Metal working – Means to assemble or disassemble – Puller or pusher means – contained force multiplying operator
Reexamination Certificate
2001-05-01
2003-10-07
Hail, III, Joseph J. (Department: 3723)
Metal working
Means to assemble or disassemble
Puller or pusher means, contained force multiplying operator
C029S825000, C029S847000, C427S096400
Reexamination Certificate
active
06629348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the manufacture of printed circuit boards having improved interlayer adhesion. More particularly, the present invention pertains to adhesiveless, flexible printed circuit boards having excellent thermal performance and useful for producing high-density circuits.
2. Description of the Related Art
Printed circuit boards are employed in a wide variety of applications. For example, they can be found inside radio and television sets, telephone systems, automobile dashboards and computers. They also play an important role in the operation of airborne avionics and guidance systems. Polyimide films are used in the production of circuit boards because of their excellent flex characteristics and good electrical properties. More particularly, it is common to attach a layer of a conductive metal foil to a surface of a polyimide film to provide a surface upon which a pattern of an electrical conductor can be provided. In such cases, it has been recognized in the art that any movement of the metal foil on the polymeric film could potentially impair the performance of the equipment incorporating the circuit board. To avoid this problem, it is necessary that the conductive metal layer be strongly adhered to the polymeric substrate to prevent any shifting of the metal layer on the film.
There have been various efforts in the art to improve the adhesion of metal foils to polymeric substrates in forming printed circuit boards while maintaining good thermal resistance and low cost of manufacture. U.S. Pat. No. 4,382,101 offers one proposed solution to this problem wherein a substrate is etched with a plasma etchant and then a metal is vapor deposited onto the etched surface of the substrate. This process requiring the vapor deposition of a metal directly onto an etched surface is very expensive. U.S. Pat. No. 4,615,763 provides a method of improving adhesion of a photosensitive material to a substrate by selectively etching resinous portions of a substrate comprising a resinous material and an inorganic particulate material. U.S. Pat. No. 4,639,285 teaches a process wherein a metal foil is attached to a surface of a synthetic resin substrate via an intermediate silicone-based adhesive layer after treating the substrate surface with a low temperature plasma. The low temperature plasma utilized is an organo-silicon compound with an inorganic gas, such as oxygen. U.S. Pat. No. 4,755,424 provides a polyimide film produced from a polyimide containing a dispersed inorganic powder. Particles of the inorganic powder protrude from the film surface to roughen the film. The film surfaces are then treated with a corona discharge treatment to alter the surface chemistry of the film. U.S. Pat. No. 4,863,808 teaches a polyimide film coated with a vapor deposited chromium layer, a vapor deposited copper layer, and followed by electroplating with copper. U.S. Pat. No. 5,861,192 provides a wet chemistry method with mechanical and projection grinding to increase the adhesion of a polyimide film surface.
The present invention provides an improved solution over those of the prior art. A process for forming printed circuit boards is provided wherein a polymeric film is coated onto at least one surface of an etched polymeric substrate followed by laminating a metal foil onto the coated film. The substrate surface may be etched with either a chemical or plasma etchant, and may comprise either the same or a different material than the polymeric film. The result is a circuit board with a substrate that exhibits high thermal resistance and excellent electrical insulating properties.
SUMMARY OF THE INVENTION
The invention provides a process for forming a printed circuit board composite comprising:
a) etching at least one of two opposite surfaces of a planar polymeric substrate;
b) attaching a polymeric film onto one or both etched surfaces of the polymeric substrate; and
c) laminating and attaching a metal foil onto the polymeric film.
The invention further provides a process for forming a printed circuit board comprising:
a) etching at least one of two opposite surfaces of a planar polymeric substrate;
b) attaching a polymeric film onto one or both etched surfaces of the polymeric substrate;
c) laminating and attaching a metal foil onto the polymeric film;
d) depositing a photoresist onto the metal foil;
e) exposing and developing the photoresist, thereby revealing underlying portions of the metal foil; and
f) removing the revealed underlying portions of the metal foil.
It is also within the scope of the invention to form multilayered printed circuit boards or composites by incorporating additional polymeric films or metal foil layers. A description of these embodiments is included herein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention provides a printed circuit board support having improved interlayer adhesion, enhanced thermal stability and excellent electrical insulating properties as compared to the prior art.
The first step in the process of the invention is to etch at least one surface of two opposite surfaces of a suitable substrate with an appropriate etchant, thereby forming a first etched surface. Typical substrates are those suitable to be processed into a printed circuit or other microelectronic device. Preferred substrates for the present invention are polymeric substrates and non-exclusively include materials comprising polyester, polyimide, liquid crystal polymers and polymers reinforced with materials such as fiberglass, aramid (Kevlar), aramid paper (Thermount), polybenzoxolate paper or combinations thereof. Of these a polyimide substrate is the most preferred. Also suitable are semiconductor materials such as gallium arsenide (GaAs), silicon and compositions containing silicon such as crystalline silicon, polysilicon, amorphous silicon, epitaxial silicon, and silicon dioxide (SiO
2
) and mixtures thereof. The preferred thickness of the substrate is of from about 5 &mgr;m to about 200 &mgr;m, more preferably from about 5 &mgr;m to about 50 &mgr;m.
Appropriate etchants are those which are capable of selectively removing portions of the substrate surface. Preferred etchants for the present invention non-exclusively include plasma etchants and concentrated aqueous etching solutions.
Preferred are aqueous alkaline solutions, non-exclusively include Group I or Group II hydroxides which include hydroxides of elements from Groups I or II of the periodic table, such as sodium hydroxide and potassium hydroxide. Ammonium hydroxide may also be used. The useful concentration of an aqueous etchant varies with the chemical composition of the substrate to be etched. Typically useful etchant concentrations range from about 5% to about 25% by weight of the etchant material, preferably from about 10% to about 20%. For example, one useful aqueous etchant is a potassium hydroxide solution having a concentration of from about 8% to about 12% of potassium hydroxide. Also suitable is a sodium hydroxide solution at a concentration of from about 8% to about 16% by weight of sodium hydroxide.
Any plasma etching technique which is suitable for etching polymer substrates may be used. This plasma etchant is a highly charged gas that bombards the film surface with positive and negative charged species causing impurities on the surface to degrade as well as ablating the film surface. These include halogen containing plasma etching materials and oxygen containing plasma etching materials. The preferred plasma etchant comprises a gaseous mixture of oxygen (O
2
) and tetrafluoromethane (CF
4
). Preferably the plasma etchant comprises at a mixture of oxygen plasma and tetrafluoromethane plasma comprising least about 3% of tetrafluoromethane, more preferably it comprises from about 3% to about 20% and still more preferably from about 7% to about 20% of tetrafluoromethane with the balance being oxygen. This minimum quantity of tetrafluoromethane is important to prevent any over etching of the substrate.
The etching step of the process of th
Andresakis John A.
Gray Jeffrey T.
Herrick Wendy
Skorupski Edward C.
Grant Alvin J.
Hail III Joseph J.
Oak-Mitsui Inc.
Roberts & Mercanti LLP
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