WO2004060033A1

WO2004060033A1 - Multilayer product to make printed circuit boards

Info

Publication number: WO2004060033A1
Application number: PCT/US2003/040871
Authority: WO
Inventors: Melvin Pasternack; Ludwig Weimann
Original assignee: SENTREX Co
Current assignee: SENTREX Co
Priority date: 2002-12-24
Filing date: 2003-12-22
Publication date: 2004-07-15
Anticipated expiration: 2005-06-24
Also published as: AU2003303474A1

Abstract

Copper foil used in printed circuit boards is protected form contamination, both prior to laying up circuit board components and during processing, by a multilayer releasable protective polymer film comprising a silicone release coating on one side of a protective polymer film and a copolymer adhesive layer on the opposite side. The side of the polymer film and the copolymer adhesive layer which contact each other are surface-modified so that when the polymer film is removed after exposure to high pressure and temperature laminating conditions the copolymer adhesive layer fully adheres to the polymer film and not remain on the copper foil.

Description

MULTILAYER PRODUCT TO MAKE PRINTED CIRCUIT BOARDS

BACKGROUND OF THE INVENTION

The present invention relates to printed circuit boards in general and more specifically to a multilayer product useful for the protection of components employed in the manufacturing of printed circuit boards.

In its elementary form, a printed circuit board includes, as a component, a dielectric layer of an epoxy resin-impregnated woven glass fiber body which is known as a "pre-preg." On one or both sides of the pre-preg is bonded a conductive copper foil sheet. Subsequently, much of the copper, through a number of processes, including photographic, is etched away to leave conductive paths on the surface of the pre-preg layer. When so assembled, the lamination is often called a core or a board. It is not uncommon to assemble a stack of such boards. The assembly is called a press lay-up and the stack is called a book. An entire press lay-up is heated and subjected to pressure. Upon curing and cooling, the then bonded individual boards are separated from each other and subjected to further processing in well known manners.

Of utmost importance in the manufacturing process is the maintenance of cleanliness or lack of contamination of the copper foil sheets. This is true whether or not the printed circuit board has one copper layer on one side of a pre-preg or is a compound board of numerous layers. A clean environment is one way to avoid, or at least minimize, contamination problems from resin dust, fiberglass fibers, hairs, bugs, and the like, which could cause defects in the copper circuit of the finished printed circuit board.

Another way to avoid contamination is to use a protective film as disclosed in US 5, 120,590 in which the marginal edges of the protective film are removably joined to a copper foil layer. During use, the entire marginal edge area (film plus copper foil) is cut off and thrown away, creating a substantial environmental/waste problem. Alternatively, if the protective film and copper foil are only joined at an end and not along the sides, there would be a substantial problem maintaining alignment between the two, resulting in a contaminated copper foil.

US 5, 153 ,050 discloses another attempt at solving this problem where an aluminum sheet is adhered along its marginal edges to two copper foil layers. This system is for use between two epoxy resin-impregnated woven glass fiber bodies. The aluminum sheet is generally thrown away leading to increased waste disposal. This system has had substantial problems with panel alignment both before and during pressing as well as image transfer. Moreover, the aluminum sheet and the extra copper adhered to its marginal edges must be removed immediately after lamination and can not remain in place thereafter to further protect the copper during further manufacturing steps.

Other attempts at solving the same problems, while avoiding some of the drawbacks of the above systems, have failed to be commercially viable, due in substantial part to an incomplete removal of adhesive material from the copper after exposure to the extreme conditions of temperature and pressure which are required for lamination. For example, US 5,057,372 discloses various poly- olefin based adhesives coating the entire surface of the protective film layer. PCT/US95/08436 (WO96/01605) and PCT/IB96/01151 (WO97/15446) disclose the use of special electron-beam curable adhesives, particularly saturated co- polyesters with terminal acrylic double bonds. Commonly assigned US 6,280,851 discloses the use of a pressure sensitive silicone polymer dissolved in an organic solvent between the copper foil and an outer protective layer.

