4 Ways to Reduce Costs of Magnetic Shields
MuMetal and other high nickel content specialty metal alloys are commonly used for high performance magnetic shielding (also known as 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 enclosure, or in the case of a plastic component, a coating such as silver or copper, will suffice.
Background on MuMetal Manufacturing Processes
To make a shield with these specialty magnetic shielding alloys, you would normally start with a raw sheet metal and process it 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) would have an effect on magnetic shielding 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 high attenuation shielding of very low power magnetic fields, like the earth's magnetic field. While these conventional materials have very high permeability which gives excellent attenuation in these low power fields, they have limited saturation performance, meaning that they are quickly "overwhelmed" by higher field strengths.
The cost of procuring and using a magnetic shield is driven by a few main areas:
- Raw Material Cost
- Manufacturing/Annealing Cost
- Integration Cost - as these shields need to be integrated
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?
4 Ways to Reduce Shield Costs
1) Reduce Material Cost: Go thinner
As was mentioned, most magnetic shields have high nickel contents (around 80%). As of February 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 rather than 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). This is a good option if you cannot reduce material thickness because of manufacturability considerations, but have some margin on the shielding attenuation side of things. That is, you can live with some modest performance reductions in order to achieve a cost reduction.
3) Reduce Shield Complexity
Shield complexity is also a significant driver of cost. Traditional shields are made from sheet metal and employ traditional metal working processes like drawing, stamping, forming, welding etc. A more complex shield will often require multiple manufacturing steps. 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.
4) Switch to a Nickel Alloy Plating on a 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 Nickel alloy with good soft magnetic characteristics. While the coating will have a high Nickel content, it is applied as a thin (typically 0.003 inches or 75 micron) coating instead of having the entire shield made from the high cost magnetic shielding material. The high magnetic saturation of the steel coupled with the high permeability of the nanocrystalline Nickel alloy plating combine to provide excellent high flux density performance.
Of course, the key to this approach is finding a supplier that can provide all of the following: Nickel alloy plating, nanocrystalline metal plating and that knows of whether the resulting product is a good soft magnetic material - that is a material with good magnetic permeability.
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 an integration step is also eliminated along with a reduction in the overall bill of materials part count. For applications where dimensional tolerances are tight, machined aluminum can also be utilized. Partial (or selective) plating is also possible, although the cost benefits of this approach are generally lost when the added labour of the masking step is factored in.
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, this approach can also be considered over injection molded plastics.
About the Author
Rich Emrich is VP Business Development at Integran Technologies Inc, a company that specializes in nanocrystalline metal electroplating including nickel alloys with good magnetic permeability (such as Nanovate EM). Integran current electroplates a wide variety of enclosure materials to create mangetic shields for sensitive aerospace, defense, industrial and autmotive applications. Click here for further information about Integran's processes.