CuBe is still relevant for EMI shielding.(EMC SHIELDING)

Traditionally, alloys of copper and beryllium (CuBe) provide the high levels of electrical conductivity for shielding and environmental protection, coupled with durability and low closure force. Applications include cabinets for telecommunications infrastructure, information technology, defense, and commercial and consumer electronics such as mobile phones and hand-held devices.

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Recently, however, issues have been raised involving the use of alternative alloys such as stainless steel, phosphor bronze, and copper-nickel-tin. Before you can make an educated shielding selection, it's important to address many critical factors such as mechanical and electrical performance, price/performance, and compliance/end-of-life issues for some materials typically used to make metal EMI gaskets.

Mechanical Performance

The purpose of a shielding gasket is to prevent the leakage of EMI from an enclosure around an electronic device operating at high frequency. Any gaps or spaces around doors, seams, or access panels larger than 1/20th of the wavelength of the signal allow EMI to escape the enclosure. Shielding gaskets fill these gaps and absorb unwanted EMI. To do this, the gasket must form a continuous electrically conducting path between the enclosure surfaces.

Table 1 shows the composition and some basic properties of four metals used for fingerstock applications: C17200 CuBe, C52100 phosphor bronze, S30100 stainless steel, and C72700 copper-nickeltin. (1) The CuBe and copper-nickel-tin materials can be heat treated to very high strength levels compared to other copper alloys.

As a fingerstock gasket compresses, it generates pressure on the opposing surfaces on either side of the gasket. This pressure is a function of the stiffness of the contact. Stiffness is determined by the geometry of the contact and the elastic modulus of the metal used.

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A higher pressure means greater force is required to seal the enclosure, so it is desirable to keep the pressure low enough to allow for easy sealing. However, if the pressure is too low, the gasket will not make good contact with the opposing surfaces and may allow radiated EMI to escape.

In a gasket of any given design, CuBe and copper-nickel-tin will provide equal pressure at a given deflection assuming there is no yielding of the material. Stainless steel will have a contact pressure 49% higher while that of phosphor bronze will be 16% lower.

During compression of the fingerstock, the stress in the metal will

increase in direct proportion to the compression. If the stress is lower than the yield strength of the material, the contact is compressed elastically. The fingerstock gasket will return to its original shape when the compressive load is removed. Compression set results when the stress exceeds the yield strength of the material. …

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