WO2001047327A2

WO2001047327A2 - Method for applying polymer film to a metal foil

Info

Publication number: WO2001047327A2
Application number: PCT/US2000/042681
Authority: WO
Inventors: John Andresakis; Dave Paturel
Original assignee: Oak-Mitsui, Inc.
Priority date: 1999-12-23
Filing date: 2000-12-08
Publication date: 2001-06-28
Also published as: WO2001047327A3; TWI284329B; AU4714001A

Abstract

This invention relates to a flexible polymer material that can be cast as a thin, unsupported, insulating polymer layer or comprising a conductive metal layer (10) and an insulating polymer layer (12). The polymer layer includes at least one polyester polyimide and does not include an epoxy resin. This invention also relates to methods for manufacturing flexible coverlay, adhesive layers or circuit board materials as well as to methods for manufacturing printed circuit boards and circuit board components using the flexible printed circuit board materials of this invention.

Description

TITLE: Method For Applying Polymer Film To A Metal Foil

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to flexible unsupported polymer layers, polymer coated films and composites comprising a conductive metal layer and a polymer layer or polymer coated film. This invention also relates to methods for manufacturing flexible polymer layers, polymer coated films and composites as well as to methods for manufacturing printed circuit boards and circuit board components using the flexible layers, coated films and/or composites of this invention.

(2) Description of the Art

Polymeric insulating dielectrics, clad or unclad are primary components in the manufacture of components of electronic devices. Generally printed circuit boards include at least one circuit plane comprising a plurality of lines for electrically connecting IC-chips, resistors, capacitors and other devices. Each circuit is typically manufactured of conductive thin copper foil lines adhered to an insulating dielectric.

Thin flexible circuit boards are finding use in a wide variety of electronic applications. Thin circuit boards allow electronic devices to be manufactured in smaller sizes. Typically, flexible circuit boards

include copper foil attached to a polymer film. The copper can be attached to the polymer film via an adhesive layer, may be laminated in foil form to the film under heat and pressure, or vacuum deposited as vapor phase plasma.

There are several drawbacks to pre-manufactured clad substrates. These include, lower operating temperatures and high moisture absorption of the adhesives, anisotorpic flexural characteristics related to stresses in the films from the casting process and brittle or pinholed copper from the vacuum process. Therefore, there exists a need for flexible printed circuits that exhibit good physical and thermal characteristics. SUMMARY OF THE INVENTION

It is an object of this invention to provide a flexible composite useful in the manufacture of flexible circuit boards that includes a high quality polymer of uniform thickness.

It is another object of this invention to provide a method for manufacturing a flexible circuit board material including a polymer film coating that is homogeneous and free of air pockets which has proven difficult to achieve with alternative technologies.

In yet another object, this invention is a flexible printed circuit board component having high physical and electrical strength with an acceptably low number of defects as defined by the industry.

In one embodiment, this invention is a flexible polymer layer consisting essentially of a polyester polyimide polymer and having a thickness of from about 0.2 to about 2.0 mils. In another embodiment, this invention is a composite comprising a layer of polymer that does not include epoxy resin and having a thickness of from about 0.2 to about 2.0 mils that is adhered to a second flexible film of about 0.5 to about 2.0 mils. In another embodiment, this invention is a flexible composite comprising a conductive metal layer, and a polymer layer adhered to the conductive layer wherein the polymer layer includes at least one polyimide and wherein the polymer does not include an epoxy resin.

In another embodiment, this invention is a flexible composite comprising a copper foil layer wherein the copper foil has a shiny surface and a matte surface, and a polymer layer having a thickness of from about 0.2 to about 2.0 mils adhered to the conductive metal layer wherein the polymer layer consists essentially of one or more polyimide polymers. In still another embodiment, this invention is a flexible composite comprising a first conductive layer, a second conductive layer, and a polymer layer located between the first conductive layer and the second conductive layer wherein the polymer layer includes at least one polyimide and wherein the polymer layer does not include epoxy resin.