Despite the desirability and demonstrated commercial need for a polymer film system which will provide the necessary protection to a copper foil during processing to prepare printed circuit boards and be completely removed without contaminating the copper surface after processing, the art has failed to produce such a system. This invention meets that need while also avoiding the formation of pimples in the copper surface which would render it unusable.

SUMMARY OF THE INVENTION

The present invention relates to a protected copper foil laminate having in sequential order (a) a copper foil, (b) a copolymer adhesive layer, (c) a polymer release film and (d) a silicone-based release coating; wherein (i) the surfaces of the polymer release film and the copolymer adhesive layer which contact each other are each subjected to an adhesion-enhancing pre-treatment before being joined; (ii) the silicone-based release coating prevents adhesion of the polymer release film to other surfaces during the high temperature and pressure laminating; and (iii) after the entire structure is subjected to high pressure and temperature laminating conditions of about 1 -2 hours at a temperature of at least 300°F. and a pressure of at least 150 psi, the polymer release film is removable from the copper foil with the copolymer adhesive layer fully adhered to the polymer film and none remaining on the copper foil.

Suitable adhesion-promoting treatments for use on the surfaces of the polymer release film and the copolymer adhesive layer include such as corona discharge, ozonation, plasma processing, and surface primer coatings.

More particularly, this invention relates to a laminate wherein a polymer release film is subjected to a corona discharge treatment and a copolymer adhesive layer comprised of polypropylene and polyethylene is subjected to ozonation prior to joining of the polymer release film and the adhesive layer.

This invention further relates to a method of protecting a copper foil being used in the manufacture of printed circuit boards, by the steps of (1) coating one side of a polymer release film with a silicone-based release coating; (2) exposing the opposing side of the polymer release film to a first adhesion promoting procedure; (3) extruding a thin copolymer adhesive layer; (4) treating the side of the thin hot copolymer adhesive layer which will contact the polymer release film with a second adhesion promoting procedure; and (5) joining together the polymer release film, the copolymer adhesive layer, and a copper foil in this order.

Preferably the method of protecting the copper foil utilizes a corona discharge procedure to adhesion promote the polymer release film, an ozonation procedure to adhesion promote the copolymer adhesive layer, and the layers are joined together by nip-rolling.

Still further, this invention relates to a protected copper foil laminate of a clean copper foil sheet and a protective deep matte polyester film laminated to said copper foil sheet, said polyester film having an outer smooth side having a uniform layer of a silicone release coating on the entire outer smooth side and an inner rough side which is exposed to a corona discharge, and inbetween the rough side of the polyester film and the copper foil sheet is located a copolymer adhesive extruded layer wherein the side which contacts the corona-treated polyester film is ozonated, and wherein after exposure of the protected copper foil laminate to a temperature of at least 300°F and a pressure of at least 150 psi, the protective film and the polypropylene copolymer adhesive layers are peelable from the copper foil sheet and leave no residues thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a fragmentary cross section of an embodiment of the present invention.

Figure 2 is a schematic drawing showing a manufacturing process suitable to prepare the embodiment of Figure 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a protected copper foil laminate having in sequential order (a) a copper foil, (b) a copolymer adhesive layer, (c) a polymer release film and (d) a silicone-based release coating; wherein (i) the surfaces of the polymer release film and the copolymer adhesive layer which contact each other are each subjected to an adhesion-enhancing pre-treatment before being joined; (ii) the silicone-based release coating prevents adhesion of the polymer release film to other surfaces during the high temperature and pressure laminating; and (iii) after the entire structure is subjected to high pressure and temperature laminating conditions of at least 1 hour at a temperature of at least 300°F. and a pressure of at least 150 psi, the polymer release film is peelable from the copper foil with the copolymer adhesive layer adhered to the polymer film and not the copper foil.

Adhesion-promoting treatments for the surfaces of the polymer release film and the copolymer adhesive layer include such as corona discharge, ozonation, plasma processing, surface primer coatings, and other such treatments known to the adhesion-promoting art.

Now referring to Fig. 1 , it illustrates an embodiment of the present invention and shows a cross-sectional view of a multilayer film product of the invention generally indicated as 10. While the preferred embodiment is described in terms of a copper foil laminate used in making printed circuit boards, it will be appreciated by those skilled in the art that, broadly speaking, this invention is applicable to copper foil laminates for other uses in which similar use requirements are present.

