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Leakage Current HiPot Testing Creepage Distance and Discharge
Due to the variety of dielectric types, thicknesses and boardlayouts,not all boards test alike. All insulated metal substratesclosely resemble a parallel plate capacitor during HiPot testing. The capacitance is equal to: Creepage distance and discharge has to be taken into accountbecause Thermal Clad dielectrics often incorporate a metal base layer. Circuit board designers should consider “Proof Testing” requirements for: conductor-to-conductor and conductor-to-circuit board edge or through holes. The graph adjacent depicts flashover: without soldermask, with soldermask and under oil. C = ∈ A/d
where: ∈ =Permittivity (Dielectric Constant)A=Surface Aread=Distance (Dielectric Thickness) The capacitance value changes with different configurations ofmaterials and board layouts. This can be demonstrated where one board fails the test and another passes, but when both are actually tested for dielectric strength and leakage current in a controlled environment, both pass. Therefore, it is very important to properly design the testing and test parameters with the material characteristics in mind. Test set-up and parameters that over-stress or do not consider reactance of the material and its capacitive and resistive components, can lead to false failures and/or test damage of the board.Another test characteristic that is generally misunderstood withinsulated metal substrates is the leakage and charge current that take place during the test. In most cases, the leakage current value on insulated metal substrates is much smaller than the measurement capability of a typical HiPot tester. What is most misunderstood is the charge current that takes place during the test. Leakage current measurements can only be realized once the board has been brought to the full test voltage (DC voltage) and is held at that voltage during the test. This current value and rate dI/dTis directly related to the capacitance of the board. Therefore, a board that has an effective capacitance higher than another board will have a higher charge currentrate than the one with a lower effective capacitance. Thisdoes not reflect the leakage current or the voltage withstand of the dielectric insulation instead, it represents the characteristic transient response of the dielectric. Therefore, one is not able to determine comparable leakage current based on the instantaneous charge current.For accurate leakage test data, bring the board up to full testvoltage and hold. FORPOSITIONONLY MEASURED CURRENT - CHARGE CURRENT = LEAKAGE CURRENT
MEASURED CURRENT - CHARGE CURRENT=LEAKAGE CURRENT 19