Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
2001-06-04
2004-04-13
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S413000, C428S447000, C428S300100, C428S312600, C442S180000, C252S008810
Reexamination Certificate
active
06720080
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to finishes for glass fabrics that are used to reinforce structures formed from epoxies and like materials. In particular, the invention relates to finishes employing silane based coupling agents for woven glass fibers. More specifically, the invention relates to glass fiber fabric reinforced circuit board laminates.
BACKGROUND OF THE INVENTION
The need for coupling agents was first recognized in 1940 when glass fibers began to be used as reinforcement in organic resin composite structures. Specific strength to weight ratios of early glass fiber resin composites were higher than those of aluminum or steel, but they lost much of their strength during prolonged exposure to moisture. The interface between such dissimilar materials as an organic polymer and an inorganic glass fiber did not allow the formation of a water resistant bond. A variety of materials have since been developed in an attempt to provide a stable interface under a varying number of adverse environments. These coupling agents can generally be described as molecules which possess two different kinds of reactivity. The siloxane portion of these molecules has reactivity with the glass, while the organic portion of the molecule is tailored to react with organic thermosetting resins used in composite manufacturing. The main function of the coupling agent is to provide a stable bond between two dissimilar surfaces.
The majority of such coupling agents have the general formula:
R CH
2
CH
2
CH
2
Si (OCH
3
)
3
Where R is a reactive organic group tailored to match the reactivity of the resin system with which it will be used.
The siloxane portion will react with the glass surface as:
More than one SiOH group may react with the glass surface, or alternatively with other silane molecules to form siloxane oligomers or polymers, which can still provide a link between glass and resin.
Epoxy resins have commonly been used in the manufacture of multilayered laminates for various applications in the electronic, recreation, marine, and aerospace industries. The most common epoxy is formed from epichlorohydrin and bisphenol A. The resin is usually provided in the form of a low molecular weight oligomer, which can be cross-linked with a bi-functional curing agent to result in a solid thermoset polymer. Catalysts are often added to accelerate the reaction with the curing agent. Multi functional epoxies are sometimes added to the resin mix to improve the high temperature resistance of the cured resin. Straight chain polymerization of epoxy can result in a solid material, which is thermoplastic and can be melted. Cross-linking with the curing agent provides a thermoset solid, which does not melt.
To make multilayered laminates, prepregs are first made by pulling glass cloth through a solution of the particular resin system chosen. The glass cloth is impregnated with the resin mix and then proceeds to a heated tower where the solvent is driven off and the resin is partially polymerized to a “B” stage. It is important that little, or no cross-links occur, before the resin can melt and flow in the laminating process. The prepreg process is tightly controlled to provide an optimum melt viscosity for lamination.
Prepregs are tested for melt viscosity, resin gel time and resin content. The gel time measurement is widely used in determining the potential reactivity of the prepreg material, as well as, the time available for resin flow in press lamination. Controlling these parameters has been thought to be critical if a void free cured laminate is to be obtained. Gel time is also an indicator of the rate of increase of melt viscosity in press lamination.
A common problem for multilayered laminates, such as circuit board is delamination during wave soldering. The most common cause of delamination during soldering is moisture absorption. The thermal energy imparted to the board in contact with 550° F. solder vaporizes any absorbed moisture and the resulting steam pressure forces the laminations apart at the line of the weakest bond. Moisture, which accumulates in even minute voids, is especially likely to produce blistering during the soldering process.
Accordingly, it is one objective of the present invention is to eliminate voids and the resulting entrapped moisture in laminated fiberglass/epoxy composites, so that rupture of the structures will not occur due to the thermal shock of the soldering process.
In the preparation of woven glass fabrics for use in a composite epoxy structure for circuit boards, organofunctional alkoxysilane finishes are applied to heat cleaned fiberglass fabrics from dilute aqueous solutions. The finish content of the dried fabric is typically 0.075% to 0.30% of the fabric weight.
Epoxy resins are usually formulated with difunctional or multi functional curing agents, which provide cross-linking thereby resulting in a thermoset polymer after curing. A catalyst to accelerate the curing is often added to the epoxy formulation. In some cases epoxy resin, curing agent, and catalyst are applied to a glass fiber substrate in solution, and dried at an elevated temperature to remove the solvent. In some applications, it is advantageous to continue heating impregnated substrates to increase the resin molecular weight, and thus its melt viscosity. This is known as a “B” stage prepreg. Any significant cross-linking in the B stage prepreg will prevent the resin from melting and flowing in a heated press during consolidation and curing of laminates. The typical epoxy resin formulation for making circuit board prepreg comprises epoxy resin dissolved in an organic solvent, dicyandiamide (dicy) curing agent, and an imidazole accelerating catalyst.
Electrical grade laminates for circuit boards are made by curing layers of epoxy/fiberglass prepreg between copper foil surface sheets. In the laminating process, multiple sheets of epoxy/fiberglass prepreg are placed between copper foil surface sheets. These lay ups are placed between metal laminating plates. A number of these assemblies are stacked to form a book, and each book is placed between heating platens in a multi-opening press. Two competing processes occur as the prepreg is heated in the press. First, the epoxy resin melts, and its viscosity is reduced with increasing temperature. As the temperature rises, the resin begins to polymerize and increase in viscosity. Finally the resin is sufficiently cross-linked that it gels and can no longer flow. Consolidation of the laminate must be completed before the resin gels. Complete cure is achieved with additional time in the press and increased temperature. The two processes must be carefully balanced to insure a void free laminate with good thickness control, and minimize resin loss at the laminate edges. If the resin gels too soon, there may not be sufficient flow to remove solvent or air trapped in the capillaries between individual filaments in a fiber bundle. Minute voids in the capillary spaces of the cured laminate are often referred to as silver streaks.
In circuit board manufacturing, the copper clad boards are coated with a photosensitive acid resist. The desired circuitry is then photo printed on the copper. The board is then subjected to a hot acid bath to remove the unwanted copper. Holes are drilled for mounting surface components, or for establishing electrically conductive connections between circuits on both sides of the board.
The holes are then electroplated. Finally, the board with its assembled components is floated across a 550° F. molten solder bath. Any moisture, which has been absorbed into a void or silver streak during the wet processing of the board, will cause it to blister. If a void stretches between two adjacent through holes, it may cause a short circuit in the finished assembly.
As with most manufacturing processes, it is desirable to maximize the productivity of capital equipment. For some applications, a high laminate glass transition temperature is required. These objectives can be achieved by increasing the catalyst, the curing agent, or the pr
Adams Richard G.
Carter H. Landis
Murari Shobha
JPS Glass and Industrial Fabrics
McNair Law Firm, P.A.
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