In accordance with the product of Fig. 1 , a copper foil 12 is present. Generally, a 0.125 to 1 ounce/square foot copper foil is used. Preferably, a 0.25 to 0.5 ounce copper foil is used. Located atop the copper foil to protect the outside thereof is a multilayer protective film composite 14. The film composite 14 includes a centrally located polymer film 15. Any suitable protective polymer film can be used and such are well known in the art. Preferably, however, a polyester film is used. Suitable polyester films generally are about 1 -5 mil thick. Preferably, the film is about 3 mils thick and has a deep matte finish on one or both sides. Such films exhibit tensile strengths in both the machine direction and the transverse direction of more than 10,000 psi, preferably more than 15,000 psi. Preferably, the polyester film has a melting point above 450°F (232°C) and a glass transition temperature above about 140°F (60°C). Preferably, the film is highly delustered by having incorporated therein one or more inert materials such as calcium carbonate, fumed silica, or the like. Most preferably the amount of inorganic material is sufficient to cause the film to be deemed "deep matte."

The polyester film 15 carries a silicone-containing release coating 16 on its outer surface. Suitable such silicone coatings are well known in the industry and only minimal details are provided herein. A silicone release coating generally comprises a silicone monomer and a metal polymerization catalyst dispersed within a solvent system. Suitable catalysts can include both base metals, e.g. tin, in the form of organo-metallic compounds, and precious metals, e.g. palladium, platinum, and rhodium. Alternatively, the composition may contain an ultraviolet light initiator. Examples of suitable commercially available release coatings include those marketed as the 164, 4,000, 6,000, and 7,000 series of products from Loparex Inc., Willowbrook, Illinois. The release coating may contain non-silicone materials in amounts which do not prevent the coating from performing the stated function.

The release coating 16 may be deposited on the protective film 15 by any conventional coating means known to the art. Most commonly, Mayer rod coating, gravure coating, or knife over roll coating. Generally the release coating 16 is used in an amount of about 0.2 to 5, preferably about 0.3 to 1 , lbs per ream dry (one ream is 3,000 ft²).

On its inner side, the polyester film 15 is subjected to a corona discharge treatment using a high frequency (typically about 10 kHz), high voltage generator, preferably one having a power output of about 5 to 40 kw, more preferably about 10 to 25 kW. The specific power output will depend on the amount of discharge rods and their power per inch. Power per inch length of the discharge rod is typically about 40 to 60 W/in, at a potential of about 1 to 100 kv, typic-ally 10 kV. Discharge is conveniently accomplished by passing the film over a dielectric support roller at the discharge station at a linear speed pre-ferably of about 1 .0 to 100 m per minute. The discharge electrodes may be positioned about 0.1 to 10.0 mm, typically about 1.5 mm from the moving film surface.

The corona treatment results in increasing a surface tension wetting test value from less than 30 dynes per centimeter (dynes/cm) to at least 35 dynes/cm, preferably about 40 to 50 dynes/cm, when measured with an Accu Dyne Test Marker. Accu Dyne markers do not differentiate surface tensions below 30 dynes per centimeter.

Between the copper foil 12 and the polyester film 15 is a copolymer adhesive layer 18 which typically contains a major proportion of polypropylene non-adhesive and a minor proportion of polyethylene adhesive. The copolymer adhesive layer adheres the polyester film 15 to the copper foil 12 as manufactured but is completely peelable from the copper foil 12 after exposure of the entire structure to high temperature and high pressure laminating conditions. Suitable olefins for use in forming the copolymer adhesive layer 18 include ethylene, propylene, isopropylene, butene, and isobutene. In addition to the olefins, the copolymer may contain minor amounts of copolymerizable aromatic monomers such as styrene, dienes such butadiene or isoprene, fluorinated olefins, and other such monomers which are copolymerizable with the ethylene and propylene. The copolymers may be either random or block configuration. Preferably the copolymer is prepared from more than 50 wt % propylene and less than 50 wt % ethylene. More preferably it is a terpolymer of propylene, ethylene, and butene. Such polymers are commercially available from a number of sources and thus further details are not provided herein.

