Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
2001-08-29
2003-07-15
Ngo, Hung V. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
Reexamination Certificate
active
06593524
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to EMF/RFI shielding for electronic components and more particularly to an improved shielded plastic enclosure part and the methods developed to provide this internal metal shield within a plastic enclosure. These objects are provided by the product of the present invention for providing an automatically shaped and assembled combination of metal foil shielding and plastic enclosure for encasing products needing EMI/RFI shielding.
BACKGROUND ART
Electronic equipment such as computers, printers, cellular phones, and most other products require surrounding shielding that serves to block electromagnetic interference/radio frequency interference (EMI/RFI). This shielding serves three major purposes.
First, various components and circuits of electronic equipment are capable of emitting electromagnetic radiation at a variety of frequencies. In developed countries, which form the most substantial markets for these types of devices, governmental agencies have set maximal acceptable limits for EMI/RFI radiation.
Second, external sources of EMI/RFI radiation can interfere with the functioning of sensitive electronic parts within such devices. Thus, EMI/RFI containment is necessary in order for these devices to perform to commercially acceptable standards. Although some progress in containing the emissions is made by adjustments to the circuits themselves, the requirement for, and use of, grounded conductive surfaces, generally involving the product's normal interior enclosure surfaces, is nearly universal.
Thirdly, grounded conductive interior case surfaces or added shields provide the electronics contained within protection against electrostatic discharge. The same shielding that protects against EMI can serve to provide a grounding path which can protect devices from this electrostatic discharge.
In order to provide shielding with respect to EMI/RFI radiation, a number of different techniques have been used in the prior art. These are commonly based on the completion of a Faraday cage, which provides an adequately grounded conductive part surrounding the electronics. The grounded conductive surfaces reflect and/or absorb the radiated magnetic energy emitted from the electronics, and serve as a barrier to external EMI/RFI, and, as a proximal low resistance path for electrostatic discharges near openings in the product. It is notable that today's higher frequency electronic circuits require only very thin conductive shields for containment.
Products could be simply enclosed by grounded sheet metal fabricated enclosures, however, they are heavy, expensive, and design shape limited. Inexpensive, light-weight, plastic molded enclosures are popular enclosure cases for non-EMI shielding purposes. However plastic, by itself, is not suitable for EMI/RFI shielding, as it is generally not electrically conductive. A workable approach, then, is either to interpose shielding material between the case and the interior components or to incorporate shielding material into the case itself.
A variety of conventional techniques have followed one or the other of these approaches. A common method for providing an interior metal shield within a plastic enclosure is to coat the inside surfaces of the plastic enclosure parts. This can be done by a number of methods. One is painting the surface using metal particle suspensions containing, for example, copper, silver or nickel, by spraying the molded plastic enclosure. Another method is vacuum metalizing, where a metal such as aluminum, is evaporated in a vacuum to form a thin film on the plastic enclosure surfaces. Still another method is electroless plating of the inside surface by metals such as copper followed by nickel. Electroless plating requires adding a catalyst material to the plastic surfaces to be plated, and subsequent immersion in a bath of plating chemicals, rinses, more chemicals, etc. Both the inside and outside surfaces can be plated, but for cosmetic reasons, usually just the inside surface is plated.
All of the coating processes employ semi-automatic or fully automated equipment systems, thus making the process commercially practicable, however, these processes are comparatively expensive in cost per square foot, and suffer from various problems. Some of these problems are: a) loose conductive particles which can short circuits, b) limited connections within particle matrix inhibits high frequency energy flow, c) poor uniformity of coating particles due to variations in coating thickness or conductive particle density, or inadequate deposition in deep cavities.
Another problem is the cost, and the handling involved to pack and transport the plastic parts to a painting or plating facility to receive the coating. Also, one of the major concerns for plating and spray coating processes is both air and water environmental pollution with the metal particles and/or solvents involved in coating application.
An alternative method of providing EMI/RFI shielding to molded plastic enclosures is by filling the molding material with conductive filler such as carbon or aluminum flakes or fibers prior to molding the enclosure. This, however, does not provide a high conductivity, without sacrificing various properties of the plastic. These methods have limitations, which have severely limited their commercial use. Recently, General Electric attempted to pre-form a metal screen(shield) part and automatically install it into the mold and inject the plastic around it.
There have also been attempts to shape metalized plastic substrates by vacuum-forming the substrates to fit into plastic enclosures. This technique relies on heating and stretching the metalized plastic substrate to shape it. Limitations of the technique, and the requirement (cost per square foot) of a stretchable substrate have curtailed popular use of the technique.
Other alternative methods of providing EMI/RFI shielding involve installing separate metal parts, known as shields, inside the plastic enclosures. The shields, which include sheetmetal, laminated foils, metalized plastic films, metalized fibers, and basic stamped sheet-metal shields. All of these materials have design and economic drawbacks. Laminated foils, such as aluminum/mylar are tenfold the price of just aluminum foil and are installed manually. This manual handling, shipping and installing also limits how delicate a part can be, and the amount of fine detail in the laminated part design. However, simple pre-manufactured shields are easily die cut and growing in popularity. Other drawbacks become evident in the packaging, shipping, and handling which can be a source of defects. Stamped metal shields are heavier gage metal, easily distorted due to handling and can contain sharp edges, which can injure someone working with it. Also, besides the weight, heavier gage metals require punch & die tooling, which takes more time and money to make.
Therefore there is a need for effective EMI/RFI shielding which has no loose conductive particles, provides good conductive paths with uniform thickness, which can be produced by automated manufacture and is inexpensive to produce. There is also a need for a device which can install such shielding in an efficient and automated fashion of layers which are so very thin that manual installation may not be practical. There is a further need for a system which incorporates such an installation device and which automates the entire process to produce shielded parts in an automated fashion.
DISCLOSURE OF INVENTION
Accordingly, it is an object of the present invention to provide an improved EMI/RFI shielded plastic enclosure, which provides good conductive paths for EMI/RFI and electro-static discharge.
Another object of the invention is to provide effective EMI/RFI shielding which is of uniform thickness and has no lose conductive particles.
A further object of the present invention is to provide an EMI/RFI shielded plastic enclosure which can be produced for very intricate and delicate forms by machine automated processes.
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