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Electrical Design Considerations

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ProofTesting

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Breakdown Voltage

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Creepage Distance And Discharge

Proof Test

The purpose of "Proof Testing" Thermal Clad substrates is toverify that no defects reside in the dielectric material. Because testing requires that voltages be above the onset of partial discharge, we recommend the number of “Proof Tests” be kept at a minimum.In proof testing, partial discharge can look like leakage current.Agency acceptance tests differentiate between discharge current and leakage current. Using soldermask can raise the test voltage where partial discharge is detected. Potting the completed assembly can eliminate partial discharge. Amuch more complete discussion of discharge and spacing is available in Application Note #130.Partial Discharges (PD) are localized releases of internal energystored in electrical insulation systems in regions of defects in the media and/or at interfaces of different materials. These discharges of energy are within the insulation system, being restricted to only a part of the dielectric material, hence not necessarily forming electrically conducting paths amongst system conductors. The series resistance limits partial discharge current in the insulation system. The term "Partial Discharge" is relatively new and includes abroad spectrum of life reducing (i.e., material damaging) phenomena such as:1.Corona discharge in gases.2.Treeing and surface contamination. 3.Surface discharges at interfaces, particularly during faultinduced voltage reversals.4.Internal discharges in voids or cavities within the dielectric.The purpose of the “Proof Test” is to verify that there has beenno degradation of the dielectric insulation due to the fabrication process or any material defects. Continued testing at these voltage levels will only take away from the life of the dielectric on the circuit board. It has been repeatedly verified that “Proof Testing” above the inception of partial discharge (700 Vac or 1200 Vac with proper use of Soldermask) will detect any and all defects in the dielectric isolation in the Thermal Clad circuit board. Any micro-fractures, delaminations or micro-voids in the dielectric will breakdown or respond as a short during the test.The use of a DC “Proof Test” is recommended, from an operatorsafety standpoint. The voltage levels typically used are 1500 to 2250 VDC. Due to the capacitive nature of the circuit board construction, it is necessary to control the ramp up of the voltage to avoid nuisance tripping of the failure detect circuits in the tester and to maintain effective control of the test. This is to avoid premature surface arcing or voltage overshoot. There is safety consideration when DC testing, in that the operator must verify the board tested is fully discharged, prior to removing from the test fixture. Amore detailed discussion of “Proof Test” is available upon request.

Breakdown Voltage

The ASTM definition of dielectric breakdown voltage is: thepotential difference at which dielectric failure occurs under prescribed conditions in an electrical insulating material located between two electrodes. This is permanent breakdown and is not recoverable. ASTM goes on to state that the results obtained by this test can seldom be used directly to determine the dielectric behavior of a material in an actual application. This is not a test for “fit for use” in the application, as is the “Proof Test”, which is used for detection offabrication and material defects to the dielectric insulation.
“Proof Test” fixture to test multiple number of finished circuit boards at one time. 18