The face of the copolymer adhesive layer which is intended to contact the polyester film is subjected to ozonation prior to that contacting and joining of the polyester film and the copper foil, generally by nipping. The ozone will be generated by ozonator set at a discharge of about 15 amps at a power of about 10 kW. Ozone flow should be about 100 to 150 cubic feet per minute (cfpm). The ozone applicator is typically located about 5 mm from the copolymer adhesive layer curtain and should have a geometrical configuration such that it will render the highest possible concentration of ozone on the surface of the melted copolymer adhesive. When a tube applicator is used, the holes on the edges of the ozone applicator tube should be blocked for about 1 inch on each end so that the ozone application length is narrower than the width of the copolymer adhesive layer to avoid ozone migration around and behind the liquid curtain of the copolymer adhesive.

The term "high temperature and pressure laminating conditions" is used herein to mean a time of about 60 minutes to about 2 hours at a temperature of at least 300°F (149°C) and a pressure of at least 150 psi. Preferably the temperature and pressure are at least 350°F (177°C) and at least 250 psi. Still more preferably the temperature and pressure are at least 350°F (177°C) and at least 300 psi. The multilayer structures of the present invention have been tested and found to be stable and useful after exposure to laminating conditions as severe as 395°F (202°C) and 350 psi.

The multilayer structure may be prepared in any manner, but it has been found particularly useful to use the manufacturing procedure shown in Figure 2 wherein nip joining between rubber roll and a chill roll is used to join a copper foil 12 to a multilayer protective film composite 14.

While the multilayer film composite 14 of the present invention may be prepared in any suitable manner, for convenience it has been found desirable to first coat a smooth side of a deep matte polyester film 14 with a commercial silicone release composition 16 using a Mayer rod coater and then to pass the coated film immediately into an oven at a temperature of about 200-400°F (93- 204°C) (depending upon the specific system and catalyst) to accomplish thermal curing.

Thereafter the non-release coated side of the film 15 is passed under a corona treatment system 20 at conditions as described.

The copolymer adhesive 18 is heated and extruded through extruder slot 21 in a conventional manner to form a molten adhesive layer 22. This layer is extremely thin, generally having a thickness of about 0.1 to 0.7 thousandths of an inch, preferably about 0.2 to 0.5, most preferably about 0.3 to 0.4 (to avoid the formation of pimples on the copper surface while still obtaining complete release of the copolymer adhesive 18 from the copper after lamination and complete attachment to the polyester film. The side of the layer 22 that will contact the corona treated side of the polyester film 15 is exposed to an ozonator 24 under conditions as described.

The polyester film 14 carrying silicone release coating 16 on one side and the copolymer adhesive layer 18 on the other is laminated onto the copper foil 12 between a rubber roll 26 and a chill roll 28 using such as nip roll pressure of about 10 to 100 psi, preferably about 20 to 50 psi. While the laminating of the protective structure to the copper foil can be performed at room tempera-ture, the use of elevated temperature, i.e. to about 250°F (121 °C), has been found advantageous. The protected copper foil product of this invention can directly substitute for unprotected copper foils as now used to manufacture circuit boards, except that the copper surface is protected before, during, and after lamination, through drilling and up to desmear in the wet area. This copper foil is clean, protected and fully supported by the protective carrier film, making it easier to handle without tearing or wrinkling than copper foil alone. This eliminates waste, eliminates the potential for rejects, reduces costs, and maximizes first pass yields. It may be used with either stainless steel or aluminum separators without the need for supplemental release films.

Preferably, the 3 -layer protective cover atop the copper foil is allowed to remain in place after the lamination procedure and provides additional protection for the copper foil during subsequent processing after lamination. So doing avoids having to perform a post-lamination scrubbing procedure after breakdown and before drilling. As a result the copper surface is protected from dirt, resin dust, airborne particles, dings, scratches, etc. far longer than previously thought possible.

