A 150 MHz conducted-emission spike shows up on the test bench the same way every time: the compliance engineer points at the cable entry point. The fix is usually a feedthrough capacitor bolted into the shield wall. But walk into any distributor and you will find threaded bodies, solder-in barrels, press-in cases and SMD chips all labelled as the same part. They are not. The mounting style changes how the component grounds, how much current it can carry, and where it physically fits. Choosing the wrong feedthrough capacitor type is one of the quickest ways to turn a simple filter into a mechanical and electrical headache.

Why the mounting style decides filtering performance

A feedthrough capacitor only filters as well as its ground path. The whole point of the part is to let the centre conductor pass through a shield wall while shunting high-frequency noise to that wall before it can travel deeper into the enclosure. For the shunt to work at 100 MHz and above, the connection between the outer electrode and the chassis must be short and low-inductance.

Threaded and press-in parts clamp directly into the metal, giving a 360° mechanical and electrical contact around the mounting hole. Solder-in parts rely on the solder fillet and the quality of the joint. Every mounting family trades something: space, current capacity, ease of rework, or environmental sealing. That trade is what this article walks through.

Feedthrough capacitor types at a glance

Threaded feedthrough capacitors

This is the default choice when you can put a hole in a metal panel. The capacitor body carries an external thread; you screw it into the shield wall and clamp it with a nut and a spring or star washer. The threaded flange forms the ground contact all the way around the hole, which keeps the ground return short even at UHF.

Threaded parts dominate high-current and high-voltage work. Typical ranges reach about 1000 VDC and 25 A, with rejection in the tens of decibels up into the gigahertz range, and resin-sealed versions add moisture and weather protection. Two filters with the same thread size can still perform very differently across frequency, because the internal circuit is independent of the thread: a C-type, L, Pi or T network can sit inside the same mechanical body.

If you need to see the full range of thread sizes and capacitance values we produce, browse the threaded feedthrough capacitors range. For the basics of how the component works before you start specifying, the feedthrough capacitor guide is a good place to start.

Solder-in and welding feedthrough capacitors

When space is the constraint, solder-in types win. These are soldered directly into a hole or inline on a PCB trace, and they come in small diameters so several can sit side by side at a connector. They are commonly used to filter signal lines, data lines and AC power lines where the panel opening is limited.

The trade-off is the ground joint. A solder-in part is only as good as the fillet you produce, so process control matters: preheat where possible, because ceramic parts can crack invisibly if heated too fast during soldering. For our solder-in product family, see the welding feedthrough capacitors category.

Press-in feedthrough capacitors

Press-in parts use an interference fit instead of a thread or a solder joint. You press the body into the hole and the metal-to-metal friction carries both the mechanical retention and the ground connection. That removes the nut, the washer and the soldering iron from the assembly step, which speeds up production and removes two common failure points.

They suit medium-volume assemblies where a clean chassis ground is needed and rework is rare. As with any press-fit component, the hole tolerance drives the contact quality, so the fit should be controlled rather than assumed.

SMD feedthrough capacitors

Surface-mount feedthrough capacitors bring the same idea to automated PCB assembly. They mount at the edge of a board so a filtered line can cross the board boundary, and they are reflow soldered like any other chip. Current capacity is lower than chassis-mounted types, so they fit board-level filtering and cost-sensitive products rather than high-power entries.

High-current and high-voltage bolt-through types

At the heavy end of the range, bolt-through feedthrough capacitors handle currents and voltages that threaded chassis types cannot. Resin-sealed bolt-through designs are rated for demanding telecom and power applications, with current in the region of 100 A and voltages up to about 2500 VDC. These are the parts you reach for on power line entries, inverters and large industrial equipment. Our high-power feedthrough capacitors cover this territory.

How to choose the right feedthrough capacitor type

Run through these four questions in order and the selection narrows itself:

• Where does the line cross the shield? A metal panel with an accessible hole points to threaded or press-in. A PCB boundary points to SMD or solder-in.
• How much current and voltage does the line carry? High current and high voltage move you toward threaded or bolt-through types with a real chassis contact.
• How much space is available at the entry? Tight connectors and dense panels favour the small diameters of solder-in parts.
• What environment will the assembly live in? Moisture, vibration and thermal cycling favour resin-sealed threaded or bolt-through construction.

Notice that mounting style and filter topology are decided separately. The mounting style handles mechanics and grounding; the internal network (C, LC, Pi or T) handles the impedance match and the attenuation shape. Pick the physical type first, then the electrical network.

The feedthrough capacitor symbol on a schematic

On a schematic, a feedthrough capacitor is usually drawn as a capacitor symbol straddling a barrier line that represents the shield wall, with the outer electrode connected to ground or chassis. When you see that symbol, it is a reminder that the part has three effective terminals: line in, line out, and the ground plane it is bolted or soldered to. If the ground connection is missing on the schematic, the filter will not work no matter which feedthrough capacitor type you choose.

Common mounting mistakes

• Ignoring the torque spec on threaded parts. Under-tightening raises contact resistance; over-tightening can crack the ceramic. Follow the manufacturer’s recommended torque for the thread size.
• Soldering ceramic without preheat. Thermal shock cracks the dielectric invisibly, and the part fails only under vibration or thermal cycling later.
• Treating SMD like a chassis filter. Board-level ground planes are not the same as a 360° chassis contact; an SMD feedthrough cannot replace a threaded one on a power entry.

FAQ

What is the difference between threaded and solder-in feedthrough capacitors?

Threaded types screw into a panel and clamp with a nut, giving a 360° chassis ground contact and higher current ratings. Solder-in types are soldered into a hole or PCB trace, are smaller, and suit compact assemblies where space matters more than raw power.

Can a feedthrough capacitor be surface mounted?

Yes. SMD feedthrough capacitors are reflow soldered at the edge of a PCB for board-level filtering. They carry less current than chassis-mounted types and are not a substitute on high-power entries.

Does the mounting style change the electrical filtering?

Not the network itself. Mounting style affects the quality of the ground connection and therefore how well the filter performs at high frequency. The internal C, LC, Pi or T network is selected separately based on the circuit impedance.