In yet another embodiment, this invention is a flexible composite manufactured by preparing a liquid polymer composition including at least one solvent and at least one polyimide polymer wherein the polymer solution doeε not include an epoxy resin. The liquid polymer composition is applied .o at least one surface of a conductive foil to give a liquid polymer composition coated conductive foil, and the liquid polymer composition coated conductive metal foil is heated to evaporate the solvent and at least partially complete the imidization of the polyimide.

In another embodiment, this invention is a flexible composite comprising one or two conductive layers, and a polymer layer as described above wherein the polymer layer is other than an epoxy resin containing polymer.

DESCRIPTION OF THE FIGURES

Figure 1 is a side cut-away view of an embodiment of a flexible composite of this invention;

Figure 2 is a side cut-away view of an alternative embodiment of a flexible composite of this invention;

Figure 3 is a side cut-away view of a flexible circuit with plated- through-hole and etched circuits manufactured from the flexible composite of Fig.2.

Figures 4A, 4B and 4C represent a method for manufacturing a rigid/flex circuit with plated-through-hole using flexible composites of this invention; and

Figure 5 is a schematic of an apparatus for manufacturing flexible composites of this invention. DESCRIPTION OF THE CURRENT EMBODIMENT

The present invention relates to a flexible circuit board composite material comprising a conductive metal layer and a polyimide polymer layer This invention also relates to methods for manufacturing flexible circuit board materials as well as to methods for manufacturing printed circuit boards and circuit board components using the flexible printed circuit board composites of this invention

Figures 1 and 2 are side views of two flexible composites of this invention Referring to Figures 1 and 2, the flexible composites of this invention include a first conductive layer 10 and a polymer containing layer 12 In the embodiment shown in Figure 2, the flexible composite includes second conductive layer 14 that is separated from first conductive layer 10 by a polymer containing layer 12 The embodiment of the invention shown in Figure 2 can be manufactured by a number of methods including, for example, uniting two composites as shown in Figure 1 such that the polymer containing layer 12 face each other Alternatively, an uncoated conductive layer may be adhered to the polymer layer in a laminating process or the like

Any number of conductive materials may be used to form conductive layers 10 and 14 The conductive materials are typically used in the form of foil rolls or sheets The conductive materials typically have a thickness ranging from about 1 microns to about 200 microns More preferably, the conductive material will be in foil or sheet form having a thickness of from 3 to about 70 microns. Copper, in essentially pure or alloy form, is a preferred conductive layer material as it is relatively inexpensive and easy to work with. Alternative conductive layer materials include aluminum, gold, silver, nickel or other known conductive materials alone or in combination or alloy. It has been found, however, that copper has an added advantage in that it has a coefficient of thermal expansion (CTE) that is substantially similar to that of polyimide material. Consequently, flexible composites having copper layers associated with a polyimide polymer coating material will generally be less susceptible to thermal stresses.

Conductive layers 10 and 14 may consist of a single conductive material layer or two or more layers of conductive material. The choice of conductive material as well as the number of layers of conductive material will generally depend upon the end use of the flexible composite of this invention.

The conductive materials used in the flexible composites of this invention may be manufactured with a shiny side surface and a matte surface. Examples of such conductive materials are disclosed in U.S. Patent No. 5,679,230, the specification of which is incorporated herein

by reference.

The flexible composite of this invention may be manufactured such that a polymer layer is applied to the shiny surface or to the matte surface of the conductive material. In a preferred article of this invention, a polymer layer is applied to the shiny side of the conductive material in applications where it is important to easily separate the conductive material layer from the polyimide polymer layer. In contrast, the polyimide polymer can be applied to the shiny or matte side of the conductive material layer which has been exposed to a bond enhancement process when it is desirable to have a strong bond between the polyimide polymer material layer and the conductive material layer.

