Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
1999-12-10
2002-08-13
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S413000, C428S414000, C428S901000, C427S386000, C156S307300, C156S307700, C525S113000, C525S528000, C528S045000, C528S052000
Reexamination Certificate
active
06432541
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to resin formulations which are useful as adhesives and in particular to resin formulations useful for adhering metallic surfaces such as copper foil to resin laminates. More particularly, the invention relates to resin formulations useful for making prepregs, resin coated copper foils, films and electrical laminates for rigid or flexible printed wiring boards (PWB). Prepregs are resin-impregnated or resin-coated cloths or sheets, which have been subjected to a preliminary cross-linking step (known as B-staging), usually prior to the lamination of a number of pre-pregs to produce a final laminate.
There is an increasing tendency toward the miniaturization of integrated circuits in the electronic industry. The industry has for some time been seeking to produce circuit boards which permit high density mounting, in order to enable maximum miniaturization of the devices. In order to achieve higher and higher density, it is desirable that the adhesion between the conductive metal tracks and the dielectric materials is improved, that the glass transition temperature (Tg) of the dielectric material is increased, and that the dielectric constant of the dielectric constant should be smaller.
The miniaturization of circuit boards requires the use of relatively recent design techniques, in which the interconnections between logic devices are routed using so-called Build Up Multilayers (BUM). There are many different technological approaches to achieving build-up multilayers.
SUMMARY OF THE INVENTION
One approach is as described in U.S. Pat. No. 5,387,495 and involves a process in which, beginning with a solidified layer of the dielectric disposed upon a substrate, alternate layers of conducting metal and dielectric are sequentially deposited. Each layer of conducting metal tracks is defined using photoresist and a photolithographic technique. After the tracks are deposited, the photoresist is removed and a second layer of photoresist is employed to define the conductive posts which function as through holes between different layers. After each layer of conductive tracks and posts is formed, and the photoresist is removed, the dielectric is flowed into place and solidified to insulate adjacent metal tracks and posts. The process may be repeated as many times as necessary to build up layers of conducting metal and dielectric, and form the completed multilayer wiring board.
Another approach, as described in JP 61118246 A2 and in JP 6118247 A2, includes the coating of copper foil with thermosetting resin and then attaching the coated side to a prepreg or a core laminate by laminating under heat and pressure. The conducting metal tracks are defined using a photoresist and a photolithographic technique. After the tracks are deposited, the photoresist is removed and holes are made by laser or plasma drilling. The tracks are further metalized, for example, by standard electroless copper-plating, or by electrolytic plating and etching. The outer layer pattern is formed and the process may be repeated as many times as necessary to build up layers of conducting metal and dielectric, and form the completed multilayer wiring board.
The laminate employed is generally fiberglass-reinforced brominated epoxy (known as Fire Retardant laminate FR-4 grade having a Tg of about 130° C. to about 135° C.) deposited upon a dielectric carrier layer through the use of heat and pressure upon lamination.
Copper foil surfaces or copper based alloy materials are frequently treated by various processes physically or chemically to improve the peel strength of copper and copper based alloy materials on the laminate or dielectric carrier layer by the use of heat and pressure upon lamination, for example, as described in U.S. Pat. No. 5,071,520. The treated copper foils are particularly needed for the high glass transition laminates in order to achieve sufficient peel strength.
One method of improving the peel strength of copper foils involves coating the surface of the copper foil with a thermosetting resin composition comprising a brominated epoxy resin grade, a cross-linker such as dicyandiamide or a multi-functional phenolic hardener and a catalyst. The coated surface is frequently brittle and can crack easily after B-staging, because the B-staged resin layer has a low molecular weight. It is also difficult to control the thickness of the B-staged resin layer during the lamination process because the melt viscosity of the B-staged material is too low. If the degree of B-staging is raised in order to obtain higher melt viscosity, the processing window during the final lamination becomes too small and as a consequence, there is insufficient time for the resin flow to fill up the gaps between the metallic tracks.
To achieve consistent dielectric thickness control, the copper foil is usually coated first with a layer of material which has been pre-cross-linked to a relatively high degree (a so-called C-staged resin layer) and followed by a second layer of material with a lower degree of pre-cross-linking (the B-staged resin layer) in order that the resulting resin flows adequately and fills the circuit tracks. The use of the C-staged resin layer is effective to ensure the minimum thickness of the dielectric layer between two adjacent layers of conducting metal tracks. However, the 2-layer coating process is time consuming and costly.
It would be very desirable therefore to provide a coating system which would allow the coating of copper foil with two layers, or if possible one layer of an adhesive coating in order to achieve one or more of the following:
a) Improved peel strength on the circuit boards after bonding.
b) Improved formability and flexibility of the coated copper foils.
c) An improved processing window so that the coating adhesive has sufficient time to flow and fill the circuit tracks whilst at the same time providing good control over the coating thickness.
d) A Tg>135° C. for the cured adhesive so that it is at least equal to the Tg of the standard FR-4 laminates.
e) A dielectric constant of the cured adhesive which is lower than 3, in order to permit a high density of conductive tracks.
It is known to incorporate thermoplastic resins in resins used to coat the copper foil to increase the flexibility/formability of the coated film and to reduce the waste resin flow-out and thereby provide better film thickness by increasing the melt viscosity of the B-staged materials produced from the coated film. A high molecular weight polymer used for this purpose is sold commercially by Phenoxy Associates (USA), under the Trademark PKHH. This material is frequently used to reduce resin flow-out by increasing the melt viscosity of the B-staged materials without shortening the gel time. However, when PKHH is used, the Tg of the resulting laminate is adversely affected because PKHH has a Tg of around 90° C. to 95° C. Also, PKHH contains a high level of polar hydroxyl groups which increases the dielectric constant of the resin systems.
There are three types of end-products (conventionally referred to as polyoxazolidones) which can be obtained in the condensation reaction of polyisocyanates with polyfunctional epoxides, namely isocyanate-terminated polyoxazolidones, linear polyoxazolidones, and epoxy-terminated polyoxazolidones. These three possible end-products, and various methods for their production, are described in U.S. Pat. No. 5,112,932. Epoxy-terminated polyoxazolidones are prepared by reacting an epoxy resin with a polyisocyanate compound using stoichiometric excess of epoxy resin (isocyanate/epoxide ratio lower than 1).
U.S. Pat. No. 4,070,416 describes a process for producing thermosetting resins by mixing one equivalent or more of polyfunctional isocyanate per equivalent of a polyfunctional epoxide in the presence of a tertiary amine, morpholine derivatives or imidazole as catalysts. The catalyst is used within a range of about 0.1 to about 2 weight percent, based on the combined weight of the reactants. The reaction temperature of 130° C. or lower is said
Dawson Robert
Dow Global Technologies Inc.
Feely Michael J.
LandOfFree
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