GB2328322A - Multilayer PCB manufacture with dielectric material matched to photo-resist material - Google Patents

Abstract

A method for manufacture of multilayer printed circuit boards comprises the steps of: (a) applying a layer of a photoimageable composition onto a copper layer; (b) exposing and developing the photoimageable composition layer to provide a relief image (20) of the composition layer and bared areas of the copper layer; (c) removing bared copper layer areas produced in step (b) to define a copper circuit pattern (26) that is coated with the photoimageable composition relief image (20); and (d) laminating the circuit pattern with coated photoimageable composition with a prepreg dielectric layer (28) to produce a multilayer board. Enhanced adhesion is found between the bonded layers where components of the photoimageable composition are matched to the prepreg material. Thus, for example, enhanced bond strengths are achieved where a photoimageable composition that contains an epoxy resin is bonded to an epoxy based-prepreg material.

Description

2328322 1 IMPROVEMENTS IN THE MANUFACTURE OF MULTILAYER PRODUCTS The

present invention relates to a method of manufacturing multilayer products, in particular printed circuit boards.

Multilayer printed circuit boards are used for a variety of applications and provide notable advantages of conservation of weight and space. A multilayer board is comprised of two or more circuit layers, each circuit layer separated from another by one or more layers of dielectric material, which is generally a resin/prepreg composite. Circuit layers are formed from copper clad is laminate. Printed circuits are then formed on the copper layers by techniques well known in the art, for example print and etch to define and produce the circuit traces.

The stack of circuit/dielectric layers is heated under pressure until the prepreg is cured. After such lamination, the multiple circuit layers are typically connected by drilling through-holes through the board surface. Resin smear from through-hole drilling is removed, for example by treatment with concentrated sulfuric acid or hot alkaline permanganate, and then the through-holes are further processed and plated to provide a conductive interconnecting surface.

Before lamination the individual copper circuit layers are often treated with an oxidizing agent to provide an enhanced circuit layer-dielectric layer bond upon subsequent lamination. The textured copper oxide that results from such oxidative treatment promotes adhesion between the copper and resin/prepreg layers during the 2 lamination cycle and can reduce mi cro-de lamination and fissuring, see, for example, U.S. Patents 2,364,993 and 4,512,818. Adequate bond strength between the circuit and dielectric layers can be essential as weak or 5 fractured bonds between the layers can cause delamination during drilling and render the multilayer board unusable for intended applications.

The use of oxide processes is well established in the printed circuit board industry even though chemical attack of the oxide can occur by process solutions leading to a board defect known as "pink- ring", where leaching of the copper oxide coating occurs laterally along through hole walls during the through hole processing and plating steps of board manufacture. Such leaching exposes pinkish copper metal thus giving rise to the term pink ring.

Additional process steps to treat a copper oxide layer have been employed to address the pink ring problem, see U.S. Patent 5, 106,454 to Allardyce et al. While such treatments can reduce the occurrence of pink ring, these approaches als o add expense and time to production of a multilayer board.

In consequence, there is a need for an improved method of adhering circuit-dielectric layers of a multilayer printed circuit board, particularly at high bond strengths. Especially desirable is a new method for bonding circuit-dielectric layers of a multilayer printed circuit board that requires fewer steps than current processes.

According to one aspect of the invention there is 35 provided a method for the manufacture of a multilayer 3 printed circuit board, comprising:

(a) applying a layer of a photaimageable composition onto a copper layer; (b) exposing and developing the photoimageable composition layer to provide a relief image of the composition layer and bared areas of the copper layer; (c) removing bared copper layer areas produced in step (b) to provide a copper circuit pattern that is coated with the photoimageable composition relief image; and (d) laminating the circuit pattern with coated photoimageable composition to a dielectric layer that coats the photoimageable composition, the dielectric layer comprising a component that has the same functional group as a component of the photoimageable composition. In a preferred method, the dielectric layer and photcimageable composition each comprise an epoxy material.

Methods in accordance with the invention are found to produce strong copper circuit layer/dielectric layer bonds while obviating the need for prior process steps of stripping of an etching resist, forming copper oxide layers and subsequent treatment of such oxide layers. As will be evident, elimination of those process steps affords substantial savings of manufacturing time and costs.

According to another aspect of the invention, a copper layer, e.g. a copper clad foil or copper clad substrate such as a copper clad epoxy-glass dielectric substrate is cleaned using chemical or mechanical processes, and a coating layer of a photoresist or other photoimageable composition applied onto the copper layer.

