How to Specify a Heating Plate for Vacuum Chamber Use?

In a vacuum chamber the absence of air removes convective heat transfer. Heat is transferred only through conduction (workpiece contact) and radiation. Heating plates for vacuum service must be built for this unusual heat transfer regime, and themselves not become a source of contamination. The design of a heating plate for vacuum applications is not trivial, as it needs taking into account thermal management, outgassing of materials, electrical feedthroughs, and safety of high voltages. A plate that works well in air may overheat, outgas, or arc when placed under vacuum.

Thermal Management in Vacuum Systems
Without convective cooling a heating plate can reach much greater temperatures than in air for the same power input. The heater components buried in the plate have to be well-coupled to the plate mass to avoid localized overheating. The following thermal concerns are important for vacuum applications.

Mechanisms of Heat Transfer
Conduction - The major method of transferring heat to a workpiece in contact with the plate. Good thermal contact (flat surfaces, optional thermal interface material) is required.

Radiation – Important above plate temperature of 150 °C. Radiant heat transmission follows Stefan‐Boltzmann rule (q=εσ(Tp4−Tc4)) . The surface emissivity (ε) of the plate effects the heating of non-contacted regions.

No convection - No air cooling of the back side or borders of the plate. Heat that is not delivered to the workpiece is stored in the plate and its temperature increases.

Coupling of Plate to Heater
Heaters should be permanently and closely linked to the plate to minimize heat resistance. Brazed or clamped cartridge heaters with high temperature conductive paste (graphite based) are favored. Heaters that use convective cooling of their own bodies (open wire heaters, for example) are not appropriate. One major design difference is that the heater watt density must be derated for vacuum service, typically 30-50% of that used for air, to avoid overheating the heater sheath.

Temperature Detection
Temperature sensors (thermocouples or RTDs) should be embedded as close as feasible to the working surface, preferably within 2–5 mm from the top face. In vacuum the surface temperature can not be obtained from back side or air side readings. Hot spots are more common without convective equalization and it is recommended to have several sensors dispersed around the plate to identify them.

Control of Outgassing in Vacuum Heating Plates
Outgassing, the release of trapped gases, moisture or volatile substances, is not acceptable in vacuum operations, particularly high or ultra-high vacuum (UHV). All materials in the heating plate assembly shall have low vapor pressure and low outgassing rates.

Plate Material
Stainless steel (304, 316, 316L) - The standard option for vacuum heating plates. Low outgassing. High strength. Electropolishing suitable to minimize surface area and trapped impurities.

Aluminum - Good for moderate vacuum (10-3 to 10-5 Torr) if properly cleaned and anodised. But aluminum has a porous oxide coating that traps moisture so it takes a long time to bake out.

Copper - Used because of its good thermal conductivity, copper oxidizes and may outgas if not coated or vacuum-fired.

Other Polymers and PTFE
PTFE coatings or solid PTFE plates are acceptable in vacuum, although special handling is required. PTFE has negligible outgassing after an initial bake-out because it absorbs very little moisture. However, PTFE is not recommended for UHV (below 10-7 Torr) because to the slow out-gassing of fluorine compounds. For most industrial vacuum applications (10⁻³ to 10⁻⁶ Torr) properly treated PTFE is acceptable. A pre‐installation bake‐out at 150–200°C for 24–48 h under vacuum eliminates adsorbed gases.

Wire Insulation & Adhesives
The wire insulation must be vacuum rated. The materials that can be used are:

PTFE (Teflon) - little outgassing after bake out Max. temperature 260 °C.

Polyimide (Kapton) - Very good vacuum compatibility, temperature rating to 400C, little outgassing.

PVC, polyethylene or nylon. Not acceptable, they outgas plasticizers and other volatiles.

Low-outgassing epoxies (e.g., EpoTek H77, Torr Seal) must be used for attaching sensors or securing wiring. Do not use silicone-based adhesives or RTV sealants since they outgas siloxanes.

Cleaning and bake out
All parts should be cleaned using vacuum grade solvents (acetone, isopropyl alcohol) and assembled in a clean environment. Before putting the plate into service, a full assembly bake-out under vacuum at 100–150°C for 24–48 hours is advised. This technique eliminates water and volatile residues deposited on the surface.

Electrical Hermetic Feedthroughs
The power and sensor wires must pass through the wall of the vacuum chamber without leaking. Standard electrical connectors with elastomer seals are not usable in vacuum because they out-gas and allow micro-leaks. Hermetic feedthrus are necessary.

