Electricity: electrical systems and devices – Discharging or preventing accumulation of electric charge – Specific conduction means or dissipator
Reexamination Certificate
2001-02-06
2004-02-24
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Discharging or preventing accumulation of electric charge
Specific conduction means or dissipator
C361S058000, C361S118000
Reexamination Certificate
active
06697248
ABSTRACT:
BACKGROUND
The proliferation of electronic devices over the past several years has created problems of radiation containment from these devices. When a digital device changes voltage, it emits electrical and magnetic energy which is comparable to frictional heat loss in any mechanical machine. When not controlled, this radiation can interfere with the normal operating use of other electrical devices. In addition, many devices need to be shielded against incoming radiation to prevent damage to the product itself The prevention of egress or ingress of electromagnetic radiation is referred to as electromagnetic interference (EMI) shielding.
The typical methods to shield electronic devices consist of surrounding the electronic components with a conductive barrier which reflects and/or absorbs the radiation. However, the problems associated with electromagnetic interference continue to challenge manufacturers due to the proliferation of devices, increasing clock speeds, and the increased density of electronic packaging resulting from continual size reductions in portable devices. The problems have been further compounded by the increased use of plastic housings. Plastics tend to cost and weigh less than alternative metal structures, are resistant to corrosion and provide much greater design flexibility. However, plastics are natural insulators and as such are generally transparent to electromagnetic radiation in the applicable frequency range. Electronic equipment manufacturers have therefore been forced to find ways to shield the plastic enclosures to protect the components inside and comply with applicable regulations.
When evaluated according to standard testing protocol, the shielding imparted by a particular structure is reported in decibels. The shielding effect is measured by the following equation:
Shielding effect (dB)=20 log (E.sub.1/E.sub.2)
Where:
-E.sub.1 is the receiving level when no shielding material is placed between the transmitting and receiving antennas, and
-E.sub.2 is the receiving level when a shielding material is placed between transmitting and receiving antennas.
20 dB is generally considered a minimum range for meaningful shielding. 30-60 dB can be sufficient to solve moderate problems. 60-90 dB represents excellent shielding solving moderate to severe problems.
Many different electromagnetic shielding methods exist. The simplest in concept is to select a metal housing or cabinet for the shield. The use of metal shields includes stamped shells or cabinets, zinc die castings, and sheet metal liners. Stamped shells can be a cost effective way to shield sensitive components inside a device. However, such shells do not necessarily protect the rest of the device from external sources of radiation and design flexibility is limited. Zinc die castings provide effective shielding, especially in high temperature environments. However, zinc die castings are costly, heavy, and design flexibility is limited. Sheet metal liners can be combined with the appeal of a plastic exterior, but sheet metal liners tend to be expensive and have to be attached to the plastic housing which complicates and lengthens the assembly process.
A number of methods have been developed to provide shielding to plastic components. Perhaps the simplest is the use of conductive paints, usually comprising copper or nickel powder or flake in a polymeric binder. These paints can be applied with simple, conventional spray painting equipment. For parts with simple designs, painting is a low cost shielding option due to low capital costs. The paints can usually be applied to most plastics although some require an adhesion promoting treatment. One major disadvantage of conductive painting is that it is a line of sight process making it difficult to successfully shield recessed or ribbed areas. Thin coatings also will not provide good shielding effectiveness so multiple coats may be required. This may be a particular problem at sharp comers where the well-known “pull-away” characteristic of paints may lead to slot antenna effects. Finally, the multiple coats which may be required result in significant material overspray waste, environmental difficulties, and increased costs. It is also difficult to achieve consistent paint thickness over the surface of an article which can result in variations in shielding performance. Finally, conductive paints often suffer from problems with durability over time. Due to these drawbacks the use of conductive paint coatings has grown very slowly over the past several years.
Another method for imparting EMI shielding capability to plastic articles is vacuum metallizing. In the vacuum metallizing process, parts are masked if necessary and then placed on rotating fixtures inside a vacuum chamber. Inside the chamber a metal, usually aluminum, is heated and vaporized. The metal will then condense on the surface of the plastic. Vacuum metallizing is generally a batch process that is best suited for small to moderate sized parts with limited geometrical complexity. For example, U.S. Pat. No. 5,811,050 to Gabower teaches the use of vacuum metallizing to shield thermoformed plastic parts. Thermoforming is a process limited to parts of relatively simple geometry. Gabower reported shielding effectiveness values of up to 60 dB for vacuum metallized, thermoformed parts. Thus, for these types of parts, vacuum metallizing provides reasonable shielding effectiveness at a relatively moderate cost. A major disadvantage of vacuum metallizing is the need for special equipment which requires a significant capital investment as well as a high operator skill level. In addition, vacuum deposition of the relatively thick films required for EMI shielding can be complex and process sensitive, as discussed in the Gabower U.S. Pat. No. 5,811,050. Like spray painting, vacuum metallizing is generally a line of sight process which makes it difficult to successfully shield recessed or ribbed areas. Finally, a base coat or ionization treatment is often required between the plastic and aluminum to minimize surface defects, and a protective coating may be required to protect the vacuum deposited metal.
An alternative method for imparting EMI shielding to plastics is electroless plating. Electroless plating involves chemically coating a nonconductive surface such as a plastic with a continuous metallic film. Unlike conventional electroplating, electroless plating does not require the use of electricity to deposit the metal. Instead, a series of chemical steps involving etchants and catalysts prepare the non-conductive plastic substrate to accept a metal layer deposited by chemical reduction of metal from solution. The process usually involves depositing a thin layer of highly conductive copper followed by a nickel topcoat which protects the copper sublayer from oxidation and corrosion. The thickness of the nickel topcoat can be adjusted depending on the abrasion and corrosion requirements of the final product. Because electroless plating is an immersion process, uniform coatings can be applied to almost any configuration regardless of size or complexity without a high reliance on operator skill. Electroless plating also provides a highly conductive pure metal surface which results in relatively good shielding effectiveness. In addition, electrolessly plated parts can be subsequently electroplated, although electroplating is generally not used unless a part also has certain decorative or functional requirements. On average, the cost of electroless plating will be higher than vacuum metallizing and conductive paints. The many steps employing harsh and expensive chemicals make the process intrinsically costly and environmentally difficult. The process comprises many steps and is very sensitive to processing variables used to fabricate the plastic substrate, limiting applications to carefully molded parts and designs. It may be difficult to properly mold conventional plateable plastics using the rapid injection rates often required for the thin walls of electronic components. The rapid injection rates
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