4 The applied photoimageable coating layer is exposed to patterned activating radiation that defines a circuit pattern. The latent image defined in the photoinageable coating layer is developed to provide a relief image of the photoimageable layer and bared areas of the copper layer. The bared copper areas are then removed, e.g. by treatment with a suitable etching solution such as a cupric chloride solution or the like. The photoimageable composition relief image then coats a copper circuit pattern.

The resist-coated circuit layer is then directly bonded to an epoxyprepreg or other material that forms a subsequent layer of the multilayer printed circuit board.

Thus, unlike prior printed circuit board fabrication methods, the coated resist layer is not stripped prior to bonding and an exposed copper layer is not treated with an oxidant to provide a copper oxide layer.

Moreover, it has been found that surprisingly good bond strengths are provided by the methods of the invention, i.e. that high peel strengths are realised between laminated layers of 1) the copper circuit pattern with coated resist and 2) the resist and resin/prepreg. Thus, for example, peel strengths provided by the methods of the invention approximate those achieved with current processes that require stripping of the etching resist and formation of a copper oxide layer.

Further, it has been unexpectedly found that enhanced adhesion between the bonded layers results from the components of the photoimageable composition being matched to the prepreg material, i.e. where the dielectric prepreg material that coats the photoresist layer comprises a component that has the same functional group as a component of the underlying photoresist. Thus, for example, enhanced bond strengths are achieved where a photoinageable composition that contains an epoxy resin is bonded to an epoxy based-prepreg material.

The photoimageable composition used in the methods of the invention is pref Parably negative-acting, although use of positive-acting resists is also possible. Additionally, the photoimageable composition may be applied as a liquid coating composition or as a dry film. Liquid resist applications are generally is preferred because the dried coating layer thickness is significantly less than required for dry film thereby offering improved resolution and minimizing material usage and waste.

other aspects of the invention are discussed infra.

The single figure of the drawing illustrates the sequential steps in performing a preferred exemplary method in accordance with the invention.

Referring now to the drawing, Step 1 of the preferred method consists of the provision of a substrate 10 which may take the form of a copper foil or a copper clad substrate, e.g. a double-sided printed circuit board substrate with outside copper layers 12a and 12b that encase a resinprepreg layer 14.

In Step 2 a photoimageable composition coating layer 16 is applied onto copper layer 12a.

The copper layer 12a does not require significant pre-cleaning prior to 6 application of a resist layer, distinct from prior processes. For example, the system reported in WO 96/19097 requires use of a composition that contains hydrogen peroxide, an inorganic acid and a so-called corrosioninhibitor of a triazole, tetrazole or imidazole. Nevertheless, the layer 12a may be optionally cleaned using a pumice scrub or standard cleaner solution.

The resist composition may.be applied by any suitable application method such as, but not limited to, curtain coating, double-sided roller coating, electrostatic spraying and dipping. The photcimageable composition may be any of a variety of known negative- acting or is positive-acting photoresists, although negative-acting photoresists are generally preferred for providing enhanced chemical resistance and thermal stability during subsequent processing of the substrate.

Suitable photoresists include, among others, acid hardening photoresists that contain a resin binder, a cross-linking agent and a photoactive component. Preferred resin binders of such photoresists include novolak resins, homo and copolymers of poly(vinylphenol) resins and homo and copolymers of N-hydroxyphenylmaleimides. Preferred cross-linking agents include amine-based materials such a melamine monomer, oligomer or polymer and various resins such as melamineformaldehyde, benzoguanamineformaldehyde, urea- formaldehyde and glycoluril-formaldehyde resins, and combinations thereof. Specifically suitable aminecrosslinkers include melamines manufactured by American Cyanamide such as Cymel 300, 301, 303, 350, 370, 380, 116 and 1130. A variety of photoactive components can be employed such as an onium salt or a halogenated 7 compound such as tris(2,3-dibromopropyl)isocyanurate.

Suitable negative-acting photoresists and components thereof are disclosed in EP0164248A and 0232972A and U.S. Patent 5,128,232 to Thackeray et al. Suitable negative-acting photoresists are commercially available including those negative resists sold by the Shipley Company.

As discussed above, a preferred resist composition is used which contains one or more components that contain the same or similar functional groups as present in a pre-preg layer to be applied subsequently as described below.

