Fabrication Process for Circuit Boards

The invention concerns circuit boards and a process for their fabrication.

Hitherto, circuit boards have been fabricated such that what is known as the inner layers, i.e. the conductive regions of the circuit board, are built from horizontal as well as vertical sandwiched insulating layers. For this purpose, various processes may be used which are described, for example, in "Handbuch der Leiterplattentechnik", Vol. 2, 1991, E.G. Leuze Verlag, Saulgau, Germany.

Such a substrate, which is normally fabricated by lamination, is coated either unilaterally and bilaterally with a metal, generally a copper foil.

Subsequently, through and/or blind holes are drilled into the metal-coated substrate and cleaned from drilling residue.

Then the holes in the substrate as well as the surface (full panel plate) or only the image pattern (pattern plate) are generally subjected to an electrodeposition of metal in, say, a copper bath. On the surface, metal is thus deposited on the metal foil, whereas in the holes it is applied directly to the substrate consisting of conductor planes and insulating material.

The substrate surface is blanket-coated with photoresist which is exposed and developed to form a pattern, while the region of the holes is "tented", meaning that the holes are covered with photoresist. Thus, the future conductors are defined in the remaining surface regions.

In the next step, the metal, i.e. , the electrodeposited layer and the foil, is removed, for example, by etching, and the photoresist is stripped, retaining the metal protected by "tenting" in the holes and that protected by photoresist in the remaining regions.

In the last step, a solder stop mask is applied to the surface of the substrate, exposed and developed. This may be followed by a test and separating steps.

In this manner circuit boards are obtained, wherein the metal layer thicknesses on the circuit board surface and in the holes are dependent on each other. As a predetermined minimum layer thickness of metal must exist in the holes, this process using thick metal layers on the surface (metal foil and electrodeposited metal) is hardly suitable for fabricating fine-line products such as those with conductor widths of less than about 75 .mu.m (about 3 mil).

It is the object of the invention to render the metallization of the holes independent of that on the surface.

This object is accomplished according to the invention in that after the holes have been formed in the substrate and prior to depositing the conductive material in the holes, a solder stop mask is applied to the surface of the substrate, exposed and developed.

The novel process permits the fabrication of circuit boards with conductors of a conductive material and with holes coated with a conductive material, wherein the layer thickness of the conductive material on one surface of the circuit board is independently selectable of the layer thickness of the conductive material in the holes.

Thus, fine-line circuit boards may be fabricated, wherein the layer thickness of the conductive material on the surface of the circuit board is about 10 to 40 .mu.m and the layer thickness of the conductive material in the holes and 5 to 30 .mu.m, and wherein the layer thickness of the conductive material on the surface of the circuit board is preferably about 15 to 18 .mu.m and the layer thickness of the conductive material in the holes is preferably about 23 to 28 .mu.m.

The inventive process and several variants are described in detail below with reference to drawings.

FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G and 1H are steps a schematic of a process variant, wherein the hole is formed prior to completion of the conductors;

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G and 2H are steps of a process variant, wherein the hole is formed after completion of the conductors in a region from which the metal foil has not been removed;

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G and 3H are steps of a process variant, wherein the hole is formed after completion of the conductors in a region from which the metal foil has been removed; and

FIGS. 4A, 4B, 4C and 4D are steps of the process variants of the invention of FIGS. 1-3, wherein the thickness of the conductive material deposited in the through hole is independent of the thickness of the conductive metal on the surface of the circuit board.

As shown in FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G and 1H, the first process variant provides for a substrate 1 with all the inner layers 2 for the circuit board to be fabricated in a known manner by, say, lamination (steps a) and b)). Substrate 1 thus consists of insulating layers (e.g., FR-4, polyimide) and conductive layers (e.g., copper) which may be differently structured.

In step b) carried out simultaneously with step a), a metal foil 3, for example, a copper foil, of small thickness, generally less than 40 .mu.m, is deposited, for example, by lamination on one or both surfaces of the substrate 1.

Then (step c)), holes 4 are formed in the substrate 1, which may be through or blind holes and which serve to connect the conductive layers. The holes may be drilled by mechanical and physical means (such as spiral drills and lasers). Suitable registration methods ensure that the holes 4 are correctly positioned relative to the inner layers 2. The holes are cleaned, for example, by chemical processes.

Then, the holes 4 are treated with a conditioning agent 5, such as gaseous sulphur trioxide, for 1 to 2 minutes at room temperature to be metallized later on (step d)). In response to the reaction between the conditioning agent 5 and the plastic surface, functional groups are formed which permit seeding with palladium nuclei (activation) and chemical metallization at a later stage. Compared with other, metallic, conditioning agents, SO.sub.3 has the advantage that it is not electrically active. Consequently, when SO.sub.3 is used, electronic effects which may lead to short-circuiting are eliminated in places where the conditioned regions are covered. In addition, the precious metal catalysts, such as PD.sup.2+, deposited in the activation step, are most effectively bound by SO.sub.3, which improves the adhesion of the chemical metallization.

In step e) , photoresist 6 is deposited on the substrate surface, for which purpose the holes 4 should be covered (tented). The photoresists employed may be conventional ones, although dry resists, which can be deposited in thin layers and which are suitable for producing fine-line products, are the obvious choice. Using, for example, a mask, photoresist 6 is exposed and developed to form a pattern. Above and around the holes 4, photoresist 6 is retained and in the remaining regions of the substrate surface it defines the future conductors. In this step, too, care must be taken by registration methods that the future conductors are correctly positioned relative to the holes 4 and that the holes are well covered.

