Composites of powdered fillers and polymer matrix

Details Composites of powdered fillers and polymer matrix Composites of powdered fillers and polymer matrix

1999-07-02

2002-05-21

Mai, Ngoclan (Department: 1714)

Specialized metallurgical processes, compositions for use therei

Compositions

Consolidated metal powder compositions

C149S005000, C149S010000, C428S548000

Reexamination Certificate

active

06391082

ABSTRACT:

TECHNICAL FIELD
The invention is concerned with methods for the manufacture of composite materials consisting of particles of finely powdered filler material bonded in a matrix of polymer material, and new composite materials made by such methods.
BACKGROUND ART
The electronics industry is an example of one which makes substantial use of printed wiring boards and substrates as supports and dielectric participants for electronic circuits, such substrates consisting of thin flat pieces produced to exacting specifications as to starting material and physical and electrical properties. The history of the industry shows the use of progressively higher operating frequencies and currently for frequencies up to about 800 megahertz (MHz) copper coated circuit boards of glass fiber reinforced polymers, such as epoxies, cyanide esters, polytetrafluoroethylene (PTFE) and polyimides, have been and are still used. At present one popular laminate material for such applications is FR-4, consisting of epoxy resin deposited on a woven glass fabric, because of its ease of manufacture and low cost. Typically this material has a dielectric constant of 4.3-4.6 and a dissipation factor of 0.016-0.022 and is frequently used in computer related applications below about 500 MHz frequencies. Mobile telephones now operate at frequencies of 1-40 GHz and some computers already at 0.5 GHz, with the prospect of higher frequencies in the future. The lowest possible value of dielectric constant is preferred in computer applications to improve signal speed. At higher operating frequencies above approximately 0.8 GHz, FR-4 and similar materials are materials, despite their low cost, are no longer entirely suitable, primarily because of unacceptable dielectric losses, heating up, lack of sufficient uniformity, unacceptable anisotropy, unacceptable mismatch of thermal expansion between the dielectric material and its metallization, and anisotropic thermal expansion problems as the operating temperatures of the substrates fluctuate. These thermal expansion problems result from the relatively large coefficients of thermal expansion of the polymers used as substrate material, and the unequal expansion coefficients of reinforcing fibers in their length and thickness dimensions. For frequencies above 800 MHz the dielectric material of the substrates become an active capacitative participant in signal propagation and here the current materials of choice are certain ceramics formed by sintering or firing suitable powdered inorganic materials, such as fused silica; alumina; aluminum nitride; boron nitride; barium titanate; barium titanate complexes such as Ba(Mg
⅓
Ti
⅔
)O
2
, Ba(Zr,Sn)TiO
4
, and BaTiO
3
doped with Sc
2
O
3
and rare earth oxides; alkoxide-derived SrZrO
3
and SrTiO
3
; and pyrochlore structured Ba(Zr,Nb) oxides. Substrates have also been employed consisting of metal powders, and semiconductor powders embedded in a glass or polymer matrix, a particular preferred family of polymers being those based on PTFE.
For example, ceramic substrates that have been used for hybrid electronic circuit applications comprise square plates of 5 cm (2 ins) side, their production usually involving the preparation of a “slip” (slurry) of the finely powdered materials dispersed in a liquid vehicle, dewatering the slip to a stiff leathery mixture, making a “green” preform from the mixture, and then sintering the preform to become the final substrate plate. The substrates are required to have highly uniform values of thickness, grain size, grain structure, density, surface flatness and surface finish, with the purpose of obtaining uniform dielectric, thermal and chemical properties, and also to permit the uniform application to the surfaces of fine lines of conductive or resistive metals or inks.
Such sintered products inherently contain a number of special and very characteristic types of flaws. A first consists of fine holes created by the entrainment of bubbles in the ceramic pre-casting slip of sizes in the range about 1-20 micrometers; these bubbles cannot be removed from the slip by known methods and cause residual porosity in the body. As an example, sintered alumina substrates may have as many as 800 residual bubble holes per sq/cm of surface (5,000 per sq/in). Another flaw is triple-point holes at the junctions of the ceramic granules when the substrate has been formed by roll-compacting of spray-dried powder; they are of similar size to the bubble holes and appear in similar numbers per sq/cm. Yet another consists of “knit-lines”, which are webs or networks of seam lines of lower density formed at the contact areas between butting particles during cold pressing. Two other common flaws are caused by inclusions of foreign matter into the material during processing, and the enlarged grains caused by agglomeration of the particles despite their initial fine grinding. The usual inclusions are fine particles due to abrasive wear of the grinding media in the mills. Both inclusions and agglomerates will sinter in a matrix at a different rate from the remainder of the matrix and can result in flaws of much greater magnitude than the original inclusion or agglomerate.
Costly mirror-finishing by diamond machining and lapping of the ceramic surfaces is required to allow the accurate deposition of sputtered metallization layers from which conductor lines are formed by etching. Mirror-finishes are required because the electrical currents at frequencies above 0.8 GHz move predominantly in the skin of a conductor line while in the lower frequencies they occupy the entire crossection of the conductor line. The thickness of the skin through which currents move at GHz frquencies becomes thinner as frequencies rise and are already as thin as 1 to 2 micrometers in copper at around 2 GHz. Any surface roughness of the conductors on the top and bottom sides will therefor contribute to considerable conductive losses. For example, at a frequency of 4 GHz, the conductive loss at of the interface between conductor and substrate is 1.65 times higher at a RMS roughness of 40 compared to a RMS roughness of 5 (See P.42 of Materials and Processes for Microwave Hybrids, Richard Brown, published 1989 by the International Society for Hybrid Microelectronics of Reston, Va.)
There is therefore continuing interest in methods for manufacturing composite materials for the production of electronic substrates and for use as electronic packaging materials, with which sintering and the high processing temperatures required together with their attendant difficulties, high cost of diamond machining and lapping, and the associated considerable costs are avoided.
The low inherent mechanical strength of the currently available matrix forming polymers and their excessive thermal expansion coefficient has made it necessary to embed reinforcing materials, such as woven glass fiber cloth, into the substrate body, to strengthen it and also to contrain its excessive thermal expansion. But such reinforcing materials unfortunately cause unacceptable inhomogenity of the structure. For example, the presence of such reinforcing material makes it difficult to incorporate powdered filler materials uniformly into the body of the substrate. Another difficulty is caused by the generally poor adhesion exhibited by the commercially available matrix polymers toward the usual filler materials, and extensive research and development has been undertaken in the past, and is continuing, in connection with known substrate-forming polymers, such as PTFE, to find coupling agents that will provide adequate adhesion between the polymer and the powder components, and thus satisfactory mechanical strength in the resultant substrates.
Dielectric materials are commonly used as insulating layers between circuits, and layers of circuits in multilayer integrated circuits, the most commonly used of which is silica, which in its various modifications has dielectric constants of the order of 3.0-5.0, more usually 4.0-4.5. Low values of dielectric constant are preferred and organic polymers

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