Particularly preferred is a resist that contains an epoxy component used in combination with an epoxy-based prepreg, i.e. such as a prepreg that includes an epoxy resin typically present with fiberglass cloth. Epoxy prepreg material is known in the art and commercially 20 available such as from Nelco. Generally preferred epoxy-containing photoresists for use in the invention include a component that is an epoxy oligomer or polymer. Particularly preferred photoresists are negative-acting and contain a photoactive component, 25 particularly a photoacid generator; a crosslinker such as the amine materials discussed above, preferably a melamine crosslinker; an epoxy component which may be an epoxide oligomer or resin, preferably an epoxidized polybutadiene resin; and optionally a further resin that 30 does not include epoxy groups such as a novolak, poly(vinylphenol) or other phenolic resin. The concentration of the epoxy component in a photoresist for use in the present invention may vary within relatively broad ranges, and in general an epoxy component will be present in a concentration of at least 8 about 3-5 weight percent of the total solids of a resist formulation, more typically from about 5 to 60 of the total solids of a resist formulation. As used herein, the term "total solids" refers to all components of a composition other than solvent carrier.

Specifically preferred epoxy-containing photoresists for use in the present invention are disclosed in U.S. Patent 5,366,846 to Knudsen et al. As disclosed in that patent, the preferred-resists contain a resin binder such as a cresol- formaldehyde novolak or other phenolic resin, a polybutadiene resin that has one or more internal epoxide groups such as Poly bd 605 Resin commercially available from Atochem North America Inc. (Philadelphia, PA), a photoacid or photobase generator and preferably a crosslinker such as a melamine resin, including the Cymel resins identified above. See, in particular, the examples of U.S. Patent 5,366,846.

Similarly, if the prepreg material is fiberglass cloth and a polyimide resin, a photoresist is preferably used that contains a polyimide component, e.g. a polyimide oligomer or resin. The polyimide component will suitably be present in such a resist formulation in the same concentrations as discussed above for an epoxide component used in a photoresist with an epoxy-based prepreg. Polyimide-based prepreg are also known in the art and commercially available.

A variety of positive-acting resists also can be employed and generally include a resin binder and a photoactive component of the type mentioned above, but without the inclusion of a crosslinking agent.

Positive-acting resists are disclosed e.g. in U.S.

Patents 5,266,440 and 5,238,776 to Zampini et al. and 9 U.S. Patent 5,302,490 to Fedynyshyn et al. As mentioned above however, negative-acting resists are generally more preferred.

In the case of a liquid resist composition, the resist components are dissolved in one or more solvents for application to a substrate. Suitable photoresist solvents and solids contents of photoresist compositions are known in the art and disclosed in the above mentioned patents and published applications. After application of a liquid coating composition, the resist layer is dried by heating to remove the solvent until preferably the composition coating layer is tack free.

Prior to application of the photoinageable composition in Step 2, the copper layer may be pre-cleaned if desired, preferably to provide a visually clean copper surface, e.g. by chemical treatments such as treating with a sodium persulfate solution, ferric chloride solution or cupric chloride solution, and/or by mechanical treatments such as pumice scrub or brushing. Such treatments are generally known in the art. For example, for a pumice treatment, a copper clad laminate can be sprinkled liberally with the pumice material Scrub Cleaner 28 available from the Shipley Company. The copper layer then is preferably scrubbed with a wet brush until the copper surface is visually clean and uniform.

During Step 3, the photoimageable composition coating layer 16 is exposed to patterned activating radiation 18 to form a latent image in coating layer 16 comprising exposed areas 20 and unexposed areas 22. Exposure is carried out in accordance with procedures recognised in the art, and the exposure should be sufficient to activate effectively the photoactive component of the photoimageable system to produce a patterned image in the resist layer. More specifically, the exposure energy typically ranges from about 1 mJ/cm2 to 1 J/cm2, dependent upon the exposure tool and the components of the resist composition.

Following exposure, the resist coating is preferably baked e.g. at temperatures ranging from about 700C to about 1600C.

During Step 4, after any such post-exposure bake, the imaged resist layer is developed, typically with an aqueous based developer such as an inorganic alkali exemplified by sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, sodium silicate, sodium metasilicate; quaternary ammonium hydroxide solutions such as a tetra-alkyl ammonium hydroxide solution; various amine solutions such as ethyl amine, n-propyl amine, diethyl anine, di-n-propyl amine, triethyl amine, or methyldiethyl amine; alcohol amines such as diethanol amine or triethanol amine; cyclic amines such as pyrrole, pyridine, etc. In general, development is in accordance with procedures recognised in the art.

