Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2000-07-18
2002-06-11
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S304400, C428S314200, C428S315700, C216S056000
Reexamination Certificate
active
06403211
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 composites including liquid crystal polymer (LCP) films.
BACKGROUND OF 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.
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.
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 insluating 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 crytalline 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 are no reports of chemically etching liquid crystal polymer films to form through holes. Chemical etching to form through holes is advantageous because it leads to the formation of unsupported or cantilevered lead structures which cannot be produced by conventional physical methods.
Since some steps of physical drilling and related processes tend to involve expensive equipment, set apart from the main flex circuit production line, there is a need for a more cost effective method for producing flexible circuits using liquid crystal polymer substrates. A further benefit would be the provision of flexible circuits including unsupported leads.
SUMMARY OF THE INVENTION
The present invention provides an aqueous-based chemical solution for controllable etching of through-holes and other shaped voids in films comprising liquid crystal polymers (LCP) as flexible circuit substrates. LCP films may be etched at rates exceeding those currently attainable with Kapton polyimide film. This results from adjusting the composition of the chemical etch solution. The new etchant makes possible the alternative use of LCP film to replace polyimide as an etchable substrate for flex circuit manufacture, especially high performance flex circuits. Chemically etched LCP flexible circuits will meet the needs of more sophisticated electronic assemblies, satisfying new opportunities that exist beyond the capabilities of polyimide and LCP film processed using drilling, laser ablation and related conventional physical methods.
The highly alkaline developing solution, referred to herein as an etchant, comprises an alkali metal salt and a solubilizer. A solution of an alkali metal salt alone may be used as an etchant for polyimide but is ineffective for developing LCP in the absence of the solubilizer. Typically the solubilizer is an amine compound preferably an alkanolamine. The efficacy of an amine in an etchant solution according to the present invention depends upon its use with a relatively narrow range of concentrations of alkali metal salts including an alkali metal hydroxide particularly potassium hydroxide. This suggests that there is a dual mechanism at work for developing flex circuits based upon liquid crystal polymers, i.e. the amine acts as a solubilizer for the LCP but preferably within a limited range of concentrations of alkali metal salt in aqueous solution. Discovery of a limited range of etchant solutions allows the manufacture of flexible printed circuits having finely structured features previously unattainable using current methods, of drilling, punching or l
Mao Guoping
Yang Rui
3M Innovative Properties Company
Ball Alan
Chernivec Gerald F.
Griswold Gary L.
Vo Hai
LandOfFree
Liquid crystal polymer for flexible circuits does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid crystal polymer for flexible circuits, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal polymer for flexible circuits will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2977597