Surface Resistance: Unit, Measurement and Limits

Surface resistance is the electrical resistance a material offers to current flowing along its surface. It is measured in ohms: two electrodes sit on the same side of the sample, a DC voltage is applied, and the voltage is divided by the current that flows (Polymer Service Merseburg, plastics testing encyclopaedia). That ohm value depends on the length and spacing of the electrodes. Surface resistivity in ohms per square (Ω/sq) takes the electrodes out of the equation and describes the material alone. With the electrode layout of the German technical rule TRGS 727 it is exactly ten times the measured value. PTFE reaches 10¹⁷ Ω/sq, polyamide 6 only 5 × 10¹⁰ Ω/sq (Professional Plastics, Electrical Properties of Plastic Materials). Anyone comparing insulation materials for motors, transformers or batteries therefore needs the electrode and the test climate behind every data sheet value. This article explains the unit, standard test methods, the limits between conductive, dissipative and insulating, and the values of common plastics and films.

Ohms or ohms per square: what the unit of surface resistance tells you

A measured surface resistance describes the test setup as well as the material. Long electrodes give the current many parallel paths and produce a low reading; a wide gap between the electrodes produces a high one. Two labs with different electrodes will therefore read different ohm values on the same material. Ohms per square solves this, because a square has the same resistance between two opposite edges whatever its size.

Electronic Design works through an example. A 2 × 2 inch square, measured with two 2 inch bar electrodes along opposite edges, reads 5 Ω, so 5 Ω/sq. Two such squares in a row give 10 Ω along the long side. Put two of these strips side by side and you have a 4 × 4 inch square; the two 10 Ω strips are in parallel, and the result is 5 Ω again. For any other electrode the general formula is ρ_s = R × P / g, with the measured resistance R, the effective electrode length P and the electrode gap g. For ring electrodes, P is the circumference at the middle of the gap. On the Keithley 8009 fixture that is π × 53.975 mm, and dividing by the 3.175 mm gap gives a factor of 53.4 (Tektronix, Keithley 6517B application note).

The arrangements differ by more than a factor of five. A film that reads 10¹² Ω on the Keithley electrode has a surface resistivity of 5.3 × 10¹³ Ω/sq, and on the TRGS 727 electrodes it would read around 5 × 10¹² Ω. A data sheet value in ohms without the electrode cannot be compared with any other value. Physically, Ω/sq is still an ohm; the notation is not part of DIN 1301 and only serves to tell the two apart (Wikipedia, Flächenwiderstand). Some glossaries give Ω·m as the unit. That is the unit of volume resistivity and wrong for a surface.

Surface resistance, volume resistance and insulation resistance are three measurements

Current has two routes through an insulating material: across the surface and through the bulk. Test standards separate them by draining the other route through a grounded guard electrode (UL, electrical properties of plastics). The separation is never perfect, as part of the surface current flows through layers of the bulk just below the surface (Polymer Service Merseburg).

Volume resistivity tells you whether a material insulates at all and converts into electrical conductivity in S/m. Surface resistance tells you what happens on the skin of the material, which is exactly where moisture and dirt act. On a finished device, the test measures the insulation resistance in megohms, which combines surface, bulk and creepage paths.

In thin-film and semiconductor work, the same quantity is called sheet resistance. For a conductive layer R_sq = ρ / d applies, and a 35 µm copper layer comes to just under 0.5 mΩ (Wikipedia, Flächenwiderstand). It is measured with four probes so that contact resistance drops out. On insulators this method fails because the currents are too small. When choosing copper foil for shielding, sheet resistance in the milliohm range is what counts; for insulating parts it is surface resistance.

