Can Electrical Resistance Be Used to Predict Shielding Effectiveness?
Conductive elastomer EMI shielding gaskets use metallic particles to create a conductive path and shield enclosures from electromagnetic radiation. A key measurement of these gaskets is Electrical Volume Resistivity. Gaskets that have a lower DC resistivity generally indicate a more conductive particle. In many cases, this lower resistance / higher conductivity is associated with better levels of shielding effectiveness. This explains why gaskets with silver particles, which are very conductive, often out-perform gaskets with graphite particles. However, this is not always the case.
A common misconception is that a measurement of DC resistivity can directly predict shielding effectiveness. Over the years, material science evolution has proven that EMI gaskets with higher DC resistance can produce higher levels of shielding effectiveness in some cases compared to gaskets with low measured DC resistance.
How can that be possible, a conductive elastomer EMI gasket with 20x greater DC resistivity that actually has a HIGHER shielding effectiveness? Well, that's because there are many factors have an impact on shielding effectiveness of a conductive elastomer EMI gasket and the volume resistivity value is only one. Other aspects of EMI gasket material and enclosure seam design which can effect shielding effectiveness are as follows:
- Particle morphology – size and shape – ability to bite through conversion coatings
- Elemental composition of the particle – i.e. permeability, absorption properties
- Elemental composition of the plating on the particle – i.e. permeability, absorption properties
- Plating thickness
- Compounding control and filler loading %
- EMI gasket surface conductivity and volume resistivity
- EMI gasket geometry
- EMI gasket footprint contact size on mating surfaces
- Gasket deflection %
- Type of metal substrate – aluminum, steel etc.
- Conversion coating or plating finish
- Fasteners/bolts – quantity, separation, generated gasket compression load
- Gasket groove (if used)
- Ancillary metal to metal contacts
Check out the Parker Chomerics Conductive Elastomer Handbook for more information on EMI Shielding Theory and the specific properties of various types of conductive elastomers.
This blog post was contributed by Ben Nudelman, market development engineer, Chomerics Division.