Magnetic shielding materials to protect sensitive electronics

Low-frequency magnetic fields can impact electronic devices, inducing unwanted current flows in sensitive paths that can add noise, introduce errors, and otherwise disrupt carefully balanced circuits. While the result in consumer systems such as cell phones and computers might not rise beyond poor communications and lost data, the impact in other systems can be devastating. In automotive systems, for example, lack of proper shielding can have catastrophic effects when magnetically sensitive sensors, such as Hall-effect sensors used in anti-lock braking systems, fail to operate as expected.

For engineers, protecting sensitive electronics from low-frequency magnetic fields requires a different approach than protecting those same devices from other sources of energy and noise. Unlike other sources of energy such as RF, low-frequency magnetic sources are pervasive. Besides the magnetic field from the Earth itself, magnetic fields arise from power supplies, transformers, and amplifiers. Indeed, as Maxwell’s equations show, they can arise from any current flow. Furthermore, magnetic fields cannot be destroyed or otherwise extinguished. They can be redirected, however, and therein lies the fundamental role of magnetic shielding materials.

Path of least resistance

Just as current will follow a low-resistance path to ground, magnetic fields will use the easiest available path to complete their own circuit. For magnetic fields, the easiest path lies through materials with the highest magnetic-permeability characteristics. Thus, a highly permeable material placed between an external magnetic field and a circuit will reduce the magnetic field reaching the circuit by redirecting the magnetic field away from it.

Although more permeable materials can effectively attenuate a magnetic field, no material can completely redirect a magnetic field, much less eliminate it. As a result, there is no material that can form a perfect magnetic shield. Even if formed into a perfect sphere, real materials will only reduce the magnetic field reaching the interior of the sphere — sometimes dramatically but never completely. Furthermore, in real applications, the need for different shapes of enclosures, such as openings for wires and seams between joints, work to compromise the ideal effectiveness of any shield design. Accordingly, magnetic shields are typically designed as one or more layer of highly permeable materials built as an enclosure or wrapped around sensitive circuits. The challenge for engineers is finding materials that are highly magnetically permeable as well as mechanically suitable for their application.

Optimized characteristics

Magnetic permeability is a characteristic of all materials, but certain alloys provide the best combination of magnetic characteristics and workability (or ability to be cut and shaped into the required form). Permeability μ of a material is a ratio of the flux density B in a material to the magnetizing force H. Typically, the magnetic characteristics of a material is expressed as relative-dc permeability, which is the ratio of μ (the permeability of the material) to μ₀ (the permeability of a vacuum) subjected to a static magnetic field. Magnetic material vendors typically use relative-dc permeability in citing permeability characteristics of their products.

Magnetic permeability of non-specialized base materials is typically low — around 100 for 99% pure nickel, for example. Formed into alloys and subjected to annealing processes, however, permeability can reach levels useful for magnetic shielding. For example, alloys of nickel can achieve permeability values in the tens of thousands (see Fig. 1 ). Furthermore, annealing alters the crystal structure in materials, removing impurities and aligning the grains to provide substantially greater permeability than the corresponding untreated material. In another example, the permeability of pure iron improves by well over an order of magnitude when annealed in a hydrogen atmosphere. However, pure iron lacks the other material characteristics required in magnetic-shield design.

Nickel-iron alloys have emerged as the material of choice in magnetic shielding, but the ratio of metals can dramatically affect their suitability. For example, NiFe alloys with low-Ni ratio (50% and below) only reach permeability levels of a few thousand. In contrast, higher Ni content alloys such as permalloy or mu-metal can exhibit permeability in the tens of thousands. Fully annealed, these alloys can achieve even higher permeability characteristics. Engineers can find a variety of such products, including Magnetic Shield’s MuMETAL, Carpenter Technology’s Mumetal, VACUUMSCHMELZE GmbH’s MUMETALL, and others.

Small is big

Even within this class of NiFe alloy, seemingly smaller differences in NiFe content can dramatically affect suitability for magnetic shielding. For example, with its approximately 80% Ni and 20% Fe content, permalloy is not easily workable, so it tends to find application in transformers or recording heads rather than magnetic shielding, where material must be easily worked into different shapes. Furthermore, high-permeability NiFe materials typically exhibit low-magnetic-saturation characteristics, resulting in less effectiveness in high-magnetic-field environments.

Fig. 1: For low-frequency magnetic shielding, engineers can combine layers of materials, using high-magnetic-saturation iron alloys, such as NETIC, to attenuate high fields, plus high-permeability annealed nickel alloys, such as MuMETAL (80%) and CoNETIC B (49%). (Source: Magnetic Shield Corp., www.magnetic-shield.com)

Engineers can use these different magnetic-performance characteristics to their advantage in designing appropriate magnetic shields. For example, when dealing with a high magnetic field, engineers can use a multilayer structure with a layer of high-Fe material such as Magnetic Shield’s NETIC on the outside of the shield and high-Ni layers on the inside. High-Fe materials, such as NETIC, do not achieve the permeability levels of high-Ni materials, but their high-saturation characteristics make them well suited for attenuating a high magnetic field to levels that will not saturate high-Ni materials.

This layered approach is common in magnetic shielding applications and allows engineers to leverage the advantages of different materials while minimizing the impact of their limitations. To address this need for variety, material manufacturers offer these specialized alloys in diverse forms including wires, rods, bars, plates, sheets, and foils. Because creating an effective magnetic shield often requires a mix of materials and types, kits such as Magnetic Shield’s Lab Kits allow engineers to evaluate different materials and test alternative multilayered designs.

Typically, commercial NiFe alloy products are provided as stress-annealed materials — a partially annealed form suitable for working into enclosures or any required shape. Because any hard forming, cutting, or welding weakens the material’s grain structure, a final annealing process is required to restore grain structure and enhance permeability to maximum levels. Because of the high-Ni content, fully annealed NiFe materials are inherently corrosion resistant and reach a bright sheen that can often be sufficient. Nevertheless, after they subject a design to final annealing, manufacturers can further paint, coat, or plate final parts for additional corrosion resistance or cosmetic requirements.

With the proliferation of magnetic sources in consumer, industrial, hand-held devices and transportation applications, the ability to shield sensitive electronics from low-frequency magnetic fields is becoming more urgent. By combining diverse types of alloys, engineers can achieve the significant attenuation required to ensure reliable operation of electronic devices.