How to Evaluate EMI Shielding Effectiveness for Stamped Cans
Shielding effectiveness is a system property, not just a material property. A can made from a highly conductive alloy can still fail if the seams leak, the grounding is poor or an aperture is too large.
This article explains how SE is evaluated and what to check before accepting a stamped shielding design.
What SE measures
SE is the ratio of the field without the shield to the field with the shield, expressed in dB. It is frequency-dependent and should be specified over the range that matters for the product.
Test methods
| Method | Use | Notes |
|---|---|---|
| Near-field probe | Quick comparison of seams and apertures | Useful for development, not a formal standard |
| TEM cell / GTEM | Small assemblies | Controlled field, limited size |
| Anechoic chamber | Formal product testing | Most representative, higher cost |
| Board-level scan | Locating leakage on a PCB | Helps identify hot spots |
What affects the result
- Material conductivity and thickness.
- Seam overlap and contact pressure.
- Number and spacing of ground points.
- Aperture size and shape.
- Cable penetrations and connector openings.
- Proximity to the noise source.
Practical acceptance criteria
Acceptance criteria should come from the product standard or the customer requirement, not from a generic number. For many consumer and industrial products, 30-40 dB may be adequate; sensitive RF or automotive modules may require more. Confirm the frequency range and the test method before quoting a dB target.
Common failure modes
- Can sits above the ground plane without reliable contact.
- Large slot or connector opening that acts as a slot antenna.
- Paint or conformal coating insulating the grounding points.
- Removable lid with insufficient contact force.
- Grounding only at one or two points.
FAQ
Can I rely on the material datasheet for SE?
No. The datasheet may give material properties, but SE depends on the complete enclosure and assembly. Testing is the reliable approach.
How much does plating affect SE?
Plating can improve conductivity and corrosion resistance, but it does not fix a poor seam or a large aperture.
Is a removable lid a problem?
It can be, if contact force and grounding are insufficient. Design the lid with spring fingers or dimples and verify the SE with the lid installed.
Related: shielding can manufacturing and request a design review.
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