The flexible composite of this invention may be manufactured using one or more curable polymers. While any curable polymer might be selected, it is preferred that the curable polymer chosen is a polyimide polymer or a mixture of polyimide polymers. Polyimides have high electrical strengths, good insulating properties, a high softening point and are inert to many chemicals. Preferred polyimides will have a glass transition temperature (Tg) of from about 160°C to about 320°C with a glass transition temperature of from about 190- 270°C being preferred. It is also important that the polymer layer not include epoxy resins. The addition of epoxy resins will detrimentally impact the flexible composites of this invention. Epoxies are thermosets and they are relatively brittle. The use of epoxy resins in the curable polymer layer will detrimentally effect the flexibility of the composite and especially the flexibility of polyimide containing composites. Also, epoxy resins typically have lower glass transition temperatures (120-200 °C) that polyimide polymers and will detrimentally lower the effective Tg of polyimide containing flexible composite polymer layers.

The flexible composites of this invention will include a layer of polymer, and preferably polyimide polymer having a thickness ranging from about 0.1 to about 3 mils. More preferably, the flexible composites will have polyimide polymer layer having a thickness from about 0.2 to 1.5 mil.

As explained below, the preferred polyimide is applied to the conductive material as a solution that allows for control of the polymer thickness and uniformity. The polyimide solution will typically have a viscosity ranging from about 5000 to about 35,000 centipoise with a preferred viscosity in the range of 15,000 to 27,000 centipoise. The polymer solution will include from about 10 to about 60% and preferably 15 to 30 wt % polymer with the remaining portion of the solution comprising one or more solvents. It is preferred that a single solvent be used in the polymer solution. Useful solvents include acetone, methyl-ethyl ketone, N-methyl pyrrolidone, and mixtures thereof. A most preferred single solvent is N-methyl pyrrolidone. The flexible composites of this invention are prepared using an unique process that provides a flexible composite including a polymer layer having few if any inconsistencies such as, air bubbles in the polymer film or varying areas of polymer thickness. A schematic of a process for manufacturing a flexible composite of this invention is set forth in Figure 5.

Flexible composites of this invention are preferably manufactured in a continuous process. The process begins with a roll of conductive or release material located on unwind roller 50. The uncoated conductive or release material 40 is continuously pulled from unwind roller 50 and directed through idler rollers 52. The idler rollers 52 position and maintain a proper angle between the foil and doctor blade 56. A polymer dispenser system 54 applies a continuous layer of polyimide polymer to a surface of uncoated conductive or release material 40. Doctor blade 56 immediately follows polymer dispenser system 54. The doctor blade is adjusted to dispense the desired thickness of the polymer onto to uncoated conductive or release material 40. The surface of uncoated conductive or release material 40 that is not coated with polymer travels over slide roller 58. Slide roller 58 preferably has a very smooth surface and is generally manufactured from polished stainless steel. The purpose of slide roller 58 is to maintain web tension providing a stable, flat surface assuring a uniform application of the polymer film. Cover 60 is located over the uncured polymer coated conductive or release material 59. Cover 60 prevents debris, dust and other material from becoming embedded in the uncured polymer. The uncured, polymer coated, conductive or release material 59 is directed into an oven 62. Oven 62 evaporates the solvent in the applied polymer film and advances the polymer to a predetermined degree of cure ranging from partial (B-stage) to total (C- stage). The oven may be any type of oven that is capable of curing a polymer. The polymer layer will typically be subjected to a temperature of from about 100°F to about 600°F in oven 62. In addition, the uncured polymer material will have a residence time in the oven of from about 1 to about 10 minutes. Oven 62 includes one or more doors 63 that provide access for ducting used to remove solvent liberated from the polymer during the evaporation and curing steps and/or to assist in oven temperature control. The process is flexible and can accommodate an additional oven 64. The purpose of additional oven 64 is to increase flexibility and capacity. A partially or fully cured flexible layer or composite 80 exits the oven(s) and is directed through idler rollers 66 before being collected in a roll on rewind roller 68. The degree of cure will depend primarily upon the end use of the flexible layer or composite. Where for examples, the flexible layer will be used as a circuit coverlay, the polymer layer will be partially cured. Where, for example, the flexible composite will be bonded to a second flexible composite, the polymer layer will be partially cured. Where, for example a circuit will be manufactured from the conductive material layer of the flexible composite, the polymer layer of the flexible composite will generally be fully cured. An optional interleave roller 70 is used to insert protective films or release papers. The flexible composites of this invention may be used in a variety of printed circuit applications. The unsupported flexible layer may be used as a circuit coverlay or intermediate adhesive layer. The flexible composites may be used in rigid, rigid/flex or flexible circuits. The flexible composites are generally used by creating a circuit pattern on the conductive layer. A second circuit pattern may be applied to the opposing polymer surface either in the form of a conductive foil, by electrodeposition, by sputtering, by vapor phase deposition or some other means. In addition, it may be necessary to generate vias in the flexible composites to electrically connect opposing circuit layers. Furthermore, the use of the flexible composites will depend upon whether the polyimide polymer layer is partially or fully imidizied.

