Cyclic Moisture Resistance Testing
Cyclic moisture testing is performed for the purpose of evaluating, in an accelerated manner, the resistance of component parts and materials to the deleterious effects of the heat and high-humidity which are typical of tropical environments. Most tropical degradation results directly or indirectly from the absorption of moisture by vulnerable insulating materials and from the surface wetting of metals and insulation. These phenomena produce many types of deterioration in constituents of materials including corrosion of metals and detrimental changes in related electrical properties.
The cyclic moisture resistance test, as performed per MIL-STD-883, test method 1004, differs from the steady-state humidity test and derives added effectiveness in its employment of slow temperature cycling, which provides alternate periods of near-condensation and drying. In addition, it produces a breathing action of moisture into non-hermetic packages. Cyclic moisture resistance testing also includes low-temperature “sub-cycles” that act as an accelerant to reveal otherwise indiscernible evidences of deterioration as stresses caused by freezing moisture tend to widen cracks and fissures. The resultant deterioration can then be detected by the measurement of electrical characteristics.
Cyclic moisture is typically run as ten 24-hour cycles. Each cycle contains two 8-hour linearly-ramped excursions at 90% to 95% humidity from 25°C to 65°C (soak temperature) and back down to 25°C. Every 65°C soak is 3-hours long and the down-ramp has a relaxed humidity specification of 80% to 90%.
Typical requirements for the ten-day test is a minimum of five -10°C sub-cycles. The procedure starts with a subcycle consisting of one 8-hour excursion (humidity uncontrolled) linearly-ramped from 25°C to -10°C (soak temperature), then back up to 25°C; each -10°C soak is 3-hours long with ramp times of 1.5-hours. It then proceeds into a subcycles of two 8-hour 25°C to 65°C to 25°C excursions which continues into an 8-hour portion of the cycle. All testing is monitored for accuracy.
Test Specifications / Standards