Then, using a suitable process, such as an etch process (step f)) , metal foil 3 is removed from those regions where no photoresist 6 exists. In addition, photoresist 6 is removed. As a result, the conductor structures are transferred to the metal foil 3 and the contact areas, such as lands (annular contact areas surrounding the holes) and pads (contact areas without holes), are formed on the metal foil 3.

In step g), a solder stop mask 7 is applied to the surface of substrate 1, exposed and developed, removing the solder stop mask in particular above the holes 4 as well as around them in a region 8 comprising at least the metal foil retained there for forming a land. The remaining surface of the substrate 1 is covered by the solder stop mask 7 which may remain there also during later processing steps. Efficient registration must ensure that the holes in the solder stop mask 7 are correctly positioned.

Substrate 1, covered on the surface, with bared hole 4 and metal foil 3 along the hole edge 8 is then ready for metallization 9 which is carried out in step h). Metallization generally takes place in a chemical metal bath, such as a chemical copper bath, although there are also other metallization processes that may be used for this purpose. Prior to copper plating, for example, it may be advantageous to nickel-plate in a chemical nickel bath, as this improves the adhesion of the metallization 9 to substrate 1.

Such a process permits selecting the thickness of the metallization in the region of the holes 4 independently of that of the conductors. By a combination of additive and subtractive processes, the advantages of both may be combined. Compared with known processes, higher aspect ratios in the holes, i.e. a more uniform metallization also in deeper and smaller holes, are obtained. With copper foils, the variation of the layer thickness on the surface is noticeably smaller than with existing processes, wherein additional metal is applied to a metal foil by electrodeposition. In addition, compared with double layers consisting of a metal foil 3 and a metallization 9, less material is wasted when only the metal foil is structured.

It is also possible to form the holes 4 in substrate 1 only after the conductors on the substrate surface have been completed. This variant is described with reference to FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H. Steps a) and b) of the latter two variants--producing a substrate 1 with finished inner layers 2 and applying a metal foil 3 thereto--are identical with steps a) and b) of the first variant.

In step c) of the second variant (FIG. 2), photoresist 6 is applied to the surface of the substrate and exposed, so that, after exposure, photoresist 6 is retained in those regions in which a hole is to be formed later on.

In the following step d), the metal foil 3 is removed from the regions in which no photoresist 6 exists, and photoresist 6 is stripped off.

Then, holes 4--through or blind holes--may be formed in substrate 1 (step e)). As previously explained in connection with step c), the second variant provides for holes 4 to be formed in a region where photoresist 6 previously existed and which thus is still coated with metal foil 3.

In step f), analogously to step d) of the first variant, hole 4 is conditioned with a conditioning agent 5.

At the stage of step g) of the second variant, the conditions are the same as those in step g) of the first variant. The remaining process steps are identical.

Accurate registration must be ensured in several process steps, namely, during the fabrication of the conductors which have to be registered relative to the inner layers 2 through holes 4 and when the solder stop mask is bared relative to the position of the holes, pads, etc.

Compared with the first variant, this approach has the advantage that the holes do not have to be tented. As a result, a major error source during the fabrication of circuit boards is eliminated, as the relatively frequent occurrence of broken tents leads to failure and rejects (excessively etched holes).

For the third process variant FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H), in contrast to the second variant, hole 4 is formed in step e) in a region of the substrate 1 from which metal foil 3 has been stripped. After conditioning (step f)), application of a solder stop mask 7 and metallization 9, which all proceed analogously to those of variant 2, there is no double layer of metal foil 3 and deposited metallization 9 in the region 8 surrounding the edges of the holes but only metallization 9.

This process allows producing all contact faces, such as lands and pads, in metallization step h). It is also suitable for what is known as a "landless design", i.e. , for connecting the conductive material of the holes 4 directly to the conductors, which saves a considerable amount of space. Compared with the second variant, variant 3 has the advantage that there is no registration problem when the solder stop mask is bared for the lands. In addition, as all conductive structures on the surface of substrate 1 other than conductors are produced only in the final metallization step, variant 3 offers the possibility of eliminating the elaborate metallization step for components rejected after the critical drilling step.

All three process variants require a solder stop mask 7 which is not electroplated, which does not poison the copper bath and which is not electrolytically infiltrated. The solder stop mask 7 may be, for example, a solder-resistant photoresist, such as epoxy resin that can be photostructured.

The above-described processes permit the fabrication of circuit boards, wherein the layer thickness of the conductive material on the surface of the circuit board is independently selectable of the layer thickness of the conductive material in the holes. Thus, the layer thickness on the surface no longer has to exceed that in the holes but both are selectable according to technical expedience. This is shown in FIG. 4A, 4B, 4C, 4D where the conductive material 9 deposited in the hole, may be thinner (4B,4D) or thicker (4A,4C) then the conductive material 3 on the circuit board surface. The layer thicknesses of the conductors may range from, say, about 10 to 40 .mu.m and in the holes from about 5 to 30 .mu.m, with the form or preferably being, for example, about 15 to 18 .mu.m and the latter about 23 to 28 .mu.m. This yields superfine conductors on the surface of the circuit board and a good metallization of the holes and may be improved by a "landless" design as per variants 2 and 3, as space savings on the surface increase the wiring density still further.