Once a flexible composite of this invention has been formed, circuit patterns may be created in the conductive layer using known etching techniques. In etching, a layer of photoimageable resist or liquid material is applied to the conductive metal layer. Using a negative photo pattern, which is overlaid on the resist, the photoresist is exposed light or other radiation sources resulting in the formation of the desired circuit pattern. The resist clad flexible composite is then exposed to film developing chemistry that selectively removes the unwanted film exposing portions of the copper surface. The flexible composite with circuit image is then exposed to known chemical etchant baths that remove the exposed conductive material leaving the final desired conductive patterns on the flexible composite.

By virtue of the above process steps a flexible circuit as shown in Figure 3 is formed. The flexible circuit includes conductive lines 20 and 22 are formed on both sides of dielectric substrate 30, and metallized through holes such as hole 32 are optionally formed to electrically interconnect layers 20 and 22. The conductive layers and metallized through holes may be constructed to have finished thicknesses which fall below the standard minimum thickness requirements of 1 mil (25.4 microns), although greater finished thicknesses, such as 35 microns, are nonetheless obtainable if desired.

EXAMPLES

The present invention is further described with reference to the following Examples, but it should be understood that the invention is not intended to be limited to these Examples.

EXAMPLE I

This Example details a method for manufacturing a flexible composite of this invention.

A liquid polyimide resin is adjusted to 25% solids, a viscosity of about 20,000 centipose, with N-methylpyrrolidone in a stainless steel mixing vat. Referring now to Figure 5, a roll of electrodeposited 18 micron foil, 25" in width is mounted on unwind roller 50. A sample of the foil is taken and a 12" x 12" cut and weighed to establish foil base weight. The foil is threaded through the tensioning rollers 52, over roller 58, through oven 62, through idler rollers 66 and onto rewind roller 68. The foil is tensioned to 4 pounds per inch width with the air brake and rewind drive.

The IR heat sources in oven 62 are turned on and set at 300°F, 450°F and 600°F, zones 1-3 respectively and allowed to stabilize. The drive motor on rewind roller 68 is engaged and set to 4 feet/minute. Edge guides are enabled and proper tracking through the oven is confirmed.

The solids adjusted liquid polyimide resin is supplied to the dispensing system 54 via gravity (a pumped or pressurized system will also work) and a film of approximately 2.0 mils is applied to the moving foil using gravity and the liquid polymer viscosity as dispensing forces.

Doctor blade 56 is adjusted to produce a wet film of 1.7 mils in thickness, this will result in a flexible composite having a dried polymer film in thickness of about 0.3 mils. At this point it is critical to maintain a continuous liquid head height and volume of dammed material on the up stream side of doctor blade 56. Both of these parameters are important in maintaining a constant flexible composite film thickness and a film free of included air bubbles.

