of the dry film over a greater area of the board facilitates its adhesion to the board during cover sheet removal. A second vacuum lamination with heat, vacuum, and mechanical slap-down after cover sheet removal acts to conform the photoimageable composition layer (and top coat) to the contours of the uneven board surface. In a typical procedure the lamination subsequent to cover sheet removal is for about half the time as the lamination prior to cover sheet removal, e.g., 30 seconds for the second lamination compared to 60 seconds for the first laminations. The additional tacking afforded by the initial lamination may prove to be particularly advantageous when using automated equipment for cover sheet removal. As a further variation to the method of the invention, instead of using a heat/vacuum lamination process, subsequent to removal of the protective sheet 22 and the cover sheet 18, to conform the photoimageable layer 20 (and top coat 24) to the contours of the board surface, a roller applying mechanical pressure may be used to accomplish the same purpose. Illustrated diagrammatically in Figure 12 is the use of the roller used to conform a photoimageable layer 20 and top coat 24 to the contours of the board. In this diagram, the board 10 and layers 20, 24 of a dry film are moved from left to right below a non-driven roller 45 with the roller in pressure bearing contact with the top coat 24. As in the processes described above, the dry film is first tacked down. Preferably, the dry film is then vacuum laminated to the board 10. In the case where the dry film has a thin photoimageable layer 20 relative to the height of the traces, the layers 20, 24 of the dry film for the most part sit on the top surfaces of the traces 16 as seen at the left-hand side of Figure 12. After passing below the roller 45, the photoimageable composition 20 and top coat 24 conform to the contours of the board 10 with traces 16 as seen on the right-hand side of Figure 12. The layers 20, 24 are pressed down across the full surface of the board 10 as the board passes below the roller. Alternatively, a roller could be drawn across the surface of a stationary board. The roller 45 used for this purpose should apply between 1 bar and 6 bar pressures. The roller 45 is heated depending on the particular application to between 40 and 110°C, generally in the range of 60 to 80°C. The surface of the conforming roller 45 has some resiliency, preferably being a polymeric material having a Shore A hardness of between 60 and 80. The surface of the roller is smooth. The relative speed of the roller 45 and board 10 will vary according to the application, generally between 1 and 10 meters per second, 3 meters per second being typical. The invention will now be described in greater detail by way of specific examples. EXAMPLE 1 A dry film is prepared in accordance with,the invention having a cover sheet which is a 50µm thick sheet of PET, a 2.5µm top coat which is 10 weight percent of the hydrolyzed polyvinyl acetate/acrylic acid copolymer described above, and 90 weight percent hydroxyethyl cellulose, a photoimageable composition forming a layer 50µm thick and a polyethylene protective sheet 25µm thick. The photoimageable composition is formulated as follows: Ingredients: Weight % Chemical Name or Trade Name 36.5 Scripset 540 (styrene/Maleic Anhydride Co-Polymer 25.8 MEK 1.2 Pigment CI 7426 0.05 Sterically hindered phenol anti-oxidant 7.54 Tripropylene glycol diacrylate 3.76 Tripropylene propane triacrylate 0.33 Adhesion Promoter of the Thiazole type 2.81 N-Methylolacrylamide 2.26 Dimethoxyphenyl Acetophenone 0.56 Coating Aid 1.18 Methyl Ethyl Ketone 18.01 Fillers 100% EXAMPLE 2 A dry film is prepared in accordance with Example 1, using LAMINAR R DM to form the photoimageable composition layer. EXAMPLE 3 Panels having copper traces and plates having tin/lead traces were each cleaned in a conventional manner. The dry film of Example 2 was applied to the boards both manually and with an automated system. Manually, the polyethylene protective sheet was removed and the four corners of the film were tacked down with only finger pressure. Using tape to obtain a grip, the cover sheet was removed by drawing off the PET cover sheet in a plane parallel to the board surface. In the automated variation, a Dynachem DFSM Model 360 laminator was used. The Dynachem apparatus uses an "off contact" method which automatically removes the polyethylene protective sheet, tacks down the leading edge of the dry film and then tacks down the trailing edge of the dry film. Application was to both sides of the boards. Then the cover sheet was removed. Lamination was effected at a platen temperature of 70°C with a cycle time of 60 seconds and a vacuum of 2 millibars. Mechanical slapdown was for 6 seconds at a vacuum of 0.3 millibar. The boards were held 20-30 minutes. Artwork was laid over the boards. Using Colight DMVL-824, exposure was for 25 seconds at 100 mJ/cm 2 measured by Dynachem's integrating radiometer model 500, achieving a Stouffer step of 8 to 9. The boards were held for an additional 20-30 minutes. The boards were developed in 1.0 weight percent Na 2 CO 3 ·H 2 O which gave a 2x break point. The boards were dried using a Chemcut 547 circuit board dryer. Curing was done using a Dynachem UVEX curing unit, an IL-390 unit and a high temperature oven. Curing steps are as follows: U.V. CURING 1. UVEX-3 lamp (78.7 watt/cm, 200 watt/inch). 2. 3.5 Joules/cm 2 (IL-390) side A. 3. Cool to room temperature. 4. 