Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2000-09-07
2002-12-17
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S325000, C428S901000, C174S258000, C174S259000
Reexamination Certificate
active
06495244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to printed circuit boards having improved fire resistance and improved environmental stability. More particularly, the invention relates to halogen-free fire retardant printed circuit boards incorporating potentially flammable polymers.
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 electronic equipment and guided missiles. In forming insulating dielectric materials for printed circuit boards, it is common to employ organic polymer films that may be flammable under certain circumstances.
To combat this problem, fire retardant halogen additives are commonly employed. The purpose of the halogens is to attain an acceptable flammability rating as determined by Underwriters Laboratory (UL) 94V0 or 94V1 flammability tests, for most standard resins. For example, Japanese abstract JP6240214 provides a copper-clad laminate having a copper foil coated with a flame-retardant adhesive. The flame-retardant adhesive comprises a poly(vinyl acetal) resin, epoxy resin, polyisocyanate resin, and brominated polyester resin. U.S. Pat. No. 6,071,836 discloses polybutadiene and polyisoprene thermosetting compositions having a bromine-containing fire retardant. However, these additives are very expensive and interfere with the physical and electrical properties of the polymer. Also, decomposition of dielectric materials having halogen additives produces carcinogenic materials such as furan and dioxins.
In order to minimize the impact on the environment of electronic materials, many countries are requiring the substrates used in circuit boards to be halogen-free. For example, Japanese abstract JP11343398 provides a laminate and metal foil utilizing a flame retardant epoxy resin composition. This flame retardant composition comprises an epoxy resin, a hardener and an additive, wherein at least one incorporated hardener comprises a polycondensate of phenols, a compound having a triazine ring and aldehydes, and an inorganic filler as an additive. Also, Japanese abstract JP10195178 discloses a halogen-free flame-retardant composition comprising a bisphenol A epoxy resin, a novolac epoxy resin, a phenolic resin curing agent, a cure accelerator and an inorganic filler. U.S. Pat. No. 5,082,727 teaches a flameproof product wherein the flameproofing agents are a combination of organic borates, salts of phosphoric acids and oxide hydrates of magnesium and/or of aluminum. However, these alternatives are also very expensive and do not have good peel strengths to foil conductors.
It has therefore been desirable to provide an affordable, non-flammable, halogen-free dielectric composition for printed circuit boards having good properties and performance. The present invention offers a solution to this problem, providing a method by which copper foils are coated with non-halogenated thermoplastic dielectric layers and thermosetting polymer layers.
In particular, a printed circuit board is provided comprising a substrate having opposite surfaces, a thermosetting polymer layer on each of the opposite substrate surfaces, a thermoplastic dielectric layer on each of the thermosetting polymer layers, and an electrically conductive layer on each of the thermoplastic dielectric layers. The thermosetting layers may have various degrees of flammability, but the thermoplastic layers are inherently flame resistant and prevent combustion of the thermosetting polymers. The thermoplastic dielectric also adds strength to the laminate, resulting in a less brittle thin core than the prior art. The result is a cost efficient, environmentally safe and flame resistant laminate having excellent properties, including a decreased probability of shorting, good dielectric breakdown voltage, a smooth surface and good electrical/thermal performance.
SUMMARY OF THE INVENTION
The invention provides a circuit board comprising in order:
a) a planar substrate having opposite surfaces;
b) a thermosetting polymer layer on each of the opposite substrate surfaces;
c) a thermoplastic dielectric layer on each of the thermosetting polymer layers; and
d) an electrically conductive layer on each of the thermoplastic dielectric layers.
The invention also provides a process for manufacturing a printed circuit board comprising:
a) depositing a thermosetting polymer layer onto opposite surfaces of a substrate;
b) depositing a thermoplastic dielectric layer onto each of the thermosetting polymer layers; and
c) depositing an electrically conductive layer onto each of the thermoplastic dielectric layers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention provides a halogen-free, fire retardant printed circuit board.
The first step in the process of the invention is to deposit a thermosetting polymer layer onto opposite surfaces of a planar substrate. Typical substrates are those suitable to be processed into an integrated circuit or other microelectronic device. Suitable substrates for the present invention non-exclusively include non-halogenated materials such as fiberglass, aramid (Kevlar), aramid paper (Thermount), polybenzoxolate paper or combinations thereof. Of these, fiberglass is the most preferred substrate. 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 10 to about 200 microns, more preferably from about 10 to about 100 microns. In a preferred embodiment of this invention, the substrate of the printed circuit board may comprise a plurality of adjacent strata including the strata of the above substrate materials, forming a complex multilayered article. In this embodiment, each stratum is attached to an adjacent stratum by a thermosetting polymer layer.
The thermosetting polymer layers are preferably deposited onto the substrate as liquids by coating, evaporation or vapor deposition to allow for control and uniformity of the polymer thickness. The liquid layers may subsequently be partially or fully cured on the substrate, thus forming a prepreg. For the purposes of this invention, an A-staged prepreg comprises a substrate having uncured thermosetting polymer thereon, a B-staged prepreg incorporates a partially cured thermosetting polymer, and a C-staged prepreg has a fully cured polymer. The most preferred prepreg for use in this invention is a B-staged, partially cured prepreg. Curing is conducted by placing the prepreg into an oven to evaporate any solvent from the polymer and either partially or fully cure the layers. Such may be done by subjecting the prepreg to a temperature of from about 100° F. to about 600° F., for about 1 to about 10 minutes. After curing is completed, the prepreg is removed from the oven and cooled.
The thermosetting polymer layers may also be deposited in the form of liquids or sheets that are laminated onto opposite sides of the substrate. Lamination is preferably conducted in a press at a minimum of about 275° C., for about 30 minutes. Preferably, the press is under a vacuum of at least 28 inches of mercury, and maintained at a pressure of about 150 psi. The thermosetting polymer layers preferably comprise non-halogenated materials such as epoxies, bis-malimide triazine epoxies, thermosetting polyimides, cyanate esters, allylated polyphenylene ethers, benzocyclobutenes, phenolics and combinations thereof. Of these epoxies and polyimides are preferred. Preferably, the thermosetting polymer layers have a thickness of from about 5 to about 200 microns, more preferably from about 2 to about 100 microns.
Next, thermoplastic dielectric layers are deposited onto each of the thermosetting polymer layers or onto the conductive layers. The thermoplastic
Andresakis John A.
Paturel Dave
Lam Cathy
Oak-Mitsui Inc.
Roberts, LLP Richard S.
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