Following Step 4, development provides copper layer 12a with areas 24 that are bared of the photoimageable composition as well as copper layer areas 26 that are coated with relief image 20 of composition 16. Those coated copper layer areas 26 define the circuit pattern to be etched into the copper layer.

If desired, in the case of a negative resist, the developed resist coating may be further exposed to 11 activating radiation and/or baked to further enhance the integrity of the crosslinked resist layer. A blanket exposure suitably may be employed, e. g. at exposure energies of f ron about 1 mJ/c=2 to 1 J/cm2. Suitable post- development bake conditions can vary widely. Suitable bake conditions include treatment at about 140 to 160C for about 30 to 75 minutes, see Example 1 below for preferred conditions. Such post-development exposure and thermal treatment can further complete crosslinking or hardening of the negative tone relief image, providing additional thermal stability and resistance to etchants.

It also should be appreciated that such a further exposure and bake steps after development are optional. In particular, the lamination process during board manufacture can provide an effective thermal treatment for further curing of the resist layer thereby obviating any need for a separate post-development bake step.

During Step 5, after development and any such optional post-development exposure and bake step, the copper layer is treated with a suitable etchant, e.g. an etchant solution such as a cupric chloride etchant solution, ammoniacal solution, etc. That treatment removes bared copper layer areas 24 to provide a circuit pattern of areas 26 with coated resist layer 20 as shown in Figure 1.

The etched substrate that contains the circuit pattern coated with resist layer is laminated to produce a desired multilayer circuit board. For example, the substrate with resist-coated circuit pattern can be sandwiched between outer board layers with prepreg material (e.g. fiberglass cloth and an epoxy resin, 12 partially cured polyimide or other known materials) and laminated in a heated press to provide a multilayer layer board that comprises circuit layer 26 coated with resist 20 bonded to resin/prepreg dielectric layer 28 as generally depicted in Step 6 of Figure 1, see also the lamination procedure disclosed in Example 1 below. The composition of the dielectric layer is such as to include a component selected from the same functional group as the resist, e.g. epoxy resin when the resist contains epoxy resin, -polyimide when the resist contains polyinide, and so on.

It should be appreciated that methods in accordance with the invention generally do not include steps of metallization (e.g. copper etc.) of the photoimageable layer coating a circuit pattern, or use of the coating photoinageable layer as a soldermask during application of solder, in distinction from prior permanent resists and photoimageable soldermasks.

As discussed above, the methods in accordance with the invention provide good adhesion between board layers. More specifically, it has been found that methods of the invention provide peel strengths of at least about 4.5 lbs/inch, more typically from about 4.5 to 6.0 lbs/inch, between a laminated copper layer and dielectric layer. References to peel strength values herein refer to values determined by standard protocols, specifically BS9760 IPC-TM-650 (2.4.8) which includes using a pull tester to measure the force required to pull a copper circuit layer from a dielectric layer at a rate of 2 inches per minute.

The following non-limiting example is illustrative of the invention.

13 Example 1

A 1 oz (36 gm) double treated copper foil, supplied by Gould Electronics (1ITC/TC11) was used as received, i.e.

the foil was not pre-cleaned prior to coating. The foil was supported, placed shiny side up onto a curtain coater and at a conveyor speed of 78 m/minute passed through a curtain of a negative-acting photoresist sold by the Shipley Company (Marlborough, Massachusetts) under the tradename XP.9500 which contains an epoxidized polybutadiene resin. The curtain was formed with a lip gap of 0. 3 mm and controlled at a temperature of 250C.

The viscosity of the photoresist was measured at 20 seconds through a No. 3 Zahn Cup. At a pump speed of rev/minute and a wet coating weight of 4 g/ft2 a final coating thickness of 15 gm was obtained. The coated copper foil was baked at 900C for 30 minutes and then imaged with a gailluin doped Hg lamp through a Mylar mesh and diazo artwork for 700 mJ/cm2 of energy at 365 nm measured near the surface of the resist. The coated foil was then baked again for a further 15 minutes at 900C, the resist developed in caustic, the foil dried and the coating exposed again for a period giving an additional dose of 1 J/cm2 of energy. After that post-development exposure, the coated foils were finally baked for 60 minutes at 1450C.

