Surface Resistivity

Static damage to electronics is controlled by how fast a charge can move across the packaging surface. Surface resistivity puts a number on that. It is defined so that the result is independent of the size of the square measured — a 1 cm square and a 1 m square of the same material give the same value — which is why the unit is written ohms per square rather than ohms. In practice the measurement is made with a concentric-ring electrode pressed onto the sample, applying 10 V for conductive and 100 V for dissipative materials, at a controlled humidity, and the meter reads out resistance that is converted to resistivity by the electrode geometry factor.

The classification limits used across the packaging industry come from ANSI/ESD S541 and the equivalent IEC 61340-5-3. Conductive materials read below 1 × 10⁴ Ω/sq; static-dissipative materials from 1 × 10⁴ to below 1 × 10¹¹ Ω/sq; insulative materials at 1 × 10¹¹ Ω/sq and above. Ordinary EPE and PU foam, polybags and BOPP tape are insulative — typically 10¹² Ω/sq or higher — and generate charge by rubbing against a product. Pink anti-static foam and film sit at the top of the dissipative band, around 10⁹–10¹¹ Ω/sq; black carbon-loaded conductive foam sits below 10⁴ Ω/sq.

Humidity is the variable that catches buyers out. Many dissipative treatments are surfactant-based: they attract a film of moisture that provides the conduction path. Tested at 50% RH they may read 10⁹ Ω/sq; at 12% RH, the dry condition specified in the standards, the same sample can read two or three decades higher and fail. A pack that passes in a Mumbai monsoon can fail in an air-conditioned Bengaluru clean room or in a Delhi winter. Ask for figures at 12% RH, and for long storage or export specify permanently compounded rather than topically treated grades.

Surface resistivity is not the only ESD property. Shielding — the ability to stop an external discharge reaching the contents — is tested separately (ANSI/ESD STM11.31) and requires a conductive or metallised layer; a pink dissipative bag has good resistivity but no shielding. Charge decay time and tribocharging are also measured for critical applications. For most packaging decisions, though, resistivity alone answers the question: bare semiconductors and exposed pins need conductive, boards and enclosed assemblies need dissipative, and finished products in their own housings usually need only that the outer pack is anti-static.

Field checks are simple. A surface resistance meter with parallel-bar or two-point probes costs little and gives a reading in seconds, good enough to confirm that a delivery is in the right decade. Certification for a customer audit needs the concentric-ring method at controlled humidity, which we arrange through an external laboratory when a specification calls for it.

Our anti-static EPE and PU are supplied with the resistivity band stated on the data sheet, and conductive black foam is offered as sheet and die-cut fitments for pin-insertion and bare-die packing.