Actually the 3 -layer protective cover (copolymer adhesive layer, polymer release film, and silicon release coating) is preferably allowed to remain in place throughout the drilling operation. The protected copper may be drilled either mechanically or by laser. This means that any plume residue that would normal-ly appear on the copper is avoided - it appears on the protective carrier film and is completely removed when the film is eventually removed after drilling. This also means that the copper surface is clean and free of debris, the drill holes are perfectly formed, and there is no exit hole chunking from laser drilling. This also means that there is little to no need for deburring of the drilled holes.

The protective polymer release film is easily removed from the laminated and preferably drilled circuit board with the copolymer adhesive layer intact by simple hand peeling or with a mylar peeler.

Claims

What is claimed is:

1. A protected copper foil laminate containing in sequential order (a) a copper foil, (b) a copolymer adhesive layer, (c) a polymer release film and (d) a silicone-based release coating; characterized in that

(i) the surfaces of the polymer release film and the copolymer adhesive layer which contact each other are each subjected to an adhesion-enhancing pre-treatment before being joined;

(ii) the silicone-based release coating prevents adhesion of the polymer release film to other surfaces during high temperature and pressure laminating; and

(iii) after the entire structure is subjected to high temperature and pressure laminating conditions of at least 1 hour at a temperature of at least 300°F (149°C) and a pressure of at least 150 psi, the polymer release film is peelable from the copper foil with the copolymer adhesive layer fully adhered to the polymer film and essentially none remaining on the copper foil.

2. A method of protecting a copper foil used in the manufacture of printed circuit boards by laminating the copper foil to a pre-preg, characterized by the steps of (1 ) coating one side of a polymer release film with a silicone- based release coating; (2) exposing the opposing side of the polymer release film to a first adhesion promoting process; (3) extruding a thin hot copolymer adhesive layer; (4) treating the side of the thin hot copolymer adhesive layer which will contact the polymer release film with a second adhesion promoting process; and (5) joining together the polymer release film, the copolymer adhesive layer, and a copper foil in order.

3. The invention of Claims 1 and 2, characterized in that the copolymer adhesive layer is prepared from at least two olefins selected from ethylene, propylene, isopropylene, butene, and isobutene

4. The invention of Claim 3, characterized in that the copolymer further includes a minor amount of an additional copolymerizable monomer selected from the group consisting of aromatic monomers, dienes, and fluorinated olefins.

5. The invention of Claims 1 and 2, characterized in that the polymer release film is a polyester film about 1-5 mils thick, is delustered sufficiently to have a deep matte finish on at least one side, and exhibits tensile strengths in both the machine direction and the transverse direction of more than 15,000 psi.

6. The invention of Claims 1 and 2, characterized in that the adhesion-promoting treatments of the surfaces of the polymer release film and the copolymer adhesive layer which contact are independently selected from the group consisting of corona discharge, ozonation, plasma processing, and surface primer coatings.

7. The invention of Claims 1 and 2, characterized in that the adhesion-promoting treatment performed on the polymer release film is exposure to a corona discharge.

8. The invention of Claims 1 and 2, characterized in that the adhesion-promoting treatment performed on the polymer release film increases a surface tension wetting test value from less than 30 dynes/cm to at least 35 dynes/cm, when measured with an AccuDyne Test Marker.

9. The invention of Claims 1 and 2, characterized in that the adhesion-promoting treatment performed on the copolymer adhesive layer is ozonation.

10. The invention of Claims 1 and 2, characterized in that the high temperature and pressure laminating conditions are about 1 -2 hours at a temperature of at least 350°F (177°C) and a pressure of at least 300 psi.

1 1. The invention of Claims 1 and 2, characterized in that the polymer release film is peeled off the copper foil after lamination and the copolymer adhesive layer adheres to the copolymer adhesive layer.

12. The invention of Claims 1 and 2, characterized in that after lamination the polymer release film is retained atop the copper foil and holes are drilled through the copper foil and the polymer release film while they are joined together.

13. The invention of Claim 12, characterized in that the holes are mechanically drilled or laser drilled and any plume residue is deposited on the polymer release film and not on the copper foil.

14. The invention of Claim 12, characterized in that the polymer release film is peeled off the copper foil after the holes are drilled and the copolymer adhesive layer adheres fully to the polymer release film.