Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
1998-03-31
2001-07-03
Cuchlinski, Jr., William A. (Department: 3661)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S034000, C174S050510, C428S364000, C307S091000, C029S739000, C029S840000, C029S843000, C029S846000, C029S850000
Reexamination Certificate
active
06255581
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to an improved electromagnetic interference (EMI) gasket. More particularly, the present invention relates to an EMI gasket which is compatible with surface mount technology installation equipment.
BACKGROUND OF THE INVENTION
An EMI gasket is a conductive interface material which is used to electrically connect an electrically conductive shield with a corresponding section of an electrical ground, such as a ground trace of a printed circuit board (PCB). Preferably, an EMI gasket should be highly electrically-conductive and conformal. Such a conductive interface material is required when mating surfaces of an electronic apparatus are not exactly conformably dimensioned, such that gaps are formed upon mating engagement of the mating surfaces. These gaps permit undesirable internal and external electromagnetic interference (EMI) which can cause the disruption of the electronic apparatus.
Presently, EMI gaskets are almost exclusively installed directly onto a conductive surface. More particularly, present manufacturing techniques for installing EMI gaskets include the following: dispensing a conductive paste or a conductive liquid material directly onto a conductive surface and curing the dispensed material in-situ; die-cutting a conductive sheet material having an adhesive backer and then transferring, positioning and adhering the dimensioned material directly to a conductive surface; or mechanically fastening a conductive material to a conductive surface.
Although the aforementioned manufacturing and installation techniques are effective in certain instances, shortcomings associated with these manufacturing and installation techniques include: complicated, cumbersome, labor-intensive and expensive automation equipment; and ineffective adhesion to certain conductive surfaces. Additionally, logistic complications may be caused by multiple or even duplicative shipments of parts and materials for processing among a diverse group of vendors.
While discrete, additional EMI gasket installation equipment is generally undesirable and labor-intensive, surface-mount technology (SMT) machines are well-known, high-speed machines which are in widespread use in the electronics industry. For example, SMT machines are widely utilized by cellular phone manufacturers to populate printed circuit boards (PCBs).
As is well understood by those skilled in the art, SMT machines utilize a vacuum head on the end of a high-speed gantry system to pick and place tape-and-reel fed PCB components onto surface-mount pads on a PCB. These pads are usually pre-screened with solder-paste and then sent through a solder reflow oven (such as infrared—IR, vapor-phase, or convection) to melt the solder joints, thereby forming an electrical and mechanical connection.
In an effort to eliminate the use of EMI gaskets, SMT-compatible “cans” were developed, which are simply formed or drawn metal shields that can be soldered to a ground trace of a PCB, thereby effectively forming a Faraday Cage. This serves, therefore, to eliminate the gasket from the entire process. Shortcomings associated with the use of soldered cans include: difficulty in the re-work of a soldered can; inspection of components underneath a soldered can be extremely difficult; and when large cans are desired, the non-flatness of the cans prevents proper solder joints from forming.
Alternatively, metal spring-finger contacts may be employed which can be SMT-fed; however, such metal spring-finger contacts provide only discrete grounding points between a shield and PCB ground trace, and thus are ineffective as operating frequencies continue to rise.
The foregoing illustrates limitations known to exist in present EMI gaskets and EMI gasket installation methods. Thus, it is apparent that it would be advantageous to provide an improved EMI gasket directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided, including features more fully disclosed hereinafter.
SUMMARY OF THE INVENTION
The present invention advances the art of EMI gaskets beyond which is known to date. In one aspect of the present invention, an SMT compatible EMI gasket is provided having a dimensioned, electrically conductive gasket material which is adhered, molded or affixed to a similarly dimensioned electrically conductive support material. The dimensioned electrically conductive gasket material and the electrically conductive support material are disposed in electrical contacting relation, one to each other. The electrically conductive support material is of a material type that effectively forms a bond with solder. In an alternate embodiment of the invention, the electrically conductive gasket material itself may be solderable, eliminating the need for the electrically conductive support material entirely.
The EMI gasket of the present invention is uniquely adapted to be installed utilizing a conventional tape-and-reel SMT compatible system. In such a system, an SMT machine's vacuum (or gripper) head picks and places an EMI gasket directly onto a ground location, such as a location on a ground trace of a PCB, which has been previously screened with solder-paste. At an appropriate manufacturing step, the solder is reflowed thereby bonding the EMI gasket to the ground. The EMI gasket assembly may be employed individually, or in combination with other similar EMI gaskets or additional assemblies, to form a suitable conductive interface.
The electrically conductive gasket material can be fabricated from any suitable electrically conductive material, such as GORE-SHIELD® brand EMI gasket material, type GS500, GS3000 or GS5200, for example. Preferably, the means for affixing the electrically conductive gasket material to the support layer comprises a conductive or non-conductive adhesive. The support layer can be fabricated from any suitable solderable material. Solder paste may also be provided on the support layer for effecting the securement of the support layer to an object of interest, such as a ground trace, during solder reflow operations.
In one embodiment of the present invention, the electrically conductive gasket material is fabricated from an expandable particulate blended into a polytetrafluoroethylene (PTFE) and conductive metal composition. Specifically, in one embodiment, the expandable particulate comprises a polymeric shell having a central core comprised of a fluid material. The central core can include a liquid material or a gaseous material. The polymeric shell has copolymers selected from a group consisting of: vinyl chloride and vinylidene chloride; vinyl chloride and acrylonitrile; vinylidene chloride and acrylonitrile; methacrylonitrile and acrylonitrile; and styrene and acrylonitrile. In another embodiment, the expandable particulate comprises unexpanded microspheres containing a blowing agent, wherein the blowing agent comprises 5 to 30 percent by weight of the microsphere, and is selected from a group consisting of: ethane; ethylene; propane; butane; isobutane; isopentane; neopentane; acetylene; hexane; and heptane. Alternatively, the blowing agent can include aliphatic hydrocarbons having a number average molecular weight of at least 26, and a boiling point at atmospheric pressure about the same temperature range or below the range of the softening point of the resinous material of the polymeric shell.
In another embodiment of the present invention, the electrically conductive gasket material is fabricated from a mixture comprising: electrically conductive particulate; PTFE, in the form of paste, dispersion or powder; and microspheres, in the form of a dry powder or solution. Specifically, the mixture is mixed in proportions of at least 20 to 90 volume percent conductive particulate, 3 to 15 volume percent microspheres, and 5 to 70 volume percent PTFE, and preferably in proportions of 60 volume percent conductive particulate, 6 volume percent microspheres and 34 volume percent PTFE. The electrically conductive particulate can be selected from a group
King David R.
Reis Bradley E.
Cuchlinski Jr. William A.
Gore Enterprise Holdings Inc.
Mancho Ronnie
Wheatcraft Allan M.
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