Posted on Mon, Feb 04, 2013
MuMetal and other high nickel content specialty metal alloys are commonly used for high performance magnetic shielding (aka ELF shielding or H-Field shielding) applications, where high magnetic permeability is important in achieving the maximum magnetic attenuation performance. This is in contrast to high frequency EMI shielding where a merely conductive coating, like silver or copper, will suffice.
Background on MuMetal Manufacturing Processes
Parts made with these specialty alloys are normally supplied in raw sheet metal format and processed using conventional fabricating processes, like stamping, with a subsequent annealing step in a controlled atmosphere. This annealing step achieves the large grain structure required for these alloys to reach peak magnetic shielding performance.
With any subsequent deformation step, like re-manufacturing, dropping or press fitting the shields, you would expect to see a drop in shielding performance. Strain (whether temporary or permanent deformation) has an effect on performance.
Other practical considerations may further decrease shielding performance of formed shields. In the case of non-welded joints (tack welded for instance) these seams lead to magnetic field leakage, further reducing shielding effectiveness from the "ideal" case based on "perfect" permeability.
These shields are excellent for shielding very low power magnetic fields, like in the case where you need to have a large attenuation of the shield earth's magnetic field, but have relatively poor saturation, meaning that they are quickly "overwhelmed" by higher field strengths.
Aside from the cost of procuring the shields themselves, customers then need to integrate these shields into existing electronics housings or enclosures, where they incur additional integration/assembly costs.
So, knowing that these shields are costly to procure as they are made from specialty alloys, and are costly to integrate, what options do we have to reduce costs of the magnetic shields or to create low cost magnetic shields?
1) Reduce Material Cost: Go thinner
As was mentioned, most magnetic shields have high nickel contents (around 80%). Currently (Feb 2013), the cost of Nickel is about $8/lb whereas steel is around $0.50 for comparison.
Reducing the thickness of the magnetic shield reduces the nickel used per shield. The thickness of many shields is selected based on manufacturability considerations, instead of attaining a minimum magnetic shielding performance. For this reason, it is often possible to get away with a thinner shield based on shielding performance.
On the manufacturability side, stamping operations usually require a minimum of 0.016" thick sheet metal starting stock. If there is a way of making a 0.008" or 0.004" shield, this would bring down the shield cost. If you were looking at a shielding cylinder, could you consider tack welding a thin foil instead of a welded, ground and drawn, thicker sleeve? This is not an option for all shields, but is worth exploring if you shield is very thick and you are passing your shielding performance target easily.
2) Reduce Material Cost: Change Composition
Another option for reducing shield material cost is to use an alloy with lower Nickel content (like Supra 50 from Magnetic Shields Limited in the UK). This is a good option if you cannot reduce material thickness because of manufacturability considerations, but have some margin on the shield attenuation.
3) Reduce Shield Complexity
Shield complexity is also a significant driver of cost. Traditional shield are most commonly made from sheet metal and therefore employ traditional metal working processes like drawing, stamping, forming, welding etc. You can imagine that a shield that is easily made in one manufacturing step, like a single stamping operation, will be significantly cheaper than one that requires multiple manufacturing processes. For instance, a more costly shield could involve multiple separate pieces that require assembly or integration, or could require an initial stamping process followed by an additional folding, trimming or machining step.
4) Switch to a Shielded Mild Steel
In cases where performance can still be met, an excellent low cost option is stamping, or otherwise forming, a low cost mild steel (remember, the base metal price of mild steel is around 1/20th that of Nickel) and having it electroplated with a nanocrystalline soft magnetic material like Integran's Nanovate EM. While the Nanovate EM plating has a high Nickel content, it is being applied substantially thinner (0.003 inches or 75 micron) than a shield that is made entirely from a magnetic shielding material. We have seen many cases where the high magnetic saturation of the steel coupled with the high permeability of the Nanovate EM combine to provide excellent high flux density performance.
Another benefit of this approach is the potential of reducing overall bill of material cost. If the mild steel can act as the enclosure or housing for the component, then subsequent integration costs are eliminated along with reducing the bill of materials by one part. For applications where dimensional tolerances are tight, machined aluminum can also be utilized. Partial (or selective) plating is also possible, although the masking step will increase the cost of a component as it adds another (often time consuming) processing step.
There is also a benefit in terms of handling as these hybrid parts are quite robust and resistant to damage (i.e., deformation) which in the case of all- MuMetal shields, can result in significantly diminished attenuation performance.
The plated steel shielding approach is particularly cost-effective for medium and high volume shield production.
Lastly, in weight critical applications where cost is less of a concern, Nanovate EM can also be applied over injection molded plastics.