Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-07-06
2002-11-26
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S196000, C156S309600, C361S818000, C257S660000, C174S356000
Reexamination Certificate
active
06485595
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic assemblies surrounded by enclosures that prevent passage of electromagnetic or radio frequency interference (RFI). Enclosing structures combine shielding covers with ground planes associated with the electronic assemblies. Shielding covers derive from shaped composite sheets comprising nonwoven mats of randomly oriented, low melting, metal fibers. The invention further relates to means for attaching shielding covers to ground planes to provide completed shielding enclosures.
2. Description of the Related Art
Electronic assemblies, containing interference-sensitive or signal generating devices, require isolation by shielding covers and ground planes to protect the devices or prevent damage by the signals they emit. Shielding covers often include an electrically conducting element as part of a composite. A variety of composites containing both metal and polymeric materials are known for use in many varied applications. Composites may include metal in the form of continuous sheet, perforated sheets, mesh, woven screen or non-woven webs of randomly distributed fibers. Similarly, polymer structures, combined with the various forms of metal, may include films, sheets, perforated sheets, woven material or non-woven layers with random fiber distribution. Regardless of the metal/polymer composite used, it must act as a shield for electromagnetic and radio frequency waves. The interference caused by such waves in electronic devices is commonly referred to as electromagnetic interference (EMI) or radio frequency interference (RFI), hereinafter jointly referred to as EMI.
Effective EMI shielding requires the formation of a uniform conductive enclosure around the EMI-sensitive or EMI-emitting device. U.S. Pat. No. 5,294,826 (Marcantonio et al) discloses a combined heat dissipation and laminated shielding cover which absorbs energy by magnetic effects and electro-conductive effects to shield against electromagnetic interference. The laminated cover combines layers of metallic copper on either side of a magnetic layer.
Suppression of radiated emissions from individual integrated circuits may involve the use of a shielded housing over an apparatus, e.g. a circuit board, that carries multiple integrated circuits. Such a housing is described in U.S. Pat. No. 4,661,888 (Jewell et al). The housing of U.S. Pat. No. 4,661,888 and the cover of U.S. Pat. No. 5,294,826 require attachment to a ground plane using conventional methods such as soldering to a grounded contact point or connection through a conductive gasket or multiple machine screws. These methods include time-consuming, additional steps.
An EMI shielding layer, associated with the conductive enclosure, may be in the form of a continuous layer or a discontinuous grid, such as a metal mesh or nonwoven fibrous metal mass. A continuous layer, such as a metallic plate, is the most effective for EMI shielding because no gaps exist to allow passage of EMI. However, when using a discontinuous grid, any enclosure formation process that significantly increases the maximum void dimension in the shielding layer, sometimes called the “slot effect”, could cause faulty EMI shielding performance of the shielding material. Void size increases in a variety of ways including, e.g. when the grid is stretched or by damage from tearing or other processes that can break the grid structure.
Previous disclosures reveal ways of producing and shaping sheet material to provide covers that have EMI shielding capability, typically using electrically conducting layers, which are required in many applications.
For example U.S. Pat. No. 3,272,292, (Nicely), discloses a non-woven unitary metallic sheet which is fabricated by extruding a molten stream from a metallic melt into an atmosphere which reacts to form a stabilizing film about the periphery of the metal stream. The spun metal filaments are allowed to solidify, and then collected as a nonwoven fibrous mass. The mass of filaments is then compressed into a sheet-like form, and given strength by binding all or selected adjacent fibers together.
U.S. Pat. No. 4,689,089 (Gaughan) discloses an EMI shielding sheet comprising a layer of nonwoven reinforcing fibers which supports a layer of metal whiskers or fibers formed from a ductile metal or metal alloy. The EMI shielding sheet is suitable for shaping of covers by stamping. Another stampable EMI shielding construction appears in U.S. Pat. No. 4,678,699 (Kritchevsky et al). This patent notes that, “The shielding layer must be able to maintain its shielding effectiveness upon stamping.” Kritchevsky further states that, “Holes formed upon tearing can dramatically reduce shielding.” This statement refers to tearing of the shielding layer. Such statements reflect the fact that stamping processes tend to disrupt fibrous networks, breaking the fibers which, in the case of EMI shielding, results in poorer shielding effectiveness of the metal layers. There is no teaching of how to reduce fiber or filament breakage to a minimum during shaping.
Stamping is one method for forming shaped EMI shielding structures. This forming technology was developed in the metal industry for forming thin metal objects. It involves rapid, almost instantaneous application of mechanical force to distort a sheet into a shaped object. Stampable plastic/metal composite sheets may require heating, to soften the plastic surrounding the metal shielding layer, prior to stamping. This reduces the modulus of the plastic, allowing it to flow while the metal shielding composite responds to the high pressure, shaping force of the stamping press. The speed of this process demands high levels of ductility for the metal and high plasticity for the remainder of the composite, to absorb the applied force without rupture. This method, applied to sheet molding compound (SMC), provides automotive body panels and business machine housings using reinforced material comprising a non-woven, glass-fiber reinforcing layer, and a mat containing conductive fibers for EMI shielding, held together with a resin such as polyester. The SMC is a flat sheet prior to forming in compression dies of high tonnage presses. Material properties limit the use of SMC to simple, relatively shallow shapes. Conditions used for sharp draws, e.g. multiple rib formation in the shaped panel, may cause ripping of the shielding layer and reduction of EMI shielding performance.
As a substitute for stamping, the use of thermoforming or injection molding may be considered. Thermoforming, as it relates to the present invention, comprises heating a sheet and forming it into a desired shape. The process includes heating a thermoplastic composite sheet until it becomes soft and pliable, then using either air pressure or vacuum to deflect the softened sheet towards the surface of a mold until the sheet adopts the shape of the mold surface. Upon cooling, the sheet sets in the required shape allowing removal from the mold. Disclosures in JP 1990-276297 (Nakanishi) suggest the use of vacuum formable EMI shielding sheets, employing sandwich structures of brass filaments between plastic films. One embodiment uses a non-woven cloth of synthetic resin to reinforce the brass filaments. Reinforcement involves needle-punching of the metal fibers into the non-woven web. Application of plastic film, on both sides of the reinforced shielding layer, completes the vacuum formable sheet. Information from JP 1990-276297 includes no evidence of the condition of the EMI shielding layer after vacuum forming.
European patent EP 529801, commonly assigned with the instant application, discloses EMI shielding, add-on sheets, comprising carrier material with a metal fiber mat at least partially embedded in the carrier material. The add-on sheets provide EMI shielding covers to selected parts of a thermoformed structure. Successful use of these add-on sheets requires that they possess or develop porosity when thermoformed in contact with the thermoformable substrate blank to which they were appli
Baker Mark G.
de Souza Jose P.
Yenni, Jr. Donald M.
3M Innovative Properties Company
Ball Alan
Ball Michael W.
Musser Barbara
LandOfFree
EMI shielding enclosures does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with EMI shielding enclosures, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and EMI shielding enclosures will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2935122