Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2002-03-07
2004-02-24
Lam, Cathy (Department: 1771)
Stock material or miscellaneous articles
Composite
Of metal
C428S209000, C428S626000
Reexamination Certificate
active
06696163
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to low dielectric constant films suitable for use in flexible circuit applications and more particularly to chemical etching of flexible films and composites that include liquid crystal polymers (LCP).
BACKGROUND TO THE INVENTION
An etched copper or printed polymer thick film circuit pattern over a polymer film base may be referred to as a flexible circuit or flexible printed wiring. As the name suggests, flexible circuitry can move, bend and twist without damaging the conductors to permit conformity to different shapes and unique package sizes. Originally designed to replace bulky wiring harnesses, flexible circuitry is often the only solution for the miniaturization and movement needed for current, cutting-edge electronic assemblies. Thin, lightweight and ideal for complicated devices, flexible circuit design solutions range from single-sided conductive paths to complex, multilayer three-dimensional packages. In a recent development, a flexible circuit provides connection to the magnetic head carried by the slider of a disk drive suspension assembly. This overcomes difficulties of connecting disk drive circuitry to small magneto-resistive (MR) recording heads.
Commonly used dielectric film base materials for flexible electronic packaging include polyimide, polyester terephthalate, random-fiber aramid and polyvinyl chloride. Changes in electronic device design create the need for new materials with properties surpassing the electrical performance and processing capabilities of the substrates listed previously. For example, a lower dielectric constant allows faster electrical signal transfer, good thermal performance facilitates cooling for a package, a higher glass transition or melting temperature improves package performance at higher temperature, and lower moisture absorption leads to signal and data processing at higher and higher frequencies.
Polyimide film is a commonly used substrate for flexible circuits that fulfil the requirements of complex, cutting-edge electronic assemblies. The film has excellent properties such as thermal stability and low dielectric constant, but represents a limiting factor to additional gain in the speed or frequency at which electronic components may operate. A major drawback to further progress using polyimide film relates to the way in which polyimide absorbs moisture to levels that interfere with high frequency device performance. Higher frequency operation will require the identification or development of substrate materials with less susceptibility to moisture absorption.
Liquid crystal polymer (LCP) films represent suitable materials as substrates for flexible circuits having improved high frequency performance. Generally they have lower dielectric loss, and absorb less moisture than polyimide films. These beneficial properties of liquid crystal polymers were known previously but difficulties with processing prevented application of liquid crystal polymers to complex electronic assemblies.
A laminate including a layer of copper bonded to a layer of liquid crystalline polymer provides a suitable starting material for printed wiring board manufacture. U.S. Pat. No. 4,737,398 describes laminate formation in a process that applies molten liquid crystalline polymer to the surface of a metal foil. International application WO 00/23987 describes the use of a high temperature laminating press to form a laminate material having a liquid crystal polymer melted for bonding a stainless steel foil to a copper foil. There is no measure of interlayer bond strength following lamination. This tri-layer material appears useful for applications referred to as flex-on-suspension (FOS) and trace suspension assemblies (TSA) and related disk drive suspension assemblies. However, device manufacture was described only in vague terms of conventional processing. Also there is nothing to show successful achievement of device performance in its intended application.
The development of multiaxial, e.g. biaxial, film processing techniques expanded the use of liquid crystal polymer film for flexible circuit applications. U.S. Pat. No. 4,975,312 describes a printed wiring board substrate prepared from a multiaxially oriented thermotropic liquid crystalline polymer film having a tailored coefficient of thermal expansion in the X-Y direction and a thickness of not more than about 100 &mgr;m. Materials of this type offer several potential advantages over polyimide films as flex circuit substrates. Such potential advantages led to the use of readily available processing techniques for producing single layer or multilayer circuit structures supported by one or more layers of a liquid crystal film substrate. A multilayer flexible circuit is a combination of three or more layers of single or double-sided flexible circuits laminated together and processed with drill and plating to form plated through-holes. This creates conductive paths between the various layers without having to use multiple soldering operations.
Reference to drilling for the formation of through-holes reflects the emphasis on physical methods such as mechanical drilling, punching, laser ablation and plasma drilling for via and related circuit feature formation in liquid crystal polymer films. An alternative to conventional drilling and related techniques for hole formation in flexible circuit substrates was introduced as Y-FLEX™ by Yamaichi Corporation. Information describing Y-FLEX™ presents it as a microvia flexible wiring board using LCP resin insulation material and employing an internal conductive bump layer connection. Interconnection of Y-FLEX™ multilayer circuits occurs by conductive bumps penetrating through insulating LCP layers without the need for through-holes.
Although the several physical methods outlined above produce holes and related shaped voids in LCP, there are no reports of chemical methods for producing flexible circuits using liquid crystalline polymer substrates. Chemical etchant solutions for polyimide substrates are well known for production of polyimide-based flexible circuits. However, as shown in European Patent Application No. EP 0832918 A1, there is no single etchant composition capable of effecting development of circuit features in all types of polyimide. It appears that selection of etchant solutions depends upon the materials used for preparing a specified polyimide. Also aqueous developable photoresists disintegrate under the vigorous attack of etchant compositions described in the published application (EP 0832918).
Having less solubility than polyimide films, liquid crystal polymer films cannot be processed effectively using in-line chemical systems and known etchant compositions. There appears to be only limited information referring directly to etchant compositions for liquid crystal polymers. U.S. Pat. Nos. 5,891,795 and 5,896,899 describe a metallization process making reference to liquid crystal polymers among a number of suitable substrates. The metallization process includes electroless deposition or vacuum deposition of a seed layer to be augmented with additional metal layers using conventional plating techniques. A function of the seed layer is to provide an adhesive link between the plated layers and the substrate. However, there is nothing to demonstrate how well the seed layer adheres to any of the substrates listed by either of the references. Published application WO 99/39021 describes electrolytic plating of elemental palladium either sputtered or ion-plated on the surface of liquid crystal polymers.
U.S. Pat. No. 5,443,865 describes a relatively complex process for applying metal seed layers to substrates. Reference to liquid crystal polymers is absent from all but the patent claims. European patent application EP 1069209 describes the plating of plastics using catalytic filler contained within the plastic. Formation of a metal layer on a liquid crystal plastic requires treatment with an alkaline solution to dissolve the plastic followed by treatment with acid to activate the exposed catalyst. The exposed
3M Innovative Properties Company
Ball Alan
Gover Melanie G.
Lam Cathy
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