Dielectric Integrity
Insulation verification protocol confirms that no unintended conductive paths exist between separate nets or layers on a printed circuit board. A substrate isolation test applies a high voltage between conductors to measure the resistance of the intervening dielectric material. This procedure is a standard part of the electrical testing phase in board fabrication.
Leakage Detection
Measurement equipment monitors the flow of current when a potential of 100 to 250 volts is applied to the circuit. The substrate isolation test identifies defects such as conductive anodic filaments, moisture trapped in the laminate, atmospheric contaminants, or residues from the etching process. If the resistance drops below a certain threshold, typically 10 to 100 megohms, the board is flagged as a failure.
This step prevents short circuits that could cause catastrophic damage once the board is powered up in a final assembly.
Pass Criteria
Environmental conditions such as humidity and temperature must be controlled to ensure the accuracy of the readings. During a substrate isolation test, the dwell time must be sufficient for the charging current to dissipate so that only the true leakage current is measured. Any contamination on the surface of the board can provide a path for electricity and lead to a false failure.
Regular calibration of the flying probe or bed-of-nails tester is required to maintain the sensitivity needed for high impedance measurements.
