Highly filled composites of powdered fillers and polymer matrix

Details Highly filled composites of powdered fillers and polymer matrix Highly filled composites of powdered fillers and polymer matrix

2001-10-05

2003-11-25

Mai, Ngoclan (Department: 1742)

Powder metallurgy processes

Powder metallurgy processes with heating or sintering

Metal and nonmetal in final product

C419S003000, C419S005000, C419S030000, C264S122000, C264S204000

Reexamination Certificate

active

06652805

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is concerned with methods for the manufacture of highly filled composite materials consisting of finely powdered filler material in a matrix of polymer material, and new highly filled composite materials made by such methods.
2. General Background and State of the Art
The electronics industry is an example of one which makes substantial use of 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. One popular laminate material for such applications is known as 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 employing frequencies below about 500 MHz. The lowest possible value of dielectric constant is preferred in computer applications to improve signal speed. Some computers now operate at 2.0 GHz, while mobile telephones now operate at frequencies of 1-40 GHz, with the prospect of higher frequencies in the future. At higher operating frequencies above approximately 0.8 GHz, FR-4 and similar materials, despite their low cost, are no longer entirely suitable, primarily because of unacceptable dielectric losses, excessive heating, 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 becomes an active capacitive 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 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 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 substantially uniform dielectric, thermal and chemical properties, and also to permit the substantially 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 cross-section. The thickness of the skin through which currents move at GHz frequencies 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 will therefore contribute to 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 with an RMS roughness of 40, compared to an RMS roughness of 5 (See P.42 of Materials and Processes for Microwave Hybrids, R. 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 coefficients has made it necessary to embed reinforcing materials, such as woven glass fiber cloth, into the substrate body, to strengthen it and also to constrain this excessive thermal expansion. But such reinforcing materials unfortunately cause unacceptable inhomogeneity of the structure. For example, their presence makes it difficult to incorporate powdered filler materials into the body of the substrate with a high degree of uniformity. 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 p

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