What’s Volume Resistivity in PCB?
Electrical Resistivity (also called Specific Electrical Resistance or Volume Resistivity) (ρ) is a fundamental property of a material that measures how strongly it resists electric current. Its inverse, called Electrical Conductivity, quantifies how well a material conducts electricity. A low resistivity indicates a material that readily allows electric current. The SI unit of electrical resistivity is the ohm-meter (Ω⋅m). For example, if a 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.
Also refer to Surface Resistivity, Sheet Resistance.
How to Measure Resistivity to Leakage Current?
- Applying a known voltage to the material
- Recording the current created by the voltage
- Using Ohm’s law to calculate the observed resistance
- Determining resistivity based on the specimen’s physical dimensions
- The final resistivity measurement depends on many outside factors, including:
- Applied Voltage: The amount of voltage applied to the material greatly alters the test’s final results. To counter this factor, sometimes a test involves varied voltage to establish voltage dependence.
- Electrification Time: The tested material charges at an exponential rate when exposed to voltage for an extended period. Therefore, the resistivity of a sample increases over time during the test. This must be accounted for to get an accurate calculation.
- Environmental Factors: High humidity levels create lower resistivity compared to lower humidity levels. The testing environment’s conditions have a large impact on potential results.
Because of these variables, these conditions should stay constant between tests when comparing multiple tests. ASTM standards recommend the commonly used method of 500V applied for 60 seconds to make results easily comparable to one another. The results of this test can measure volume resistivity, depending on their application.
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In PCB industry, volume resistivity is always measured in Ω⋅cm, and used in dielectric materials, like CCL, FCCL, Coverlay, Bonding Sheet, PP and etc.