As the coated foil first enters oven 62, an initial temperature drop should be anticipated. Material processed through the oven under these conditions is tagged on the take-up as scrap along with the uncoated leader material.

Once steady state temperatures are achieved in oven 62, film thickness is checked by taking a foil sample and comparing the coated weight to the base weight of the foil using the resin density to convert from weight to film thickness. Adjustments to the rate of resin dispensed and or doctor blade height over the foil are made based on this measurement. This process is repeated until the desired film

thickness, in this case 0.3 mils is attained.

The flexible composite produced by this method includes a metal foil layer with 0.3 mils of semi-cured polyimide containing 2-3% residual solvent. The finished flexible composite is accumulated in roll form on rewind roller 68 and converted into printed circuitry in sheet or

roll form.

EXAMPLE II

The flexible composite manufactured as described in Example 1 was used to manufacture a rigid polyimide, glass reinforced, double sided, printed circuit board by the following method.

The flexible composite material, conductive metal layer side facing outwards, was laminated onto each side of 2 sheets of a nonflame retardant, polyimide resin impregnated woven fiberglass. The package was cured at a temperature of 425°F for 180 minutes under a pressure of 300 psi and 28" of vacuum. The resulting clad laminate had a dielectric thickness of 0.014". The extended cure time is necessary to cure the thermoset resin used to impregnate the woven glass core material. A substantially shorter cure cycle would be used for the composite material itself. Circuits are applied to the metal layers by standard imagining and etching techniques to form a double-sided circuit boards. A mechanical drill was used was used to clear through holes; (the flexible composite is laser ablatable but the reinforced core is not).

A thin layer of electroless copper was applied to all surfaces, and more specifically to the walls of the through holes.

A plating resist was applied to the composite to define the areas to be plated-up with electrodeposited copper. Copper was subsequently plated onto the exposed surfaces to attain minimum 0.8 mils of thickness in th 3 through hole. The plated up, exposed copper areas were covered wiϊh a tin/lead resist via an electrodeposition process. The plating resist from the ED copper deposition process was chemically removed and the panel passed through an etching process. The unprotected copper surfaces were removed forming circuits with plated through holes in the areas protected by the tin/lead.

The tin/lead resist was chemically stripped (etched away), a solder mask applied and solder applied to the exposed areas to form a circuit board product. The circuit board was inspected after each intermediate process step for evidence of cracking, blistering and delamination. None was observed. The finished board was subjected to a thermal stress of 550°F for 10 seconds. It was again examined for evidence of cracking, blistering and delamination, none was observed. The bond strength between the foil and dielectric was then measured using a force gauge. The peel strength was reported to be 5 lbs/inch width, a value slightly higher than rigid construction made without the composite material.

EXAMPLE III

The flexible composite of Fig. 1 was used to manufacture a double-sided flexible circuit substrate. This was accomplished by laminating two composite pieces together, polymer layers face-to-face, under a pressure of 300 psi and temperature of 520°C for 15 minutes after which the composite was allowed to cool to room temperature. This double clad composite is represented in Figure 2.

A clad composite as in Fig.2 was processed using known techniques as described in Example 2 to form a flexible circuit as in Figure 3.

The flexible circuit was inspected after each intermediate process step for evidence of cracking, blistering and delamination, none was observed.

The finished circuit was exposed to a thermal stress of 550°F for 10 seconds. It was again examined for evidence of cracking, blistering and delamination, none was observed. This service temperature is considerably higher than composites that use adhesive layers. The bond strength between the foil and dielectric was then measured using a force gauge. The peel strength was reported to be 5 lbs/inch width. A sample of the composite was etched free of copper and tested for flame retardency using UL 94 method. It received a V-0 rating, self extinguishing, nonflammable.

EXAMPLE IV A rigid/flex circuit is a hybrid in which an integrated "flex leg" is used to connect two rigid constructions. These hybrids are used because they are reliable and because they take up only a small amount of space. A fully cured flexible composite of Fig. 1 was processed using known techniques as described in Example 2, to form a single sided flexible circuit.

