Select the Right Material
Engineers understand that proper material selection is crucial and must take care to ensure consistency, quality, and shielding efficiency of the materials used for production parts.
Throughout the decades and increasingly today, design and electrical engineers, OEMs, metal suppliers and fabricators worldwide refer to mumetal as the industry standard. Mumetal, the most desired high nickel content alloy specified for low-frequency shielding applications, is used to resolve electro-magnetic interference (EMI) problems. Professionals may use the mumetal name as a generic term but not realize that it is in fact a registered trade name. Engineers understand that proper material selection is crucial and care must be taken to ensure consistency, quality, and shielding efficiency of the materials used for production parts as other companies may misrepresent this brand. A refined mumetal formulation and exacting spec provides maximum magnetic permeability (highest degree of shielding) for use in most electrical/electronic applications found today.
Across a wide array of industries such as appliances, OEMs, hand-held devices, telecommunications, automotive and medical, design and electrical engineers face challenges of increased complexity when creating new product design drawings for all types of electronic device and component layouts. Designers often encounter problems with densely packed electronic assemblies that have internal components interfering with each other. When the electromagnetic interference (EMI) includes low frequency fields (30 to 300 Hz), magnetic shielding is essential to assure proper operation of the electronic equipment. Engineers must also design and manufacture products to meet applicable regulations or standards. Mumetal is an industry reference material with different characteristics than proprietary alloys. When the specification or the application calls for a magnetically soft or high permeability material, mumetal can frequently be successfully applied.
Technical professionals can incorrectly believe that a magnetic shield will block the field’s effects. There is no known material that blocks magnetic fields without attraction to the magnetic force itself. Magnetic fields can only be re-directed, not created or removed. To do this, high-permeability shielding alloys are used and the shielding material attracts and absorbs the magnetic field lines.
One of the most important design questions is whether the source field is internal or external, and which field is most economical and practical to shield. Internal interference is created by designed-in components located within a circuit, system, enclosure, piece of equipment or room. External interference is created by unwanted nearby fields radiating from power lines, circuits, electrical enclosures, transformers, electro-magnetic machinery and the earth’s magnetism.
Secondly, should the source of interference or the sensitive device be shielded? The answer to this question depends on several factors. Shielding the source may involve stronger fields, and therefore thicker materials. All interference sources must be shielded, or the sensitive device will still be affected. The usual approach is to shield the sensitive device. This prevents interference from both present and future sources. A hands-on approach to solving a shielding problem can be accomplished with the use of a lab kit and testing.
Over several decades, we have performed hundreds of laboratory tests and calculated thousands of shielding equations—all in an effort to continually validate the theoretical application of magnetic interference control in both AC and DC applications, and to aid engineers in shield design. Theory behind the B-H curve is well documented, and it remains an effective reference tool. Using the B-H curve, shielding alloys have been developed and refined to offer the most effective shielding characteristics for a variety of applications and markets. The B-H curve chart depicts material capabilities to attenuate and absorb a wide range of magnetic fields.
You may be interested to learn the history of “µ (Mu)” metal. The 12th letter of the Greek alphabet µ (Mu), pronounced “mew,” is used in physics and engineering formulae to represent permeability, the measure of a material’s ability to support the formation or absorption of a magnetic field within itself. In other words, permeability (µ) is a value representing the degree of magnetization obtained in a material from an externally applied H-field. Because mumetal provides maximum permeability in magnetic shielding, it became permanently named after the Greek letter µ (Mu). However, mumetal is a registered trade name known worldwide.
Shielding efficiency is a result of material composition, specific gravity, thickness, grain structure within the alloy, and hardness. Material selection will depend on strength of the unwanted field (HO) and the amount of field attenuation you need.
Mumetal is the preferred solution because of its reliable performance and unique properties of this family of alloys resulting in the highest possible attenuation, making these shielding alloys the materials of choice for reducing low-frequency electromagnetic interference (EMI). Available as stress annealed (partially annealed for ease of machinability) sheet stock, it is used for fabricated shields such as enclosures, cylinders, cans, channels or 3-D boxes. Complex cylindrical shaped shields are the most efficient for redirecting and reducing magnetic fields. Product forms that may be available are foil, sheet, plate, bar, rod, and wire.
Typically, atmospherically controlled final annealing is required after stamping, machining, bending, rolling, severe forming or welding, and is crucial in developing the alloy’s maximum shielding performance. During fabrication, a material’s grain structure weakens. After fabrication processes are completed, grain structure is modified by controlled atmospheric annealing to relieve stress in the material, resulting in larger grain structure and softer temper. Furnace conditions must allow for careful control of the annealing temperature and cooling rate. Optimum properties are reached in a pure hydrogen atmosphere.
Mumetal is an industry reference material, with different characteristics than proprietary alloys. Engineers understand that proper material selection is crucial and must take care to ensure consistency, quality, and shielding efficiency of the materials used for production parts. When the specification or the application calls for a magnetically soft or high permeability material, mumetal is frequently and successfully applied.
Many valuable online tools (such as slide rule calculator, online calculator, and B-H curve) are available to help you compare shielding alloys, different thicknesses, shielding efficiencies, and select the size of shielding required to reduce (attenuate) unwanted fields.
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