Electrical Opens and Shorts
Verification can be done with two methods.
1. For single-layer boards using A.O.I. (Automatic Optical Inspection) is the most cost-effective method. Using original Gerber data to compare to the etch panel will
find any anomalies, even in an etch-down condition.
2. The more traditional “Bed-of-Nails” testing is also available. This requires a fixture charge and is a higher cost method. It is the only viable method for two-layer constructions.
The purpose of "Proof Testing" Thermal Clad substrates is to verify no defectsreside in the dielectric material. Because testing requires that voltages be
above the onsetof 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. A much
more complete discussion of discharge and spacing is available upon request.
Partial Discharges (PD) are localized releases of internal energy stored 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.
Leakage Current HiPot Testing
Due to the variety of dielectric types, thicknesses and board layouts, not all boards test alike. All insulated metal substrates closely resemble a parallel plate capacitor during HiPot testing. The capacitance is equal to:
The capacitance value changes with different configurations of materials 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 with insulated 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/dT is 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 current rate than the one with a lower effective capacitance. This does 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 test voltage and hold.
Proof Testing or (HiPot)
Testing is done to verify dielectric strength integrity of a T-Clad board.
1. Proof testing done in panel form is the most cost effective method. This form of testing is done at post-etch condition, prior to surface finish or final fabrication.
2. The recommended method for safety agency requirements on T-Clad assemblies is an individual piece-part or array-part test.This method 100% tests and marks each finished board and/or array. This method requires a fixture charge.
3. When higher test voltages are required to meet safety agency requirements; standard clearances for fabrication may not be enough to allow for “Creepage Clearance” to meet test voltages. These minimum creepage distances can be found in safety agency standards or IPC- 2221 for reference.
The term "Partial Discharge" is relatively new and includes a broad 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 fault induced voltage reversals.
4. Internal discharges in voids or cavities within the dielectric.
The purpose of the “Proof Test” is to verify that there has been no 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 operator safety 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. A more detailed discussion of “Proof Test” is available upon request.
Breakdown voltage is the stress at failure when AC at power frequency is applied. The usual rate of rise is 500 Volts/second. The dielectric in Thermal Clad is prepared in separate layers to insure a void free coating. Breakdown voltage testing does not relate to proof stress “Proof Testing”. The ASTM definition of dielectric breakdown voltage is: the potential 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 of fabrication and material defects to the dielectric insulation.
Creepage Distance and Discharge
Creepage distance and discharge has to be taken into account because 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.