Electrical Feedthroughs - Introduction
In the vacuum components industry there can be few more specialized technologies than the production of ceramic-to-metal seals. Ceramic-to-metal seals, the bonding of metals to ceramics are fundamental to the manufacture of thousands of components in applications where electricity and optics must interface with high and ultrahigh vacuum environments.
The electrical feedthroughs offered on this website are electrically rated for operation with one side in dry atmospheric conditions while the opposite end is in a stable vacuum environment with a maximum system pressure of 1x10-4 Torr.We recommend that users make allowances for deviations from these stated operating parameters and take adequate safety precautions when working with high voltages or currents.
- High Current
- High Voltage
- Breaks and envelopes
Coaxial feedthroughs are those products constructed with two concentric conductor paths. One outer metal tube or shield path, concentric with, and enclosing, a cylindrical center conductor path. The inner and outer paths are separated and insulated with a high purity alumina ceramic dielectric. In this catalog, MDC offers coaxial components with military and industry standard BNC, MHV, SHV, Type N and SHV Bakeable connector interfaces.
The BNC coaxial connection, also referred to as the bayonet naval connection, is commonly used in 50 and 75 ohm low power instrumentation lines. Due to restrictions in geometry and the relatively high dielectric constant of alumina ceramics, BNC feedthroughs offered on this site are not impedance matched or rated.
The MHV coaxial connection, also referred to as miniature high voltage or high voltage BNC’s, are ideally suited for medium to high power applications with higher voltage requirements. Caution should be exercised in electrical systems fitted with both BNC and MHV connections. BNC and MHV connections are almost identical in appearance and geometry and should never be cross-mated since their electrical ratings are not compatible. BNC connections are rated for 500VDC while MHV connections can handle voltages as high as 5000VDC.
The SHV coaxial connection, also referred to as safe high voltage connections, are also rated for service to 5000VDC. The difference between these feedthroughs and their MHV counterparts are the pin and contact geometry. SHV cable connectors have recessed female contacts with the male mating pin located in the feedthrough. The exact opposite is true with BNC and MHV connections. SHV cable-connector center contacts do not protrude beyond connector ends as they do on an MHV. This makes the SHV safer if accidentally powered while disconnected. Additional coaxial feedthroughs, including SMA, SMB, SHV-15, SHV-20, Triaxial, Microdot type and various others are all available through our Insulator Seal ceramic to metal feedthroughs product line.
Power feedthroughs are used to transmit either high voltage, high current or a combination of both. These products can be used for a multitude of vacuum applications including vacuum furnaces, sample heating or biasing, in-vacuum coating applications such as electron-beam evaporation, resistive heating evaporation, and DC plasma sputtering. Proven and time tested designs feedthrough family are employed to optimize the electrical performance of MDC power feedthroughs. Where space is not a limitation ceramic surfaces are made as long as possible to maximize strike and creep distances. If space is limited, ceramics are convoluted in order to achieve increased surface distances with minimal impact on an insulator’s overall length. Convoluted or fluted ceramics are recommended for environments where moisture or other surface contaminant may hinder electrical performance of conventional straight wall insulators. All power feedthrough air-side ceramic surfaces are glazed with a high temperature glass coating. This glass coating reduces ceramic surface roughness and minimizes surface contamination thus enhancing an insulator’s electrical surface tracking characteristics. The power feedthroughs offered in this catalog are constructed with exposed, bare metal conductors on both the air and vacuum sides. Connectors for these feedthroughs are available, but must be purchased separately.
Included in the Insulator Seal ceramic feedthrough product line are tubular conductor feedthroughs that can be used to transmit both power and coolants simultaneously. These products are referred to herein as watercooled feedthroughs. Watercooled electrical components should be used with grounded, closed-loop cooling systems and / or the use of nonconductive coolants such as deionized water or ethylene glycol. Although inefficiently, tap water will conduct electricity.Water cooling lines must therefore be electrically grounded and constructed of nonconductive material such as polypropylene tubing. Properly grounded water lines will provide a safe dissipation path for any power conducted by the water.
Watercooled power feedthroughs can carry higher current loads than solid conductors of equal size and material when adequately cooled. Current or power ratings are not given for watercooled feedthroughs because these ratings are dependent on a coolant’s flow rate and its heat dissipating capacity. Since tap water temperatures can vary dramatically from one location to another, so too will water’s heat dissipation capacity. Users are advised to establish safe and practical coolant flow rates based on the power requirements for their specific application and coolant heat dissipation capacity.
Breaks & Envelopes
Breaks and envelopes are tube like adapters with metal tube hardware bonded to the ends of a ceramic tube. Components with diameters below and including 2.50 inches are referred to as vacuum breaks, while those above are referred to as vacuum envelopes. The bonded metal tube ends provide a means of attaching the breaks and envelopes to vacuum tube lines using flange mounts or welding. The central ceramic portion of a break or envelope provides electrical insulation between the two conductive metal ends. In other words, the ceramic produces an electrical break in an otherwise continuous and conductive metal tube geometry.
The joining of ceramics to metals is a compromise between materials with dissimilar expansion coefficients. Low expansion metals combined with careful joint design bring expansion coefficients to an acceptable match and effectively minimize the stresses caused by differential expansion between the ceramic and metal components being bonded. Minute variations in expansion coefficients can be detrimental if ceramic to metal seals are subjected to severe thermal gradients. The maximum recommended thermal gradient for any ceramic to metal seal should not exceed 25°C per minute.
Multipin instrumentation refers to any feedthrough product containing more than one conductor path or pin that is also fitted with fastening air-side connectors. These feedthroughs are commonly used for the transmission of signal voltages and currents. They are commonly referred to as instrumentation feedthroughs because of their use in instrument control applications such as electron microscopes, electron-beam evaporation, electron microscopy, surface science analysis and semiconductor process controls. MDC multipin instrumentation feedthroughs are fitted with industry standard MS threaded circular connectors which comply with MIL-C-5015 specifications.
Complete air and vacuum connectivity are standard with MDC’s new D-Subminiature instrumentation feedthroughs. Nine, fifteen and twenty-five pin geometries are hermetically sealed using the latest in glass-ceramic bonding technology. Air side connections are designed to interface with standard off-the-shelf serial cable connectors. Vacuum connectivity is made possible with MDC’s unique UHV compatible connectors and ribbon cables. These instrumentation feedthroughs provide the same conveniences of circular type multipin products, but offer higher pin density in a smaller footprint.
Connector accessories are available for most electrical feedthroughs.
Insulator Seal (ISI) brand catalog and custom engineered ceramic to metal seal products are manufactured to ISO 9001:2008 standard utilizing our qualified hermetic brazing and joining processes.