As shown in Figure 4A, imaged cores consisting of circuitized conductive layer 10 and polymer containing layer 12 are collated with B-stage prepregs 41 and copper foil layers 40, into package ready for lamination using the prepared flex leg 52 as an interconnecting circuit for the individual rigid packages. The materials were laminated at appropriate temperature and pressure to form the intermediate package shown in Figure 4B. Using known processes microvia interconnects 50 and plated through holes 32 were formed into the package.

The rigid/flex circuit was inspected after each intermediate process step for evidence of cracking, blistering and delamination, none was observed. The finished circuit was exposed to a thermal stress of 550°F for 10 seconds. It was again examined for evidence of cracking, blistering and delamination, none was observed. This service temperature is considerably higher than composites that use adhesive layers.

Claims

What we claim is:

1. A flexible composite comprising; a conductive metal layer; and a polymer layer adhered to the conductive layer wherein the polymer layer includes at least one polyester polyimide and wherein the polymer does not include an epoxy resin.

2. The flexible composite of claim 1 wherein the conductive layer is a metal selected from the group consisting of copper, gold, aluminum, silver, and alloys thereof.

3. The flexible composite of claim 2 wherein the conductive layer is a copper foil layer.

4. The flexible composite of claim 1 wherein the conductive layer includes at least two conductive metal layers.

5. The flexible composite of claim 1 wherein the polymer layer has a thickness of from about 0.2 to about 2.0 mils.

6. A flexible composite comprising; a copper foil layer wherein the copper foil has a shiny surface and a matte surface; and a polymer layer having a thickness of from about 0.2 to about 2.0 mils adhered to the conductive metal layer wherein the polymer layer consists essentially of at least one polyester polyimide.

7. The flexible composite of claim 6 wherein the polymer

layer is partially cured.

8. The flexible composite of claim 6 wherein the polymer layer is fully cured.

9. The flexible composite of claim 6 wherein the conductive metal layer has a matte surface and a shiny surface and wherein the polymer layer is associated with the conductive metal layer shiny surface.

10. A flexible circuit manufactured using the flexible composite of claim 1.

11. A rigid/flex circuit manufactured using the flexible composite of claim 1.

12. A rigid circuit manufactured using the flexible composite of claim 1 as a cap layer.

13. The flexible composite comprising; a first conductive layer; a second conductive layer; and a polymer layer located between the first conductive layer and the second conductive layer wherein the polymer layer includes at least one polyester polyimide and wherein the polymer layer does not include epoxy resin.

14. A method for manufacturing a flexible composite comprising the steps of;

(a) preparing a liquid polymer composition including at least one solvent and at least one polyimide polymer wherein the polymer composition does not include an epoxy resin;

(b) applying the liquid polymer composition to at least one surface of a conductive foil to give a liquid polymer composition coated conductive foil; and (c) heating the liquid polymer composition coated conductive metal foil for a period of time sufficient to at least partially cure the polyimide polymer to form a flexible composite including a conductive foil layer and a polyimide polymer layer.

15. The method of claim 14 wherein the liquid polymer composition coated conductive metal foil is heated until it is fully cured.

16. The method of claim 14 wherein the polyimide polymer layer has a thickness of from about 0.2 to about 2.0 mils.

17. The method of claim 14 wherein the liquid polymer composition consists essentially of from about 10 to about 60 weight percent polyimide polymer and from about 40 to about 90 wt% solvent.

18. The method of claim 17 wherein the solvent is N-methyl pyrrolidone.

19. The method of claim 14 wherein the flexible composite is manufactured in a continuous process.

20. The method of claim 14 wherein a resist layer is applied to the flexible composite conductive foil layer, imaged, and developed to expose at least a portion of the flexible conductive foil layer; and wherein the exposed flexible conductive foil layer is etched from the flexible composite.