Electronic assemblies with elastomeric members made from...

Details Electronic assemblies with elastomeric members made from... Electronic assemblies with elastomeric members made from...

1996-05-17

2002-06-11

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Composite

Of silicon containing

C428S209000, C428S450000, C439S086000, C525S478000, C525S479000, C528S015000, C528S031000, C528S032000

Reexamination Certificate

active

06403226

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic assemblies for electronic interconnect applications. More particularly, the present invention relates to electronic assemblies which include an elastomeric member made of a cured, room-temperature curable polysiloxane composition. When the assemblies are used to electrically interconnect a first contacting site on a first electronic device to a second contacting site on a second electronic device, the stress-relaxation resistant properties of the elastomer enhance local contact force to maintain a reliable connection. In addition these polysiloxane compositions exhibit exceptional stress-relaxation resistance during high temperature aging.
2. Description of Related Art
Conventional electrical connectors using metal pin and spring beam contacts cannot be easily miniaturized to satisfy the anticipated pin count density for high performance electronic devices. The electrical characteristics of these connectors cannot meet requirements such as propagation delay, risetime degradation, reflection, and crosstalk. The increasing demand for higher speeds and higher I/O contact density in electronic devices has led to the development of new connectors which utilize non-metallic components to produce and maintain contact force.
For example, some connectors are no more than elastomeric matrices loaded with electrically conductive materials. The elastomeric matrix is placed between the contacts on a first electronic device and the contacts on a second electronic device, the devices are pressed together, and the conductive materials provide electrical interconnection. Such connectors are well known, and examples are shown in U.S. Pat. No. 5,049,085 to Reylek, U.S. Pat. No. 4,008,300 to Ponn, U.S. Pat. No. 5,037,312 to Casciotti et al., U.S. Pat. No. 5,275,856 to Calhoun, and U.S. Pat. No. 4,003,621 to Lamp. The connectors described in these patents utilize a wide variety of elastomeric materials, including butadiene-styrene, butadiene-acrylonitrile and butadiene-isobutylene rubbers, chloroprene and polysulfide polymers, polyvinyl chloride, vinyl acetates, polyurethanes and silicone rubbers. The '621 patent to Lamp states that silicones are preferred, and these materials may be selected from dimethyl, methyl-phenyl, methyl-vinyl or halogenated siloxanes. These silicones may be cured with peroxides or metal salts. The Lamp '621 patent further states that a useful silicone should not deform under its own weight and should not plastically deform after curing.
In some applications, particularly with flexible circuits, the standard metal pin and metal spring socket contact is replaced with a contact in which electrical interconnection is established by mechanically pressing a first contact pad on the circuit to a second contact pad on the connector, device or other circuit. The pressure connections are normally made with a resilient pressure applicator, such as an elastomeric member. The elastomeric member is compressed to bias at least one of the components to be electrically interconnected toward the other components to hold the contact pads thereof in electrical contact. Examples include U.S. Pat. No. 5,009,607 to Gordon et al., U.S. Pat. No. 5,186,632 to Horton et al., U.S. Pat. No. 5,059,129 to Brodsky et al., U.S. Pat. No. 5,313,368 to Volz et al., U.S. Pat. No. 4,636,018 to Stillie, and U.S. Pat. No. 3,967,162 to Ceresa et al. These patents teach that a wide variety of polymeric materials may be used as the elastomeric member, and silicone rubbers are in many cases preferred. For example, the '129 patent to Brodsky states that important properties of the elastomeric material include long-term stress retention, low magnitude pressure against the contacts, and resistance to high temperatures, solvents and humidity. The preferred elastomeric material in the '129 patent is a low compression set polysiloxane (silicone) rubber.
In addition, it is well known that elastomeric compressive members may be used to bias a component against a connector, a circuit, or another device. Examples include U.S. Pat. No. 5,345,364 to Biernath, U.S. Pat. No. 4,867,689 to Redmond et al., and U.S. Pat. No. 4,548,451 to Benarr et al. For example, the '451 patent to Benarr states that any elastomeric material which maintains a “uniform compressive force” may be used as the compressive member, such as silicone or polyurethane. The '364 patent to Biernath states that the elastomeric component may comprise rubbers, foams and the like.
Therefore, it is well known to use rubbery materials, particularly silicones, with low compression set as an elastomeric member in an electronic connector. Compression set resistance is defined as the ability of an elastomeric material to recover its pre-stressed shape after removal of the stressing members (ASTM D 395). The compression set resistance is a measure of a dimensional change in an elastomeric material following removal of an applied stress.
However, the principal function of an electrical connector is to maintain electrical interconnection between a first set of contacts on a first device and a second set of contacts on a second device. If reliable electrical interconnection is to be maintained, the force applied by the connector at the contact interface must remain substantially constant, especially when the connector is exposed to an externally applied mechanical force, or to environmental stresses such as heat, humidity, solvents, and the like. If an elastomer is used as a component part of such a connector, the elastomer selected must have the ability to maintain the normal force at the contact interface, which is referred to in the art as the “contact force,” rather than simply maintaining its pre-stressed dimensional shape.
Force-bearing elastomers in electronic components must have stable force-bearing capabilities at high temperatures for long durations of time (e.g., 1000 hours at 125° C.). These requirements are dictated by their usage and standardized by standards organizations (See, for example: Military Standard 1344A
Test Methods for Electrical Connectors
.).
If the resistance at the contact interface is to remain low and the contact force is to remain high, the normal force exerted by the silicone elastomer at the contact interface must remain high following extended exposure to mechanical force and to the environment. Therefore, for electronic connectors, a silicone elastomeric material is needed in which a high percentage of this normal force is retained in the portion of the elastomer adjacent to the contact interface following exposure to mechanical and environmental stress. The proper parameter to measure a silicone elastomer's suitability for use in electronic connectors is the stress relaxation resistance, which is a measure of the percent of the applied mechanical force retained by the material after exposure to both mechanical stress and the environment.
The references discussed above teach that an elastomer with low compression set, preferably a silicone elastomer, is well suited to maintain electrical interconnection in an electronic device. However, there is no direct correlation that can be established between compression set resistance (a dimensional property) and stress relaxation resistance (a force/pressure property). For example, an elastomer that exhibits 100% initial size recovery (thus, 0% compression set) after aging may require only a fraction of the initial force loading to re-compress the material. The compression set resistance of a silicone elastomeric material is therefore an insufficient measure of its suitability to maintain contact force in an electronic connector application.
In addition to the requirement of excellent stress relaxation resistance, a silicone elastomeric material selected for use in an electronic connector must be easily moldable to a wide variety of highly precise shapes. The silicone must flow easily to adapt to the precise dimensions of the mold. During the curing process, the

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