Photocurable form-in-place gasket for electronic applications

Details Photocurable form-in-place gasket for electronic applications Photocurable form-in-place gasket for electronic applications

2001-09-04

2003-12-30

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Circular sheet or circular blank

Seal, gasket, or packing

C522S015000, C522S083000, C522S104000, C522S107000, C522S170000, C522S181000, C523S466000, C528S089000, C528S090000, C528S091000, C528S093000, C528S408000

Reexamination Certificate

active

06670017

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to gasket materials used for attaching and sealing covers to enclosures. More particularly, the invention relates to form-in-place gaskets, applied to surfaces of containers for sensitive electronic components. Gasket compositions, according to the present invention, may be cured by a process including exposure to actinic radiation to convert them to a condition substantially free from deleterious effects of outgassing and ion contamination.
2. Discussion of the Related Art
Conventional methods for gasket manufacture substantially comprise either die-cutting the gasket out of an elastomeric sheet material, or shaping the gasket by injection-molding of an elastomeric mix or the like. Both these methods require expensive tools such as punches and molds, which add cost to the final product. Newer manufacturing methods deposit a bead or thread of a fluid elastomer from a nozzle onto a substantially planar surface. The pattern adopted by the fluid elastomer thread may be controlled using automated equipment, programmed according to selected coordinates to provide a gasket having a desired shape. After forming to a desired gasket pattern, the fluid elastomer thread may be cured, either at ambient temperature or in an oven, with or without accelerators or other additives.
Fluid elastomer compositions, suitable for form-in-place gaskets, include condensation-reaction curing silicone rubbers and addition-reaction curing silicone rubbers. These compositions have viscosities suitable for application using robot applicators which apply a sealing bead of material to at least one surface of at least one of the joining members. According to U.S. Pat. Nos. 4,643,863 and 4,643,864, fluid elastomers, suitable for controlled automated dispensing from a nozzle, include polyurethane, monocomponent or bicomponent silicone, and even polyvinylchloride compositions. An apparent disadvantage, of previously cited, nozzle-dispensed materials, is the need to provide support for the extruded bead of fluid elastomer. This problem was overcome with the development of a silicone rubber composition described in U.S. Pat. No. 5,684,110. Upon application of this silicone rubber composition to a substrate, it exhibits excellent resistance to distortion, under pressure, immediately after being applied and while curing to a highly pressure-resistant and strongly adhering silicone rubber gasket. In this case the silicone gasket composition cures via a combination of condensation-reaction curing and addition-reaction curing, the latter catalyzed with a platinum catalyst. The patent (U.S. Pat. No. 5,684,110) further reveals that the silicone rubber composition is a two-part formulation requiring a first silicone containing fluid to be added to a second fluid, comprising a silicone and catalyst, with intimate mixing immediately before robot application.
Another two-part silicone formulation, disclosed in U.S. Pat. No. 5,679,734, relates to compositions, which can be crosslinked by hydrosilylation at room temperature, in the presence of a metal compound catalyst. Crosslinking proceeds via an addition reaction involving hydrogen substituents and alkenyl radicals of the vinyl type. Storage stability is achieved by providing a system having at least two component parts. Upon mixing the component parts, a gel forms between a few minutes to 1 hour 30 minutes depending on the cure temperature which may be between room temperature and 180° C.
Silicone materials, of the type previously discussed, meet the needs of a variety of applications including use as sealants, shock-absorbing elements, anti-vibration elements and gaskets in electrical and electronic components. One special and particularly interesting form of gasket is the electromagnetic interference (EMI) shielding gasket. EMI shielding gaskets perform the dual protective function of sealing cover assemblies and enclosures to prevent ingress of contaminants and, at the same time, exerting control over interference from electromagnetic energy. Protective sealants, effective in controlling EMI, may be used as gaskets that typically require a flexible, elastomeric, rubber-like matrix filled with a conductive material that is readily distributed throughout the flexible matrix. The conductive material may take the form of particles, flakes or fibers having intrinsic conductivity, or electrically conductive surface coatings. U.S. Pat. No. 5,641,438 discloses conductive sealant materials for application using form-in-place methods, which accomplish accurate positioning of the sealant bead. Similar compositions and methods are revealed in related published European applications, EP 0643551 and EP 0653552. Each reference describes EMI shielding sealant compositions, consisting of two or more components, requiring storage in separate containers and mixing just before applying and curing the gasket in place.
The previous discussion addressed primarily silicone-based fluid elastomer compositions suitable for use in a variety of applications including form-in-place gaskets. One disadvantage of using silicone elastomers is the presence of relatively low molecular weight siloxane contaminants in cured materials. Such contaminants tend to deposit on surfaces of an electronic assembly with the potential to cause device failure. Problems of contamination may be avoided using fluoroelastomer gaskets. Since they are subject to formation by injection molding, fluoroelastomer gaskets represent a costly approach for preventing contamination. Difficulties associated with non-fluid gaskets and fluid silicone elastomers, for gasket formation, suggests the need for a non-silicone, dispensable, fluid material for contaminant-free, low cost, sealing of containers of electronic components and associated devices.
SUMMARY OF THE INVENTION
The present invention provides fluid elastomer compositions having reactive functionality, preferably in the form of epoxy groups. Dispensable elastomer compositions provide dispensable, form-in-place gaskets designed for containers such as enclosures for hard disk drives. For accuracy of dimensions, placement and final positioning, various patterns of form-in-place gaskets, according to the present invention, require the dispensing of elastomer compositions using automated liquid dispensing followed by in-place curing to soft, resilient gaskets, exhibiting moisture resistance, minimal compression set, and adhesion to selected substrates. The elastomer composition, before curing, should have sufficiently low viscosity to be readily dispensed. Curable compositions may be prepared as one-part formulations that include a curative or as two-part formulations requiring addition of a curative before curing. Regardless of the components included in an uncured gasket composition, the curing process may be initiated thermally, photonically, a combination of both, and/or, in the case of two part formulations, by simply combining the parts under ambient conditions. One preferred embodiment uses photocuring to initially solidify a gasket bead, which is then heated to complete the cure and aid removal of remaining volatile components. Preferably uncured formulations provide non-slumping dispensed beads of gasket material having good dimensional stability, as applied, with essentially no change in shape or position during and after curing.
For electronics grade cleanliness, properties of these elastomer compositions, after curing, include low outgassing and low extractable ionics. These properties surpass those of commercially available, silicone-based form-in-place gaskets which, as previously discussed, typically contain low molecular weight siloxanes that can damage electronic components following contamination of device surfaces. Since this invention utilizes a flexible epoxy based material, possible damage due to siloxane contamination is avoided.
More particularly the present invention provides a non-silicone composition for form-in-place gaskets comprising a liquid polyolefin oligomer, a reactive d

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