Direct Answer
Choose CF flanges when your system must reach pressures at or below 10⁻⁷ torr, requires bakeout above 150°C, or must meet a helium leak-rate specification at or below 1×10⁻⁹ atm·cc/sec. Choose KF flanges for rough vacuum, high vacuum up to about 10⁻⁷ torr, and any connection that benefits from fast assembly, frequent reconfiguration, or lower cost. Never use KF as a primary boundary seal in a UHV system.
The Decision Starts with Pressure
Vacuum system design decisions cascade from the target base pressure. A system pumped only to 10⁻³ torr for a rough process has very different component requirements than a surface science chamber targeting 5×10⁻¹¹ torr. As a practical guide:
- Above 10⁻³ torr (rough vacuum): any seal type works; KF is standard.
- 10⁻³ to 10⁻⁷ torr (high vacuum): KF is commonly used; CF is acceptable but rarely necessary.
- 10⁻⁷ to 10⁻⁹ torr (UHV threshold): CF is strongly preferred; KF should be limited to non-critical areas.
- Below 10⁻⁹ torr (UHV and XHV): CF required on all primary boundaries; no elastomeric seals in the UHV volume.
Bakeout Requirements
Reaching UHV base pressure almost always requires bakeout – heating the entire vacuum system to 150–300°C while pumping, to drive off adsorbed water and hydrocarbons from internal surfaces. KF O-rings cannot survive extended bakeout above approximately 80–150°C (depending on elastomer). A single KF joint in the vacuum boundary will limit bakeout temperature for the whole system. If bakeout is required, use CF on all joints that will be exposed to heat.
Leak Rate Specifications
Many UHV system specifications include a helium leak-rate requirement, often expressed as ≤1×10⁻⁹ atm·cc/sec per ASTM E498/E499. KF assemblies with elastomeric O-rings are generally not able to meet these specifications reliably due to permeation through the polymer. CF flanges with copper gaskets readily achieve these leak rates when properly assembled and torqued. If your system or component has a published leak-rate specification, verify that your flange choice can meet it before finalizing the design.
Assembly and Maintenance Considerations
KF Assembly
KF connections require only a centering ring, an O-ring, and a single clamp. Assembly takes seconds with no tools for smaller sizes. O-rings can be replaced, and the same flanges reused indefinitely. This makes KF ideal for components that are frequently swapped, such as roughing-line connections, vent valves, and temporary instrumentation.
CF Assembly
CF connections require a new copper gasket and a full bolt circle. Proper assembly requires torquing bolts in a star pattern to a specified torque — for example, typically 15–20 Nm for a CF40 — to ensure even compression of the knife edge into the gasket. Torque requirements vary by flange size and bolt count; always consult the manufacturer’s torque specification for the specific flange size in use. A damaged or reused gasket will not seal reliably. Flanges themselves are reusable, provided the knife edges are not damaged.
Mixed Systems: Using CF and KF Together
Most practical UHV systems combine both flange types. A typical architecture uses CF on the main chamber body, all chamber ports, and any component that sits inside the UHV vacuum boundary – feedthroughs, viewports, gauges, and valves. KF is used on the foreline (roughing pump connection), vent lines, and any diagnostic port that is temporarily blanked off and not part of the UHV boundary.
When mixing flange types, map your vacuum boundary carefully. The leak rate of the entire system is limited by the weakest joint. A single KF O-ring on a vent valve that is part of the UHV boundary will prevent the system from reaching UHV regardless of how well all the CF joints are sealed.
Cost and Availability
KF hardware is less expensive to purchase and maintain. CF flanges and gaskets are more expensive, and gaskets are consumable. For large systems with many ports, the material cost difference between all-CF and mixed CF/KF configurations can be significant. However, the cost of a failed UHV pump-down or a contaminated process chamber far exceeds the cost of using CF where it matters. Optimize cost by using CF only where the pressure and temperature requirements demand it.
Decision Checklist
- Target pressure below 10⁻⁷ torr? Use CF.
- Bakeout above 150°C required? Use CF on all heated joints.
- Helium leak-rate spec at or below 10⁻⁹ atm·cc/sec? Use CF.
- Connection reconfigured weekly or more often? KF is acceptable if not in the UHV boundary.
- Roughing line or backing pump connection? KF is appropriate.
- Feedthrough, viewport, or gauge in a UHV chamber wall? CF required.
Getting It Right
Flange selection is one of the most consequential decisions in vacuum system design. An incorrect choice discovered during commissioning can mean rebuilding chamber ports, replacing flanged components, or accepting a system that never reaches its design base pressure. MPF Products can assist with component selection, including CF and KF-compatible hermetic feedthroughs, viewports, and flanged assemblies matched to your vacuum boundary requirements.