One of the most common causes of electronics failure is moisture. Because water and electronics do not mix well, several strategies exist to protect mission-critical components from moisture and condensation. One of the most successful methods involves hermetically sealed electronics cavities enabled by moisture-blocking component assemblies such as hermetic feedthrough technology.
Reliable sealing involves two major considerations: accounting for moisture sources in the ambient environment, and choosing a robust hermetic feedthrough that will keep moisture from wicking into the electronics cavity while allowing power and signal conductors to enter.
Electronics failures due to water vapor and condensation can occur in a number of ways. Among the most common are corrosion of solder joints, shorts caused by water droplets, and stress corrosion cracking of seals and electrical interface areas. Glass seals with unseen defects and those under high residual stresses can crack and fail over time, especially in cold temperatures.
Water vapor within hermetic packages may arise from several sources. With die attach adhesives, water is often generated as a byproduct of the curing process itself. This is due to certain bonding agent chemistries that produce water during polymerization reactions when adhesives are curing. To avoid this issue, the curing process must be completed before sealing, or an epoxy formula that does not contain water byproducts should be chosen. Prebaking materials can also eliminate any trapped moisture in components.
Water issues can also occur during the sealing of gas containers. To attack this potential issue, manufacturers vacuum purge and backfill hermetic cavities before sealing, typically using helium, nitrogen, and dry air. However, improper gas control or faulty sealing techniques can lead to water vapor becoming trapped in tanks and cavities. Yet another source of moisture is fine package leaks. This is especially tricky because the pass-fail mark for helium fine leak testing is traditionally set at <1×10-8 atm-cc/sec to prevent moisture ingress, but even smaller amounts of vapor ingress can cause failure in sensitive devices over time. Note that the helium leak rate can be easily converted to a water vapor leak rate by using a 0.469 multiplication factor.
To avoid these water vapor issues in hermetically sealed electronics cavities, it is important to choose the most suitable materials for all components, use careful process control when sealing, and specify reliable hermetic feedthroughs such as epoxy-based units. Stringent testing is also important to evaluate the moisture resistance of various components and subassemblies. MILSTD-883 method 1004 and MIL-STD-750 method 1021 are the most common U.S. military standards used for determining the moisture resistance of electrical components. There are no universal acceptance criteria for internal water vapor content, although MIL-STD-883 method 1018 defines less than 5000 ppm — and many real-world applications require water vapor content of less than 1000 ppm. Whatever the acceptance criteria, end use environment and service life are critical in evaluation of tolerance to moisture.
In addition, MIL-STD-883 method 1014 and MIL-STD-750 method 1071 define various methods for testing hermetic seals, such as dye penetrant, bubble emersion, pressure decay, radioactive testing, and mass spectrometry. Pass-fail criteria are typically listed at 5×10-8 cc-He/sec. The helium mass spectrometer method is widely considered one of the most reliable, sensitive, and accurate measurement methods. Overall system leak and moisture permeation must be considered in addition to electrical cavity moisture ingress.
Electrochemical migration caused by corrosion often results in spotty performance and short circuits. In order to avoid failure issues, it is important to pay attention to the assembly environment and hermetic packaging materials. Ambient air containing moisture can become trapped in electronic devices and cavities during assembly. Further, temperature drops during shipping or storage can also cause water condensation within devices.
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