The next logical step is to see how a perforated soldered can would perform using the same 1-mm hole size on a 1.7-mm grid. Using the test method described earlier, a 16.3 × 22.5 × 3.1-mm can, completely soldered around its perimeter, was used as the shield. Using the model generated from the plane-wave experiment, a prediction was made for (127) 1-mm holes (Fig. 9). The surprising result of an overall lower SE is added to the fact that the response is flat with frequency, even at low frequencies. This shows that even at low frequencies, where the size of the apertures are extremely small compared to the wavelength, the SE remains unchanged. Cooperative research with the University of Delaware, using custom method of moment software, has confirmed these results (and will be published formally at the 2003 IEEE EMC symposium in Boston, MA). If the aperture spacing is increased by a factor of 2, thus reducing the total number of holes by a factor of 4, the SE is increased, or shifted by about 12 dB, the same as the plane-wave case.

Perforations in cans allow for heat transfer during solder reflow. Since these cans are attached to a PCB by the same SMT assembly process used for the components, the solder reflow of the components under and around the shields should not be impeded by the can. In typical applications, hexagonal arrays of sufficiently sized holes are placed over the full top surface of a can. A 1.4-mm or larger hole is typical with the hexagonal grid yielding a hole spacing of 4.4 mm and smaller.

To simplify the addition of shielding to PCBs, Gore has developed the "snap-on" snapSHOT shield,™ which consists of a metallized thermoformed shell that is attached to a PCB using standard BGA solder spheres. This approach provides an excellent mechanical and electrical connection and yet allow the user to easily remove the can to get access to the components within the shield. This patented technology (US patent No. 6,377,475; other US and foreign patents pending) offers a PCB-level shield that can be easily attached to a populated board after it has gone through the solder reflow process.

The shield is metallized on the outside with an insulator on the inside. The BGA spheres snap through holes in the shield, creating a robust electrical and mechanical connection to the PCB. The periodicity of the sphere placement is determined by the required shielding performance. Since the inside surface is not conductive, any components that may come in contact with this surface will not be electrically shorted. The snapSHOT™ shield provides significantly improved shielding over similar perforated solder cans with a great deal of design flexibility (Fig. 10).

REFERENCES

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2. Thomas Clupper and Joseph Wheeler, "The effects of gaps introduced into a continuous EMI gasket," ITEM Magazine 2000, Robar Industries, West Conshohocken, PA, 2000.
3. Thomas Clupper, "Effects of Apertures in Multiple EMI Enclosures on RF Printed Circuit Boards," Workshop W8, 2001 IEEE EMC Symposium, Montreal, Canada.
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