3.5 Joules/cm 2 (IL-390) side B. 5. Cool to room temperature. THERMAL CURE 1. Oven bake at 150°C for 60 minutes. EXAMPLE 4 FIGURE 6 shows a solder mask formed from a 50µm layer encapsulating a 50µm trace. FIGURE 7 shows a solder mask formed from 50µm layer encapsulating a 75µm high trace. Excellent coverage at the regions of the knees is seen. FIGURE 8 shows a solder mask formed from a 50µm layer encapsulating a 50µm high trace having overhangs. Solder mask material filling the regions below the overhangs is seen. FIGURE 9 shows a solder mask formed from a 50µm layer encapsulating 50µm high trace. Conformance of the solder mask to the contours of the circuit board is seen. EXAMPLE 5 A dry film is prepared in accordance with Example 1, using Laminar R AX to form a photoimageable composition layer. The resist was laminated over raised connector tabs, as per Example 3, and exposed to an image to a solid copper 7-9 on a Stouffer R 21 Step Sensitivity guide. A covered trace is seen in Figure 10. The exposed panel was then developed in a Chemcut 547 spray developer containing 91 liters water, 3% by weight potassium carbonate and 1 to 5% Butyl Cellosolve R which bared about 3/4 of each connector pad. Developer temperature was held between 27-33°C. After thorough rinsing, the board given a 7.6µm (0.30 mil) nickel strike followed by 30 min. gold plating cycle in a Technic RC-80 bath. The resist was then stripped in a 3-5% NaOH at 54-60°C. The gold plated tabs were then inspected for wicking in the areas around which the resist had conformed. No traces of Au or Ni were found anywhere above the gold plated areas. A control, not vacuum laminated in this fashion, showed wicking on about 25% of the tabs. The excellent conformability achieved by the laminated dry film in accordance with the present invention might also be achieved with a dry film comprising a layer of photoimageable material backed by a cover sheet of deformable material, e.g., heat-conforming vacuum film. Such a dry film would be applied in the manner of conventional film, heat/vacuum laminating being effected either during or subsequent to application to the circuit board. The cover sheet, in this case, would provide the desired conformance; handling problems of the more flexible sheet might be encountered. In a variation of the method of the present invention, after the cover sheet is removed and prior to vacuum/heat lamination, a flexible sheet, e.g., Tedlar or Teflon®, is laid over the top coat. During vacuum lamination, the flexible sheet conforms to the top coat, but can be and is removed prior to or after exposure to allow development. A reason for applying such a flexible sheet is to lengthen the permissible period of time between lamination and exposure of the photoimageable layer. Flexible sheet material may be a polymeric material or a thin sheet of paper.

Claims: A method of making a composite comprising a non-conductive substrate having circuitry traces thereon an a layer of a photoimageable composition overlying said traces, comprising the steps of: a) providing a non-conducting substrate (10) having a flat surface with circuitry traces (16) extending upwards from said flat surface; b) laying over said surface a dry film (21) comprising a cover sheet (18) and a layer (20) of a photoimageable composition having an average thickness equal to or less than the height of said circuitry traces above said surface, said composition being soluble in a developer and capable of being rendered insoluble in the developer when exposed to actinic radiation, and being curable after sequential steps of exposure to patterned actinic radiation and development in the developer to provide a hard, permanent solder mask overlying said non-conducting substrate; c) tacking said photoimageable composition layer to said surface and d) laminating the photoimageable composition layer to the surface of said printed circuit board, using heat and vacuum and/or mechanical pressure to conform the layer to the surface and encapsulate the circuitry traces to a thickness of at least 18 µm, characterised in that the dry film (21) comprises, between the cover sheet (18) and the photoimageable composition layer (20), a top coat (24) which is selectively adherent to the photoimageable composition relative to its adherence to the cover sheet, said top coat having a thickness of 1 to 5 µm, being soluble in a developer to the photoimageable composition layer and forming a continuous oxygen barrier layer, and in that the cover sheet is removed from the top coat after the tacking step (c) and before the lamination step (d), leaving the top coat covering and protecting the photoimageable layer as it is laminated. A method according to claim 1 wherein said laminating step (D) is carried out by causing a heated roller to pass across the photoimageable layer, applying pressure thereto. A method according to claim 1 or claim 2 wherein the photoimageable layer is first laminated to the surface of the printed circuit board, using heat, vacuum and mechanical pressure, prior to removal of the cover sheet from the top coat. A method according to any preceding claim wherein the following additional steps are carried out to produce a photoimaged layer on the surface of the printed circuit board: (E) exposing said photoimageable composition layer to patterned actinic radiation; and (F) developing said photoimageable composition to remove unexposed portions from said printed circuit board, leaving either exposed portions remaining on said substrate surface. A method according to claim 4 wherein said remaining portions of the photoimageable composition layer are cured to form a hard, permanent solder mask on the surface of the printed circuit board. A method according to claim 4 or claim 5 wherein said photoimageable layer (20) is developed in an alkaline aqueous solution and said oxygen-barrier layer (24) is soluble in the alkaline aqueous solution. A method according to any preceding claim wherein said circuitry traces (16') have flat upper surfaces and lateral knees (28), the thickness of said photoimageable composition layer in the region of said lateral knees being at least 70% of the thickness of said photoimageable composition layer over the top surfaces of said circuitry traces over about 90% of the linear extent of said circuitry traces. A method according to any preceding claim wherein said photoimageable composition layer (20) and said continuous oxygen-barrier layer are vacuum-conformed to said non-conducting substrate (14) and said upwardly extending circuitry traces (16).

Inventors: AXON FREDERICK JOHN, BRIGUGLIO JAMES JOHN, CANDORE AMEDEO, CROOKS CLANCY PATRICK, LIGHTFOOT LAWSON, ROOS LEO, AXON, FREDERICK JOHN, BRIGUGLIO, JAMES JOHN, CANDORE, AMEDEO, CROOKS, CLANCY PATRICK, LIGHTFOOT, LAWSON, ROOS, LEO
Applicants: MORTON INT INC, MORTON INTERNATIONAL, INC.
Application year: 1989
Main classification: G03F7
IPC codes: G03F7, H05K3, B32B37, B30B15