Some Noise Cannot Be Fixed by Shielding Alone
The basic principle of EMI shielding is reflection by a conductive material. But reflection does not destroy noise energy — it redirects it. Covering an IC with a metal shield (SUS cover) reduces outward emission, yet the reflected noise can bounce around inside the cover, causing secondary interference in nearby circuits or leaking through seams.
When the problem sits right at the source — CPU, AP, power IC, RF chip — you need an approach that converts the noise energy itself into heat. That is the job of the EMI absorber.
How Absorbers Work: Magnetic Loss
An EMI absorber is a sheet of magnetic powder dispersed in a polymer. As electromagnetic waves pass through, the material's magnetic loss converts the wave energy into minute amounts of heat. Because it dissipates noise instead of trapping it, it is especially effective against near-field coupling right at the source.
Permeability indicates how readily the material takes in magnetic energy — the higher, the greater the absorption potential. Effective frequency characteristics differ from product to product, so always match the absorber's characteristic curve to the frequency band of the problem noise.
Shielding vs. Absorption Compared
| Shield (conductive tape, etc.) | EMI Absorber | |
|---|---|---|
| Principle | Reflection (conductivity, low surface resistance) | Absorption (magnetic loss, heat conversion) |
| Noise handling | Blocks the path and drains noise to ground | Attenuates the energy itself |
| Best for | Blocking outward emission, cable/seam leakage, grounding | Countermeasures at the source, cavity resonance inside covers, circuit-to-circuit interference |
| Caveat | Reflected noise can cause secondary interference inside enclosures | Alone, weaker at blocking outward emission than a shield |
In Practice, the Answer Is Usually a Combination
- SUS cover + absorber — apply an absorber on or inside the metal cover to soak up the internal resonance and near-field noise the cover cannot handle.
- Absorber + Cu tape multilayer — the absorbing layer attenuates energy while the Cu layer reflects the remainder: shielding and absorption in a single sheet, dust-free when die-cut, well suited to volume production.
- Division of labor with conductive tape — conductive tape handles seams and grounding while the absorber covers near-field noise above the chip.
A Suggested Selection Sequence
- Identify the noise source and frequency band (near-field scan, etc.).
- If outward emission is the issue, start with shielding (path blocking and grounding); if interference near the source, start with absorption.
- If a cover shield is already in place with no improvement, suspect internal resonance and consider adding an absorber.
- Confirm production conditions — thickness, adhesion, converting — before finalizing the material.
FAQ
- Does an absorber generate heat?
- The absorbed noise energy does become heat, but the amount is negligible — nowhere near enough to affect device temperature. If chip heat itself is also a concern, consider combining with a heat-spreading sheet — see Thermal Management Basics.
- What frequency range can you cover?
- Effective bands differ by product. Solueta absorbers maintain absorption across a wide band; tell us your problem band and we will recommend a product backed by measurement data.
Struggling with EMC certification because of stubborn noise?
We will review everything with you, from source analysis to a combined shield-plus-absorber design.
