Mu-Metal vs Nickel Silver: Choosing EMI Shield Material by Frequency
Nickel silver is the RF shield-can workhorse; mu-metal is for low-frequency magnetic fields. Learn why the right choice depends on frequency and field type — not raw conductivity.
Key Takeaways
At RF, shielding is dominated by reflection, so nearly any conductor works and nickel silver is chosen for formability, solderability, and cost. At low frequency (below ~100 kHz), magnetic fields must be absorbed or diverted, which requires a high-permeability material like mu-metal. The deciding factor is frequency and field type, not raw conductivity.
Why it matters:
- Above ~100 MHz, material choice barely affects shielding — reflection already dominates
- Low-frequency magnetic fields need permeability (mu-metal), not a better conductor
- Mu-metal work-hardens when formed and loses permeability, so it's rarely used for stamped board-level cans
Quick Reference:
| Factor | Recommendation |
|---|---|
| RF / plane-wave (>100 MHz) | Nickel silver (or tin-plated steel) — chosen for formability & cost |
| Low-frequency magnetic (<100 kHz) | Mu-metal or other high-permeability alloy; consider thickness |
| Corrosive environment | Stainless or appropriate plating; RF performance still adequate |
Ask two engineers which metal makes the best EMI shield and you may get two confident, opposite answers. They are usually both right — for different frequencies. Shield material selection only makes sense once you separate two very different shielding mechanisms.
Two mechanisms, two materials
Shielding effectiveness is the sum of reflection loss (the impedance mismatch a wave sees at the metal surface) and absorption loss (attenuation as it travels through the wall).
- At RF (MHz to GHz), reflection dominates. A good conductor reflects so strongly that even a thin, modest-conductivity sheet provides far more shielding than a board-level can ever actually delivers in practice.
- At low frequency (below ~100 kHz), reflection of magnetic fields is weak. Shielding then depends on absorbing and diverting magnetic flux, which requires high magnetic permeability.
That single split explains the whole material question.
Why nickel silver wins at RF
Because RF shielding barely depends on the metal, the real selection drivers are formability, solderability, corrosion resistance, and cost. Nickel silver checks every box: it solders without pre-plating, forms to tight radii, resists corrosion, and stamps as thin as 0.10 mm. That is why it dominates board-level shield cans.
When you actually need mu-metal
Mu-metal earns its place only against low-frequency magnetic fields — the noise from motors, transformers, switching at low rates, and current-sensing circuits. Here, high permeability lets the material absorb and redirect magnetic flux around the protected region.
Mu-metal work-hardens when stamped or bent and loses much of its permeability, usually requiring re-annealing after forming. That makes it impractical for ordinary stamped board-level cans — it's reserved for dedicated low-frequency magnetic enclosures (for example, around sensitive magnetic sensors).
Skin depth: why thickness rarely matters at RF
The reason thickness is almost irrelevant at RF is skin depth — the depth at which the field falls to about 37%:
δ = 1 / √(π · f · μ · σ)
At 1 GHz, copper's skin depth is only about 2 µm, so a 0.15 mm wall is thousands of skin depths thick — absorption is enormous and reflection already dominates. Near DC, though, skin depth is millimetres, so a thin non-magnetic can barely touches a magnetic field. That is the quantitative version of "use permeability, not thickness, at low frequency."
Decision summary
- RF / plane-wave shielding (>100 MHz): nickel silver (or tin-plated steel) — choose for formability and cost.
- Low-frequency magnetic (below 100 kHz): mu-metal or another high-permeability alloy; thickness matters here.
- Corrosive environment: stainless or appropriate plating; RF performance stays adequate.
- Always confirm performance by measurement on the real geometry and frequency.
Where POCONS fits
POCONS precision-stamps board-level shield cans — predominantly nickel silver, plus other alloys and platings on request — at an IATF 16949 facility in Korea, with sales, stock, custom tooling, and design-in support from San Diego. We can advise on material for your frequency and environment, publish real part dimensions, and do not publish shielding-effectiveness figures we have not measured for a given configuration. Request a sample or material review.
Frequently Asked Questions
Why is nickel silver the standard EMI shield-can material?
Because at RF nearly any metal reflects fields extremely well, so material rarely limits shielding. Nickel silver (a copper-nickel-zinc alloy) wins on the practical factors: it solders without pre-plating, forms to tight radii, resists corrosion, and is available very thin. Material choice at RF is about manufacturability and cost, not raw shielding capability.
When do I actually need mu-metal?
For low-frequency magnetic fields — roughly below 100 kHz, from sources like motors, transformers, and current sensing. There, reflection is weak and shielding works by absorbing and diverting magnetic flux, which requires a high-permeability material such as mu-metal. For ordinary RF board-level shielding, mu-metal is unnecessary.
Doesn't higher conductivity mean better shielding?
Only for RF electric and plane-wave fields, where reflection loss scales with conductivity — and even modest conductors already provide far more than a board-level shield needs. For low-frequency magnetic fields, conductivity is nearly irrelevant; permeability is what matters. So 'more conductive' is not a universal rule.
Can I just stamp a mu-metal shield can like a nickel silver one?
Not easily. Mu-metal work-hardens when formed and loses much of its permeability, often requiring re-annealing after forming. That makes it impractical for typical stamped board-level cans, which is why mu-metal is reserved for dedicated low-frequency magnetic enclosures rather than general RF shields.