How to measure surface resistance

The measurement itself is simple: apply a voltage, measure the current, divide. It gets difficult because insulators carry currents of picoamps and less, and humidity, dirt or a different electrode shift the result by orders of magnitude. A reliable result takes six steps:

1. Condition the sample. The test lab Measurlabs stores 125 × 125 mm plastic plaques for four days at 23 °C and 50 % relative humidity and measures three of them in the same climate. For ESD materials, ANSI/ESD S11.11 specifies 48 to 72 hours at 12 % humidity (Electronic Design).
2. Keep the surface free of fingerprints, dust and release agent. The powder coating maker IGP requires contamination to be removed without residue before testing (IGP Pulvertechnik, TI 101).
3. Place the electrode and load it with the standard weight. The Warmbier ring electrode 880 to IEC 61340-2-3 is 80 mm in diameter and is brought to 2.5 kg; IGP also tests with 2.5 kg electrodes.
4. Choose the test voltage to the standard: 100 V to IEC 62631 unless another voltage is specified, typically 500 V under ASTM D257 (Tektronix). ESD materials are tested to ANSI/ESD STM11.11 at 10 V first, switching to 100 V once the reading reaches 10⁶ Ω (Electronic Design).
5. Read the value after the electrification time, typically 60 seconds. The reading rises with time because the material charges and the current falls (Tektronix).
6. Record electrode, voltage, time, temperature and humidity. Without these five details the value cannot be reproduced.

The standard has to match the material. When ASTM D257 was applied to dissipative ESD materials, five labs testing identical samples ended up about four orders of magnitude apart. The ESD method S11.11, with 10 and 100 V and a dry climate, cut the spread to plus or minus a quarter of an order of magnitude (Electronic Design). When comparing materials, think in powers of ten: two insulating materials only differ significantly when their values are at least one order of magnitude apart (Polymer Service Merseburg).

Limits: when a material counts as conductive, dissipative or insulating

The three classes sound like a fixed scale, but the boundaries depend on the rulebook and the climate. TRGS 727, the German rule for explosion protection, gives two upper limits for the same dissipative object, depending on whether it was measured at 50 or 30 % humidity. The ESD method S11.11 tests at 12 %, drier than either TRGS level. ESD guides based on the first edition of IEC 61340-5-1 from 1998 put both limits one order of magnitude higher than newer sources.

Sources for the table: TRGS 727 no. 2, Industrie-Fachwissen and IGP Pulvertechnik for ESD practice, the Kärcher ESD handbook for the 1998 classes. Individual items have tighter windows. An ESD work surface should have a surface resistance of 10⁶ to 10⁹ Ω (Industrie-Fachwissen), and a dissipative powder coating should not drop below 10⁴ Ω so that discharges do not happen too fast (IGP Pulvertechnik). Above 10¹⁰ Ω the Kärcher handbook recommends measuring the charge decay time as well.

For purchasing, this means a limit such as "below 10⁹ Ω" only becomes a specification once standard, electrode and humidity are named. Put all three in the order, or the supplier will test to their own standard and be formally right. How plastics reach the dissipative or conductive range with carbon black, carbon fibre or intrinsically conductive polymers is covered in the article on electrically insulating plastics.

Typical values: plastics, insulating films and ESD films

The table shows the surface resistivity of unfilled and filled plastics from the Professional Plastics property table, plus a coated ESD film. The class follows ESD practice from the table above.

The insulating films used in electrical engineering sit far up the scale. Polyimide, the material of Kapton film, is five orders of magnitude above the insulating limit, a polyester film still two. Polyamide is the odd one out: it absorbs moisture from the air, and PA 6 already sits below 10¹¹ Ω/sq in this table. For an insulating part in a humid environment we therefore advise against polyamide. Professional Plastics gives neither climate nor standard for its values. The table shows how the materials rank; for a specification, use the data sheet of the actual semi-finished product, as single values shift by an order of magnitude or more with climate, electrode and voltage.

Humidity and dirt lower surface resistance by orders of magnitude

Every surface picks up a thin film of water from the air. Dissolved salts and dust make it conductive, and surface resistance falls. TRGS 727 puts it plainly: as humidity drops, surface resistance as a rule rises considerably. The reverse case hits ESD materials. A study by Finland's VTT tested ESD protective materials between 5 and 70 % humidity. Below 20 to 30 %, some of them can turn into insulators (Paasi et al., Journal of Electrostatics 2001).