Heating Plates – Types of Feedthroughs
Glass-to-metal sealing - A glass insulator is bonded to a metal housing and conductor pins. Suitable for temperatures up to 250–400 °C, depending on the type of glass. Common for low to medium power heating plates (single phase up to 10 A per pin).

Ceramic glued to a metal flange (alumina or beryllia). Ceramic to metal seals. Higher temperature rating (500°C+) and improved vacuum integrity (leak rate < 1×10-10 atm cc/sec). For high power or UHV applications.

Compression glass sealing - Cheaper, but temperature limited to roughly 150°C.

Number and Rating of Feedthroughs pins
The vacuum chamber heating plate specification shall specify the number of electrical circuits:

Power to each heating zone (2 wires/zone)

Temperature sensor wires (two wires per thermocouple or RTD)

A single multipin feedthrough may have several pins. Each pin must be rated for a current larger than the maximum heater current. For example, a 500 W heater at 120 V uses 4.2 A. A 10 A feedthrough pin would do.

Internal Wiring
Important: Wires from the feedthrough to the heating plate in the vacuum chamber must be vacuum rated (PTFE or Kapton insulated) and free of hot surfaces. Secure connections using ceramic terminal blocks or standoffs. No plastic cable ties or adhesive tapes are permitted.

Paschen's Law (Electrical Breakdown in Vacuum)
According to Paschen's law the breakdown voltage between two conductors depends on pressure and distance. At atmospheric pressure breakdown occurs at high voltages (kV range for tiny gaps). The breakdown voltage is greatly reduced at moderate vacuum (10-1 to 10-3 Torr) - in certain cases to a few hundred volts for a 1 mm gap. At high vacuum (<10^5 Torr) the breakdown voltage increases again.

The following precautions are necessary for vacuum heated plates:

Maintain sufficient separation from grounded surfaces to active conductors (e.g. heater terminals, feedthrough pins). A minimum of 5 mm for 240 V a.c. or 10 mm for greater voltages is suggested.

Avoid sharp edges - Sharp points concentrate the electrical field and reduce the breakdown voltage. Preferred are rounded terminals, smooth conductor routes.

In the case of long internal lines, use shielded wires, grounded at the chamber feedthrough.

Before full power application, test insulation resistance under vacuum.

Vacuum Design Checklist – Hot Plates
Below is a summary of the major points to include in a specification for a vacuum chamber heating plate.

Category RequirementNormal Solution
Plate material 316 stainless steel, low outgassing, electropolished, cleanable
Type of heater Intimate coupling to plate; derated for vacuumBrazed cartridge heaters (Incoloy jacket)
Watt Density Reduced to avoid overheating ≤5 W/cm2 (vs 8-10 W/cm2 in air)
Temperature detection Embedded close to working surface Type K thermocouple or PT100 RTD
Wire insulation PTFE (Teflon) or Kapton Low outgassing, vacuum rated
Sealants / adhesivesLow outgassing epoxy EpoTek H77, Torr Seal (no silicones)
Electrical feedthroughs Hermetic, leak-tightGlass-to-metal or ceramic to metal
Feedthrough current rating Exceeds heater load ≥10 A per power pin for typical 1‑2 kW plates
High‑voltage clearance Prevents Paschen breakdown ≥5 mm gap (240 V), rounded terminals
Pre‑service bake‑out Removes adsorbed gases 100‑150°C under vacuum for 24‑48 h
Special Cases: Radiant Heating Plates
In some vacuum processes, the heating plate is not intended to contact the workpiece directly. Instead, the plate radiates heat to the workpiece across a gap. In this configuration, the plate's surface emissivity becomes critical. A high‑emissivity coating (e.g., black anodize on aluminum, or Pyromark 2500 on stainless steel) is applied to increase radiant heat transfer. However, coatings must be vacuum‑compatible and not outgas. Pyromark 2500, a ceramic‑based coating, is often used for vacuum radiant heaters up to 600°C.

Conclusion
Vacuum chamber heating plates require specialized design for thermal management and contamination control. The absence of convective cooling means heaters must be well‑coupled to the plate and operated at reduced watt density. Outgassing is controlled by selecting low‑vapor‑pressure materials (stainless steel, PTFE, Kapton) and performing pre‑service bake‑out. Hermetic electrical feedthroughs (glass‑to‑metal or ceramic‑to‑metal) are mandatory to maintain chamber integrity. Paschen's law imposes clearance requirements to prevent arcing at moderate vacuum levels. The application environment-vacuum level, temperature range, and process sensitivity-dictates design specifics. A properly specified vacuum heating plate provides reliable, contamination‑free heat transfer in an environment where convection is absent and every material choice matters.