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Automobile engine compartments are exposed to much wider temperature and moisture-level changes than passenger compartments. Therefore, for electrical components housed in the engine compartment, protection of printed circuit boards is extremely important in order to prevent open or short circuits caused by electrochemical reactions. It is well known that silicon oxide accumulates on electromagnetic relay contacts, and may cause degraded circuits once volatile low-mass cyclic polydimetylsiloxane from a commonly used silicone gel waterproofing material reacts in a direct-current arc that occurs when the contacts open and close. Material selection for relay modules is critical in order to avoid this phenomenon. We used a gel material jointly developed with a supplier, and evaluated its reliability compared to silicone in terms of relay operation. This material is a polymer resin that consists of poly(n-butyl acrylate) as the main component, linked through silicon.

Compact PCB power relays were tested in repetitious on/off operations to define their reliability through a quantitative analysis of failure cycles and an investigation of relay property changes and contact erosion forms. Operation voltage was 14 V and relay temperature was 120 °C. Ten relays were used in each of two types of load test. First failure cycles were found to be high enough for practical use, 2.2 million in the lamp load (11 A) test and 7.5 million in the test using an integrated load (10 A) comprised of engine control components. Failure mode of all relays in the lamp load test revealed contact welding, while those in the integrated load test revealed significant contact resistance due to excess erosion. Wide differences between them in contact life cycle distribution and failure mode were found to be clearly distinguishable from load current characteristics.

Power relays for mounting on printed circuit boards with just half of the case volume compared to low-profile ISO microrelays were tested in repetitious switching load operations. Operation voltage was 14 V and relay temperature was 120 °C. The tests for normally open type relays used lamps (11A) and horns (8A), and those for transfer (changeover) type relays used wiper motors (8.5A, intermittent operation). First failure cycles in each of the 10 relay tests were found to be of sufficiently high value, 2.2 million for lamps, 3.3 million for horns, and 2.4 million for wiper motors. Cycles at cumulative failure rates determined that the durability margins of the relays were acceptable for use.

Newly developed small-size power relays are described and their test results on switching load durability are presented. They are half the case volume compared to plug-in low-profile ISO Microrelays. Test temperature for the relays was 120 °C keeping in mind engine room conditions. Operation voltage was 14 V. Contact resistance of the normally-open type kept stable values ranging from 3 to 4 mΩ in lamp load tests (11 A, 300k cycles) and in horn load tests (11 A, 150k cycles). Contact resistance in the transfer (change-over) type kept stable values ranging from 3 to 9 mΩ in windshield wiper motor load tests (8 A, INT operation, 600k cycles).