Find resistance in EMI gaskets
W.L. Gore tests the effectiveness of EMI shields used to protect sensitive cell-phone circuits.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 4/1/2003
EMI gaskets used to shield sensitive circuits in cell phones. Each gasket is a custom design that fits between a PCB and a cell phone case. The gaskets must retain their shielding effectiveness when compressed between a cell-phone cast and a circuit.
THE CHALLENGEDesign a test system that measures DC resistance in 162 segments of the gasket. Write software to control instruments, record results, and report resistance of each segment with a color code. From the resistance measurements, engineers can develop statistics about the gasket's hardness, compression, and deformation.
THE TOOLS- Keithley 2750 DMM and 7002 switching card, which provide 6½ -digit resolution and switching for up to 200 differential channels. www.keithley.com.
- SAS/Graph plotting software, SAS Institute. www.sas.com.
EMI gaskets shield electronic components from emissions, but the effectiveness of a gasket varies when installed into a product. To measure a gasket's shielding effectiveness, engineers at W.L. Gore & Associates (Elkton, MD; www.wlgore.com) developed an automated test system to test EMI gasket designs. With the test system, engineers can determine the compression of a gasket, its hardness, and its deformation by measuring the gasket's DC resistance under numerous conditions such as mechanical stress and temperature.
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A switching system connects 162 sets of wires to a segmented PCB ground trace, from which a DMM measures DC resistance. |
The figure shows the test system. To measure DC resistance at numerous points on the gasket, Gore engineers designed a matching PCB trace broken into 1-mm by 2-mm segments. Above the gasket resides a plated plastic shield containing screw holes that match the PCB. The screws hold the gasket under test and the PCB together so they make contact. The PCB and shield also provide contacts for connecting test equipment.
To measure the gasket's resistance, the test system uses a DMM and switch cards. The mainframe contains switch cards that connect the DMM to each segment of the PCB. The Source+ line and the Sense+ line of the DMM connect directly from the DMM to the shielding plate. From the mainframe, 162 pairs of switches connect 162 pairs of wires to 162 pairs of pads on the underside of the PCB. The mainframe switching system scans the segmented pads along the PCB to measure the Source– and Sense– values. The DMM pumps 10 mA through each pad and measures the pad's resistance with the 4-wire ohms method.
A computer running Visual Basic (VB) controls the DMM and switch system through an IEEE 488 port. The DMM stores the test data, and the PC loads the data under control of the VB program. The VB program plots an image of the gasket, breaking it into 162 segments. The color of each segment represents a resistance range, and black indicates an open circuit. Values greater than 1 Ù—represented by red—indicate areas of concern, and an EMI engineer will evaluate the test results to determine if the gasket still effectively shields emissions.
RESULTS
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| Figure 2 Visual Basic code downloads test data and produced a color-coded plot of gasket resistance at each segment. |
The VB software will also store data for further analysis by either Excel or SAS/Graph, a data-graphing package. Gore engineers use SAS/Graph to produce 3-D plots of the DC resistance in a gasket. From the plots, engineers draw conclusions about a gasket design after testing up to 50 gaskets per design.
Martin Rowe, Senior Technical Editor m.rowe@tmworld.com
