Reliability qualification test for circuit boards exposed to airborne hygroscopic dust

The insulation resistance of circuit boards was monitored after different hygroscopic salts were deposited. By choosing different salts and concentrations, various resistance versus humidity characteristics were achieved. Insulation resistance as high as 10¹¹ Ω was lowered to 10⁶ Ω at 40% relative humidity after controlled contamination with calcium chloride. For purposes of designing a qualification tool to identify design robustness, the salt and contamination technique should give controllable and reproducible characteristics. Spin coating and spraying of atomized particulates were investigated. Sodium sulfide or, as alternatives, potassium carbonate or calcium chloride was identified as a suitable salt for the test. Although these materials are not typically found in airborne hygroscopic dust, their use offers a controllable means to simulate the loss of surface insulation resistance and the failures that can result. After being covered by the test salt, the board under test is plugged back into its host system via an extension card. The local environment around the circuit board is controlled and maintained at 25°C and high humidity. A functional test in normal operation of the circuit board is then performed