The exposed copper areas were etched to define the inner layer circuitry using an ammoniacal based etchant. The foils were laminated with epoxy/glass prepreg supplied by Nelco (1080 NS3205) and FR-4 resin at a temperature of 1850C for 60 minutes and a pressure of 250 p.s.i.

(1725 kPa).

Those foils laminated to the epoxy/glass prepreg layer typically exhibited peel strengths between 4.5-6.0 14 lbs/in (8-10.2 N/cm).

A similar range of peel strengths was achieved using a comparable copper foil-epoxy prepreg lamination process, but where the photoresist was stripped after etching of the copper layer, and the copper circuitry was treated with a commercially available oxidant to provide a copper oxide surface layer which layer was laminated to an epoxy prepreg layer.

The foregoing description of the present invention is merely illustrative thereof, and it is understood that variations and modifications can be made without departing from the spirit or scope of the invention as 15 set forth in the following claims.

Claims (25)

1. A method for the manufacture of a multilayer printed circuit board, comprising:

(a) applying a layer of a photoimageable composition onto a copper layer, (b) exposing and developing the photoimageable composition layer to provide a relief image of the composition layer and bared areas of the copper layer; (c) removing bared copper layer areas produced in step (b) to provide a copper circuit pattern that is coated with the photoimageable composition relief image; and (d) laminating the circuit pattern with coated photoimageable composition to a dielectric layer that coats the photoimageable composition, the dielectric layer comprising a component that has the same functional group as a component of the photoinageable composition.

2. The method of claim 1 wherein the dielectric layer and photoimageable composition each comprise an epoxy material.

3. The method of claim 2 wherein the photoimageable composition comprises an epoxidized polybutadiene resin.

4. The method of claim 1 wherein the photoimageable composition comprises a phenolic resin, an epoxidized polybutadiene resin, a crosslinker and a photoacid generator.

5. The method of claim 1 wherein the dielectric layer and photoimageable composition each comprise a 16 polyimide material.

6. The method of claim 1 wherein the circuit pattern is laminated together with a resin layer comprising a partially cured polyimide or an epoxy resin.

7. The method of any preceding claim wherein the photoimageable composition is a negative-acting 10 photoresist.

8. The method of any preceding claim wherein the copper layer is not treated with an oxidant solution prior to laminating.

9. The method of any of claims 1 to 7 wherein the copper layer is not treated with a solution containing a peroxide and an azole prior to applying the photoimageable composition.

10. The method of any preceding claim wherein the copper layer is a double treated copper foil.

11. The method of any preceding claim wherein the photoimageable composition layer is baked after exposure and before development.

12. The method of any preceding claim wherein the photoimageable composition relief image is exposed to activating radiation after development in step (b).

13. The method of claim 11 wherein the photoimageable composition layer is exposed from about 1 Mj/CM2 to 1 j/CM2 Of energy after development.

17

14. The method of claim 11 wherein the photoimageable composition relief image is baked after the radiation exposure after development.

15. The method of any preceding claim wherein the photoimageable composition relief image undergoes curing during lamination in step (d).

16. The method of claim 15 wherein the photoimageable relief image is not baked after development and prior to lamination.

17. The method of claim 1 wherein the photoimageable composition relief image is baked after development.

18. The method of any preceding claim wherein the photoinageable composition is applied as a liquid composition.

19. The method of any preceding claim wherein the photoimageable composition layer has a dry layer thickness of from about 5 to 25 gm.

20. The method of any of claims 1 to 17 wherein the photoimageable composition is applied as a dry film.

21. The method of any preceding claim wherein bared copper layer areas are removed by treatment with a cupric chloride etchant.

22. The method of claim 1 wherein metal is not deposited on photoimageable composition coating the circuit pattern.

18

23. A method of treating a copper surface comprising.

(a) applying a layer of a photoimageable composition onto a copper layer; (b) exposing and developing the photoimageable composition layer to provide a relief image of the composition layer and bared areas of the copper layer; (c) removing bared copper layer areas produced in step (b) to provide a copper pattern that is coated with 10 the photcimageable composition relief image, (d) laminating the circuit pattern with coated photoimageable composition to a prepreg layer that coats the photoimageable composition, the prepreg layer comprising a component that has the same functional group as a component of the underlying photoimageable composition.

24. Methods substantially as hereinbefore described with reference to the drawings.

25. A multilayer printed circuit board produced by the method of any preceding claim.