The surface reacts more strongly than the bulk (Tektronix). Even the bulk of an aramid paper loses two orders of magnitude at high humidity: 0.25 mm Nomex 410 drops from 2 × 10¹⁶ Ω·cm at 50 % to 2 × 10¹⁴ Ω·cm at 96 % humidity (Nomex 410 data sheet, DuPont, now Arclin). On the finished part, influences appear that no lab specimen has:

• Condensation when a cold component meets warm, humid air
• Fingerprints that leave salt and grease on the film
• Cutting dust and conductive particles from processing
• Antistatic agents that migrate from the plastic to the surface and bind water there (Wikipedia, Antistatic agent)
• Tracking marks from earlier discharges, which are carbonised and conduct

This is where surface leakage current comes in. Surface resistance is measured on a clean, conditioned sample and describes the starting condition. Tracking resistance (CTI) to IEC 60112 tests the highest voltage at which a surface withstands 50 drops of electrolyte without forming a conductive track. A high surface resistance therefore does not guarantee a high CTI, and design needs both, together with the creepage distance of the assembly.

Surface resistance on the finished part: what we watch in production

A common objection is: the higher the surface resistance, the better. For a slot liner or a phase separator that is true. For a film that runs over rollers, is unwound and stacked, it is only half true. A strongly insulating surface charges up in dry air as soon as air moves across it (Kärcher, ESD handbook). The charged film attracts dust, and that dust later lowers the surface resistance of the installed part. For films meant to drain charge in electronics packaging or handling, a dissipative value is therefore the right specification. Insulating parts still need a high value at the real operating climate; to keep them from charging during processing, the Kärcher handbook recommends humidity above 30 % and ionisers.

We slit and die-cut insulating films and papers in thicknesses from 7 µm to 3 mm depending on the material, as rolls, sheets, blanks, die-cut parts or formed parts, from a single piece. In contract slitting to widths of 5 to 1,600 mm with plus or minus 0.1 mm, we keep edges burr-free and the process low in contamination, so that neither burrs nor particles pull down the surface resistance of the finished part. Films with an antistatic coating are available on request, for example for the printing industry. With material in stock, we deliver within two weeks at most.

For a quote we need the drawing, quantity and thickness, and for rolls also width, running length, core diameter, core material and tolerances. If a surface resistance is required, add the limit, standard, electrode and climate. Our technical consulting team will check with you whether an insulating or a dissipative material suits the installation.

FAQ on surface resistance

Can I measure surface resistance with a multimeter?

No. Insulating plastics such as PET or PTFE sit between 10¹³ and 10¹⁷ Ω/sq, and even for ESD materials the S11.11 method calls for an instrument with 10 V and 100 V test voltage and a range up to 10¹³ Ω. That takes a high-resistance meter, plus a ring electrode with a guard electrode for insulating materials to IEC 62631-3-2. Setups such as the Keithley 8009 measure up to 10¹⁷ Ω/sq (Tektronix).

At what surface resistance does a material count as dissipative?

Under TRGS 727, between 10⁴ and 10⁹ Ω at 23 °C and 50 % humidity, or between 10⁴ and 10¹¹ Ω at 30 % humidity. ESD practice to IEC 61340 mostly uses 10⁴ to below 10¹¹ Ω. Below that a material is conductive, above it insulating.

How much does humidity change surface resistance?

By orders of magnitude. TRGS 727 therefore allows a limit two orders of magnitude higher at 30 % humidity than at 50 %. Below 20 to 30 % humidity, dissipative ESD materials can turn into insulators. A reading without humidity and temperature cannot be compared with anything.

How do you convert ohms to ohms per square?

Multiply the measured resistance by the geometry factor of the electrode, which is the effective electrode length divided by the electrode gap. For the TRGS 727 electrodes and the ANSI/ESD STM11.11 ring electrode the factor is 10, for the Keithley 8009 ring electrode about 53.4. For laminated, coated or metallised materials, S11.11 warns that the conversion often does not hold (Electronic Design).

Why do data sheet values differ so much?

Because electrode, test voltage, electrification time and climate differ. The same film reads about five times fewer ohms on a Keithley ring electrode than on the TRGS 727 electrodes. Only values in ohms per square at the same climate are comparable, and even then only once they differ by an order of magnitude.