Large and bulky electronic products are a thing of the past. Today’s customers expect equipment to be small and elegant powerhouses. The 40-lb “portable” computer of a decade ago has been on a fat-free diet to become the current sleek 6-lb notebook-sized unit.
Shrinking a product, however, increases the manufacturing and engineering challenges. These smaller products come in many shapes, are made of lighter-weight materials, and must contain or keep out a wider range of frequencies to comply with the current EMI/RFI standards.
The EMI gasketing industry helps electronic manufacturers meet these new stricter emission and susceptibility standards by providing a perfectly fitted gasket for each item. One of the newest production techniques makes customized shielding gaskets by printing an electrically conductive material on metal or plastic housings. This specialized procedure also eliminates waste by squeezing out exactly the right amount of material.
The materials are typically a mix of an elastomer base or polytetrafluoroethylene (PTFE) compound that returns quickly to their initial dimensions and a conductive material like silver-nickel, silver-copper, silver-aluminum, silver-glass or inert aluminum. The compound is often placed on the surfaces of a product with automated or semi-automated machinery.
The gasket size, volume and placement are inherently custom processes, said Ray Borgersen, Product Manager at Tecknit. The procedure uses a computerized numerically controlled machine that dispenses a precision bead of conductive silver- plated copper silicone elastomer on a customer-supplied electronic housing. The gasket can be comprised of any geometric shape, including a line, arc, circle or ellipse.
The gasketing equipment downloads the geometry, speed and pressure information from a computer-aided design/computer-aided manufacturing system to print the precisely shaped gasket. The Chomerics Form-In-Place (FIP) process, for example, applies a viscous conductive compound with 0.001″ accuracy to the predetermined parts, and cures them in-line.
Instrument Specialties prints the gasketing components repeatedly over the surface. With this process, gasket thicknesses can reach 1/2″ while providing the required shielding.
What does this technology mean to you? Basically, it allows you to customize gaskets for smaller snap-together enclosures and to provide appropriate shielding for your higher-frequency devices.
The process supplies the accurate gasket application required by the small hand-held products, using the adhesive material to compensate for uneven surfaces and complex geometric patterns. This technique really shines at production facilities making new models every six to nine months because the gasket equipment is easily updated.
Some automated gasketing systems cure conductive gaskets in-line by sending the products through a hot-air oven for a few minutes, then cooling them to complete the process. The whole cycle—from printing the gasket to exiting the assembly line—takes approximately one hour, said Walt Dolbier, Market Manager at Parker-Chomerics.
The shielding effectiveness for the gasketing depends on the conductive material you use and the energy field you need to attenuate, including the plane wave, the E-field and the H-field. For example, typical shielding provided by the Instrument Specialties’ gasket for the H-field at 200-kHz ranges from 59 dB for silicone with inert aluminum to 126 dB for silicone with silver-copper gasketing. E-fields are attenuated by similar amounts up to frequencies exceeding 500 MHz; plane waves are shielded in the GHz range with attenuation ranging from approximately 30 to 90 dB.
Applications
Several industries such as telecommunications and medical electronics have discovered the advantages of FIP gasketing. It is especially good for applications where there is little room for traditional-style gaskets.
These types of applications require soft, small cross-sectional gaskets (measuring 0.040″ W ´ 0.340″ H) that must be placed with 0.004″ accuracy, said Mr. Dolbier. Common examples are base-station covers and PC cards.
The technology is also used to shield wireless radios connecting local area networks. Space is at a premium in these wireless networks that use small radios attached to the rear of a computer. FIP meets the volume, EMI-shielding and space requirements of this application, providing a computer-controlled process that dispenses gasketing onto surfaces as narrow as 0.03″-wide with shielding effectiveness of approximately 75 dB.
This is the right process if your current application uses gaskets cut from sheet stock, resulting in a copious amount of wasted material, said Dave Bannet of Instrument Specialties. By printing the gaskets on the hardware, multiple gaskets are installed in one operation and delicate material is handled less.
This process, however, is not a solution for everyone. For example, operations with gasket thicknesses greater than 0.040″ or products that need gaskets on a non-flat surface are not appropriate, said Mr. Bannet.
The FIP process produces gasketing that is superior to techniques using molded, rule die-cut or small cross-section gaskets that are too large for the new-generation products. Conventional gaskets also are difficult to install, said Mr. Borgersen.
This technology is not limited to hand-held devices, added Mr. Borgersen. It is used for larger castings such as linear amplifiers for cellular-phone transmission sites.
The cost to build a mold for a conductive gasket with a complex configuration is prohibitive, said Mr. Borgersen. The scrap factor is high because of tears that occur when the gasket is released from the mold.
Housing configurations for complex designs are also more intricate because designers are fitting more into smaller areas. Conventional gaskets are extremely difficult to install in the thin-wall partitions of these small units.
When a die-cut or molded gasket is placed on a 0.050″-wide ground plane, the gasket is positioned very close to the active devices on a surface-mount board, said Mr. Borgersen. When the gasket is compressed, it has a tendency to flow outside its allocated boundary and short the neighboring components. Form-in-place gasketing is very effective for this situation because it dispenses a thinner bead of conductive compound more accurately and can be programmed to follow intricate patterns on two- or three-dimensional planes.
EMI/RFI Gaskets
Conductive EMI Gasket
Dispensed In Place
Cho-Form Technology is an automated system dispensing conductive elastomer gaskets in place to provide EMI shielding and grounding for metal and plastic. The system dispenses programmed gasket beads in three axes and compensates for uneven surfaces. A silicone-base resin and a silver-plated copper conductive filler combine to offer a shielding effectiveness of 80 to 100 dB from 20 MHz to 10 GHz. Gaskets can be applied in straight strips, continuous 360° paths or any combination of these. Beads are dispensed with an accuracy of 0.001″ with a cross-sectional tolerance of ± 0.004″. Parker Chomerics, (617) 935-4850.
Printed Conductive Elastomers
Made for Metal or Plastic
A technique produces an Electrically Conductive Printed Elastomer (EcPE) Gasket for printing shielding material on metal or plastic substrates or component hardware. Gaskets have a typical thickness from 0.02″ to 0.04″. They are available in silver-nickel, silver-copper, silver-aluminum, silver, nickel-graphite and inert aluminum filler materials. Shielding effectiveness ranges from 59 dB to 126 dB at 200 kHz, from 65 dB to 120 dB at 100 MHz, and from 33 dB to 93 dB at 1 GHz. The gaskets are made in a variety of forms from simple prints to complex subassemblies. Instrument Specialties, (717) 424-8510.
Form-In-Place Gasket
Provides 75-dB Shielding
A form-in-place gasket material made from silver-plated copper-filled resin cures to create a flexible EMI shield and an environmental seal. The material is applied on any of three planes and provides shielding effectiveness to 75 dB @ 10 MHz. It adheres to metal or plastic housings and is applied with a computer-controlled process that dispenses the gasketing onto flanges as narrow as 0.03″. Tecknit, (908) 272-5500.
Conformable Shielding Material
Provides Environmental Seal
GORE-SHIELD™ EMI gasketing suppresses radiated EMI and RFI while maintaining an environmental seal. The expanded PTFE material provides protection in applications as high as 18 GHz and remains conductive under vibration. It is pliable and can be formed around corners. Shielding effectiveness is up to 100 dB when tested to MIL-G-83528. The operating temperature range is -200° C to +200° C. Thicknesses range from 0.01″ to 0.125″ and widths from 0.125″ to 10″. W.L. Gore & Associates, (800) 231-4EMI.
Copyright 1996 Nelson Publishing Inc.
August 1996
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