Heat-shrink tubing can provide a versatile EMI shielding solution.
Emil E. Millas
Methode Development Company
Chicago, IL, USA
With the proliferation of and demand for high-speed computer and data communications equipment, the susceptibility of such equipment to electromagnetic interference (EMI) and radio frequency interference (RFI) has become a growing problem and challenge. In today’s world of sophisticated electronic systems, this susceptibility usually is manifested in one of two ways: conducted EMI/RFI which causes the system to malfunction from within, or radiated EMI/RFI, which causes other nearby equipment to malfunction.
Today, EMI/RFI control is considered so important that the Federal Communications Commission (FCC) in the US, the European Union (EU), and other nations have imposed strict guidelines mandating allowable EMI/RFI limits in electronic systems. Manufacturers of electronic equipment must conform to stringent EMC certification regulations if they wish to sell their products globally. The regulations are quite comprehensive, ranging from directives covering sophisticated medical equipment and computers to those regulating common household items.
Designing EMI/RFI compliance into a new piece of equipment may appear to be just another design task; but in reality, that task may become quite complicated. For example, the EMI/RFI performance of a product prototype may be completely different than that of the actual production unit. The manufacturer may discover that a new piece of equipment is not EMI/RFI compliant until pilot production units have been built and tested.
SHIELDED CABLE
Shielded cable is often used to mitigate EMI/RFI, and EMI/RFI problems frequently occur at the junction between a shielded cable and its connector. Common solutions include using a metal or metallized plastic connector and soldering the cable shield to the connector or, alternatively, wrapping the junction with copper tape and then soldering it to the cable shield. Even though these types of solutions may work, they are costly, labor-intensive, and can produce inconsistent shielding performance. In a worse case scenario, the junction between the shielded cable and the connector may be improperly wrapped. If the copper tape fails to cover even a very small area of the junction, the cable assembly may emit unacceptably high levels of EMI/RFI. Attempts at repair may rip or tear the foil shield of the cable, thereby rendering the entire cable useless. Moreover, if the cable and connector junction are not the source of the interference, solving the shielding problem is likely to be even more difficult.
HEAT-SHRINK TUBING
One solution to these cable-related problems is to use shielded conductive heat- shrink tubing with a metallic conductive ink coating on the inside of the tube. The inner coating provides electrical continuity and EMI, RFI, and ESD shielding around the joints being connected. This tubing can solve a number of EMI/RFI problems easily and inexpensively and does not require that the designer, the engineer, or the user have in-depth knowledge of electromagnetic and radio frequency interference and their attendant shielding issues.
In use, the appropriate diameter of tubing is simply placed over the components or assemblies to be shielded, and heat from a heat gun, an oven, or any other conventional heating device is applied to the tubing. After the tubing shrinks, the inner metallic layer provides an electrical connection between the outside surface of the objects that are joined by the tubing, thereby creating an almost 100% effective, 360° circumferential shield.
Heat-shrink tubing requires no soldering. The tubing is a multi-purpose polyolefin rated at MIL-STD-R-46846. It meets stringent aerospace industry specifications for outgassing such as ASTM E- 595, NASA SPR-0022A, and ESA PSS- 010702, which cover the vacuum stability of polymeric materials in spacecraft applications of mass loss and collected volatile condensable materials. It can be used with solderable and non-solderable surfaces such as stamped metals, plated finishes, and shielding paints. Standard inside diameters (IDs) range from 1/8″ to 5″ but custom materials are available for application specific programs.
Shielded conductive heat-shrink tubing provides a low resistance shield of less than 1 Ω/inch. Shielding effectiveness varies according to tube diameter and frequency (Figure 1). The operating temperature range is -55° to +135° C.
Heat-shrink tubing can be obtained in EMC compliance kits comprised of a selection of 6-inch lengths of 1/4″, 3/8″, 1/2″, 3/4″, and 1″ inside diameter tubes (Figure 2). These kits are ideal for use in prototypes and pilot production environments, and production quantities can be ordered as required.
EXAMPLES OF VERSATILITY
The versatility of heat-shrink tubing and its shielding effectiveness have been demonstrated in diverse applications.
One such application is its use in a firefighter’s high-tech helmet. The manufacturer had developed a new design incorporating advanced systems such as night vision, heat sensing, and two-way communications into the helmet. However, when the helmet was manufactured, the various electronic systems in the finished product were in close proximity, and were interfering with each other. The resulting crosstalk was so bad that it was virtually impossible to use more than one component of the system at a time.
Using heat-shrink tubing to cover the cables running inside the helmet easily solved the crosstalk problem. That simple solution avoided the design and fabrication of custom cable shields or metal enclosures, thus reducing the complexity and the weight of the helmet. The heat-shrink tubing was also able to withstand the rapid thermal changes experienced when firefighters leave the hot environment inside a burning building and emerge into severe winter weather.
Another manufacturer designed a cable assembly using an RJ45 telephone jack. A circular grommet in the cable assembly acted as a strain relief. The irregular shapes within this configuration—the rectangular jack and the round grommet—created an EMI/RFI problem, and both had to be shielded. The shielding of irregularly shaped combinations of components can be extremely difficult. A solution was achieved by forming a shrinkable transition boot to slide over the entire assembly. Once shrunk to size, the shrinkable boot solved the noise problem.
An agricultural sensor system manufacturer designed and tooled a housing for one of its products. However, when the sensor and its associated circuit board were assembled, EMC testing of the unit showed unacceptable EMI/RFI performance. The dilemma facing the manufacturer at this stage was that he already had a significant investment in tooling and that the design of a new housing would be cost-prohibitive to the project
The simple solution was to place the sensor and the circuit board inside a special piece of thin-walled heat-shrink tubing. With this solution, the entire assembly still fit inside the existing housing and solved the EMI/RFI problem without forfeiting the substantial investment in tooling for the original housing.
CONCLUSION
Although not all EMI/RFI cable shielding problems can be fixed with a single product like heat-shrink tubing, it can be the cost-effective and easily implemented solution to EMC challenges.
ABOUT THE AUTHOR
Emil E. Millas is Sales & Marketing Manager at Methode Development Company, Chicago, IL, a division of Methode Electronics, Inc. Visit www.methode.com.
