Patent Publication Number: US-2005139282-A1

Title: Microwave-absorbing form-in-place paste

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to the electronics industry, and more particularly, to the absorption of undesired radio-frequency (RF) emissions, such as microwave radiation, from electronic equipment, such as computers, telecommunications equipment, and the like.  
      2. Description of the Prior Art  
      Problems caused by stray electromagnetic (EM) fields and radio frequency (RF) emissions in electronic equipment, such as computers and telecommunications equipment, are well known in the art. A variety of approaches are taken to shield housings for such equipment to reduce the emissions coming from components within the housings, and to prevent emissions originating outside the housings from penetrating inside and interfering with the operation of the components within.  
      Among the approaches is to reduce radio-frequency (RF) emissions, such as microwave radiation, by including radiation-absorbing pads within the housings. Pads of this type have traditionally been cut from large rolls or sheets of the pad material and sized to fit cavities, spaces or areas within the housings. The pads may have a high magnetic permeability and a high electrical resistivity.  
      The present invention provides an alternative to the pads of the prior art, enabling a housing for electronic equipment to be provided with microwave-absorbing material in a more straightforward and convenient manner.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is a microwave-absorbing material which can be dispensed from standard form-in-place dispensing equipment. Such equipment may include a syringe-like applicator of the variety used to produce form-in-place gaskets and the like.  
      The microwave-absorbing material comprises a liquid resin system and a magnetic or lossy dielectric filler. In a preferred embodiment, the liquid resin system is a silicon resin system and the magnetic filler is iron carbonyl powder.  
      The present invention is also a method for absorbing radio-frequency (RF) emissions in electronic equipment. The method comprises the steps of providing a liquid resin system; dispersing a magnetic or lossy dielectric filler in the liquid resin system; dispensing the liquid resin system having the filler into a housing for the electronic equipment; and curing the liquid resin system having the filler thereby forming a radio-frequency (RF) absorbing pad in situ in the housing.  
      The microwave-absorbing material, that is, the liquid resin system having the filler is self-leveling and may be dispensed into a cavity in the housing. Alternatively, because of its thixotropic properties, the microwave-absorbing material may be dispensed onto a surface in the housing, where it will form a pad when cured.  
      The present invention will now be described in more complete detail, with reference being made to the figures identified hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a dispensing system for the microwave-absorbing material; and  
       FIGS. 2A and 2B  are perspective and side views, respectively, of a dispensing system depositing microwave-absorbing material onto a surface. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Accordingly, the present invention is a microwave-absorbing material which can be dispensed from standard form-in-place dispensing equipment. That is to say, it may be dispensed from a syringe-like applicator of the variety used to produce form-in-place gaskets and the like.  
      The microwave-absorbing material comprises at least two components, the first being a liquid resin system, which may be a silicone resin system. Alternatively, the liquid resin system may be a urethane, epoxy or other organic resin system.  
      The other component is a filler, such as a magnetic or lossy dielectric filler. The magnetic filler is a filler material having a high magnetic permeability and a high electrical resistivity. The magnetic filler material may be iron carbonyl powder. Alternatively, the magnetic filler material may be iron powder; iron silicide; a ferrite (a compound of ferric oxide with another oxide); PERMALLOY® (an alloy containing 78.5% nickel and 21.5% iron and having a high magnetic permeability and electrical resistivity); SENDUST (an alloy containing 85% iron, 9.5% silicon and 5.5% aluminum); carbon, that is, conductive carbon powder; fibers, such as graphite or metal fibers; and other fillers with electromagnetic (EM) properties, that is, high magnetic permeability and high electrical resistivity.  
      The microwave-absorbing material is loaded into a dispensing system, such as one incorporating a syringe-like dispenser, in tubes. Such a system is shown in  FIG. 1 , where a dispensing system  10  dispenses the microwave-absorbing material  12  into a cavity  14  in a housing for electronic equipment. Within the cavity  14 , the microwave-absorbing material  12  self-levels to a desired thickness, thereby forming an RF (radio-frequency) absorbing pad in the cavity  14  when cured. The thickness desired is predetermined to be that required to attenuate surface currents and EMI (electromagnetic interference) within the housing.  
      The microwave-absorbing material  12  of the present invention has a viscosity sufficient to maintain the filler material in a suspended state in the liquid resin system. The amount of filler material used, and the thickness of the microwave-absorbing material  12 , deposited in the cavity  14 , are controlled to optimize the absorption of microwave energy at a given frequency or frequencies.  
      The microwave-absorbing material  12  of the present invention also has thixotropic properties. Thixotropy is the ability of certain colloidal gels to liquefy when agitated and to return to the gel form when at rest. In other words, gels of this type hold their shape when at rest and not under pressure, but flow freely on application of slight pressure.  
      This property may be exploited to produce a pad of the microwave-absorbing material within a housing without the presence of a cavity. Referring to  FIGS. 2A and 2B , which are perspective and side views, respectively, of a dispensing system  10  depositing microwave-absorbing material  12  onto a surface  16  in a housing for electronic equipment to form a pad  18  thereof. Because of its thixotropic properties, the microwave-absorbing material  12  maintains its shape as a pad  18  on the surface  16  and cures while keeping that shape.  
      Modifications to the above would be obvious to those of ordinary skill in the art, but would not bring the invention so modified beyond the scope of the appended claims.