In an unconventional move, a manufacturer chose an SMT gasket solution to address critical design and manufacturing factors.
Conventional EMI gaskets such as form-in-place gaskets provide adequate shielding performance and acceptable failure rates for many applications. But conventional gasketing approaches can fall short when high levels of shielding performance, very low failure rates, and low installed cost are required.
That’s what a major manufacturer discovered when designing a new series of RF power amplifiers for wireless base stations. The company wanted its product to have high EMI shielding performance along with shielding failure rates well below the industry average of 5%. After extensive testing, it concluded that the existing EMI shielding solution, a chromate aluminum enclosure with a silver-filled, dispensed-silicone gasket, would not provide the required combination of high performance, low failure rate, and low installed cost.
Company engineers zeroed in on the EMI gasket as the weak link and searched for alternatives. The following design and manufacturing factors were identified as critical in achieving the shielding objectives:
- Gasket material.
- Gasket width.
- Gasket thickness and consistency.
- Ease of gasket design and manufacturing.
The solution was decidedly unconventional—small pieces of nickel-filled polytetrafluoroethylene (PTFE) backed with a solderable metal shim. These gasket pieces or building blocks are easily arranged in whatever gasket configuration is required, installed directly on the PCB ground trace with standard surface-mount technology (SMT) pick-and-place machines, and soldered with standard reflow equipment.
Material
A fundamental challenge was to ensure that the gasket would make consistent, effective electrical contact with the chromate aluminum enclosure whose surface layer is a relatively poor conductor. For the gasket to conduct current effectively, the conductive metal particles in the gasket must literally cut through the chromate surface layer and make contact with the underlying aluminum.
The silver particles in dispensed gaskets typically are round and relatively soft. Generally, they provide acceptable contact initially, but in accelerated life testing, their performance degrades substantially over time.
The nickel-filled gasket material is designed to perform effectively during the accelerated life testing process. The nickel particles in the SMT gasket parts are harder and sharper than the silver balls in the dispensed gasket. In accelerated life testing, they provide excellent conductivity as shown in Figure 1.
Width
All types of EMI gaskets have better conductivity and higher levels of shielding when they are wider in size. Because the manufacturer required very high shielding effectiveness, wider gaskets were essential.
Producing wide gaskets with dispensed gasket technology requires multiple dispensing cycles, adding incrementally more width to the gasket with each cycle. Multiple gasket dispensing cycles add substantially to the time required to install the gasket on the enclosure and increase the installed cost of the gasket. These time and cost penalties were unacceptable to the manufacturer.
SMT gaskets are available off the shelf in standard widths from 0.75 mm to 3.2 mm. As a result, designers were able to experiment with different gasket widths and inexpensively find an optimal width.
Thickness
Mating surfaces of EMI enclosures require gaskets between them because of tolerance stack-up. The thicker and more consistent the gasket, the better it is able to take up this tolerance.
However, areas of uneven compression due to thickness variations in dispensed gaskets can lead to lower conductivity and EMI leakage. Thicker, more uniform gaskets provide more take-up tolerance and don’t contribute to lower conductivity and EMI leakage.
Producing thicker gaskets with dispensed gasket technology has the same issues described for gasket width. Again, the manufacturing time and cost penalties associated with multiple passes and curing cycles were unacceptable to the manufacturer. Moreover, SMT gasket parts are available off the shelf in standard thicknesses from 0.45 mm to 2.0 mm.
Ease of Design and Manufacturing
The resources required to design and manufacture EMI gaskets vary significantly with the chosen gasket technology or approach. SMT gasket technology provided substantial benefits to this manufacturer in terms of design and manufacturing costs and time to market.
Design Issues
SMT gasket technology allows virtually unlimited design flexibility. Designers manually arrange and rearrange SMT gasket parts on the PCB ground trace until they determine optimum part sizes and placement.
The gaskets can easily provide different levels of shielding in various areas of the PCB, which is very difficult to do with conventional dispensed gaskets because of issues with starting and stopping dispensing machines. Perhaps most importantly, the design flexibility provided by SMT gaskets allowed the manufacturer to quickly and inexpensively design gaskets that delivered the high levels of shielding performance required. For cents per part, an effective, flexible, cost-effective solution was reached.
Manufacturing Issues
Conventional form-in-place gaskets require specialized machinery for application on an EMI enclosure. After application, they must cure for up to 24 hours before the enclosures can be installed. This cure time adds significantly to per-unit manufacturing time and affects per-unit cost. This type of conventional gasket is cost-effective only when installed in relatively high production volumes because of the additional machine and operator costs.
SMT gaskets are packaged in EIA standard tape-and-reel format compatible with existing SMT pick-and-place machinery. They integrate directly into the manufacturer’s PCB assembly lines as ordinary pick-and-place components, requiring no special equipment or handling and producing no scrap. Typical pick-and-place rates are eight to 10 parts/s.
Summary
The SMT gasket proved to be the superior EMI shielding solution for the manufacturer by meeting all of its critical design and manufacturing factors. In accelerated life testing, the SMT gaskets continued to be most effective in shielding the enclosure, producing a field failure rate of 1% that is significantly lower than the 5% industry average.
The wider size provided better conductivity and higher levels of shielding, and the thickness made take-up tolerance less of a concern. The design flexibility of the SMT gaskets allowed the manufacturer to deliver a high-level shielding solution quickly and inexpensively even at low production volumes.
About the Author
William Candy is a product manager for the SMT EMI gasket product line at W. L. Gore & Associates. He earned a B.S. in mechanical engineering from Lehigh University in 1992. After receiving his master’s degree in mechanical engineering in 1997 from Villanova University, Mr. Candy joined Gore. He also is a certified professional engineer. W.L. Gore & Associates, 402 Vieve’s Way, P.O. Box 160, Elkton, MD 21922-0160, 410-506-4041, e-mail: [email protected]
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Published by EE-Evaluation Engineering
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January 2003