White Paper: Power
Use of Adhesives and Sealants for Hermetic Wire Feedthroughs
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The hermetic sealing of electrical and optical feedthroughs is a critical area for assuring the longevity and reliability of devices such as optical assemblies, thermocouples, EV battery packs, power supplies, transformers, and generators. Epoxy adhesive sealing or potting offers exceptional processing flexibility, especially for hermetically sealing wire feedthroughs composed of many conductors through a small diameter housing. Epoxy polymer is inherently electrically insulative and exhibits high adhesion to materials including metals, ceramics, glass, and some plastics, providing a greater range of adhesion compared with glass and ceramic sealing. The epoxy fabrication and curing process allows for a wide array of different conductors to be hermetically sealed within a housing. Download this white paper to learn more about this process and the adhesive requirements for hermetic wire feedthroughs, EV battery pack applications, and a case study from NASA.
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Overview
This whitepaper from Master Bond Inc. discusses the use of adhesives and sealants, particularly epoxies, for creating hermetic wire feedthroughs critical to the longevity and reliability of electronic devices such as optical assemblies, thermocouples, electric vehicle (EV) battery packs, power supplies, transformers, and generators. Hermetic feedthroughs ensure that conductors carrying power or signals remain sealed from moisture, air, and contaminants, which could degrade device performance.
Traditional hermetic sealing methods involve glass-to-metal or ceramic-to-metal seals that provide high chemical, temperature, and pressure resistance but are complex, expensive, and limit conductor density due to spacing requirements. In contrast, epoxy adhesive sealing offers greater flexibility, easier fabrication, cost-effectiveness, and higher signal density, while maintaining strong adhesion to metals, glass, ceramics, and some plastics.
The paper highlights critical factors for using epoxies in hermetic sealing: viscosity suitable for back-filling, proper mixing and curing schedules to maximize glass transition temperature (Tg) and minimize shrinkage, low permeability to prevent gas ingress, chemical and temperature resistance, and matching coefficient of thermal expansion (CTE) among epoxy, conductors, and housing to avoid stresses and leaks during thermal cycling.
Case studies underscore epoxy advantages: MIT's LIGO project used an all-epoxy seal with stainless steel threads for vacuum feedthroughs, finding durability and low outgassing compliant with NASA standards essential for clean vacuum environments. NASA developed epoxy-based hermetic feedthroughs for thermocouple wires in refrigerant compressors, solving fluid intrusion issues while withstanding harsh temperatures and chemical exposure. EV battery packs demand hermetic electric feedthroughs with strong chemical resistance and shock/vibration durability to protect sensitive battery management system electronics.
The whitepaper also reviews relevant certifications: all Master Bond products comply with RoHS 3 and REACH EU standards restricting hazardous substances. Many formulations meet NASA’s low outgassing requirements (TML <1%, CVCM <0.1%), vital for vacuum applications. Flame retardancy standards such as UL 94 and FAR 25.853 certify flame resistance, with options for halogen-free fillers to avoid toxic combustion byproducts, important in aerospace and electronics.
In conclusion, epoxy-based adhesives provide a versatile, cost-effective, and reliable approach to hermetic sealing of wire feedthroughs with customizable properties that meet demanding mechanical, chemical, thermal, and regulatory requirements across diverse high-tech applications.