Stamped EMI Gaskets and Fingers: Designing for Shielding
Short answer: a stamped EMI gasket works by surrounding an opening with a continuous, low-resistance metal path so interference has no gap to cross. Fingers or a folded gasket wall create many parallel contacts that close the seam between enclosure halves, and the shield's effectiveness is set by contact resistance, finger force, and — above all — the size of any remaining aperture. Keep every opening and the pitch between fingers below about one twentieth of the wavelength of the highest frequency you must block, and the gasket does its job. Materials are usually beryllium copper, phosphor bronze or stainless, plated tin, nickel or gold.
Shielding is often treated as a sheet-metal problem, but the weak points are the seams, slots and connector cut-outs. A stamped finger gasket or folded gasket strip is how those weak points are closed repeatedly through thousands of open-and-shut cycles. This guide covers the mechanism, the material and plating choices, the frequency rules that size the geometry, and the design rules that keep force and continuity stable.
How Stamped EMI Gaskets Work
An EMI shield is a conductive enclosure. Interference radiates in through any break in the enclosure — a seam, a vent, a slot, or the gap between two housing halves. A stamped gasket fills that break with metal and turns many small gaps into many small contacts, so the seam behaves like solid wall. The gasket does not need to be a single continuous piece; a row of spring fingers that each press against the mating surface creates a parallel array of low-resistance contacts that act together.
Two properties determine shielding performance. The first is contact resistance: even a few milliohms per finger is a voltage drop for the currents induced by interference, and high resistance lets more through. The second is aperture size: any hole or gap larger than a fraction of the wavelength leaks energy, which is why finger pitch and slot size follow the frequency you must block. Clamping force sets the contact resistance, and geometry sets the aperture, so force and pitch are the two design levers. For the electrical fundamentals, our EMI shielding contacts guide is a useful companion.
Materials and Plating for Shielding Contacts
The gasket material must be springy enough to stay in contact, conductive enough to ground, and stable enough not to corrode. Copper alloys give the best combination; stainless is used where corrosion resistance or rigidity matters more than conductivity.
| Material | Conductivity | Spring force | Corrosion | Typical use |
|---|---|---|---|---|
| Beryllium copper C17200 | Very good | Excellent | Good | High-cycle finger gaskets |
| Phosphor bronze C51000 | Good | Good | Good | Finger stock, general shielding |
| Stainless 301/304 | Lower | Excellent | Excellent | Rigid clips, shield cans, no plating |
| Copper alloy + tin plate | Very good | Good | Very good | Most board-level gaskets |
| Copper alloy + gold plate | Excellent | Good | Excellent | Low-resistance premium contacts |
Plating is the deciding detail for contact resistance. Tin is the standard: cheap, solderable, and gives a few milliohms per contact, but it can oxidize and its resistance creeps in a dry joint. Nickel holds up better at temperature and under wear but has higher resistance. Gold gives the lowest and most stable contact resistance and is reserved for signal-critical or high-reliability gaskets. Silver is the most conductive but tarnishes, so it is used only where the resistance requirement is extreme. The plating must be ductile enough to bend with the finger without cracking, which rules out very thick, brittle deposits on tight radii.
Sizing Fingers and Apertures for Frequency
Shielding effectiveness falls as the frequency rises, because a fixed aperture becomes electrically larger. A slot leaks badly once it approaches a twentieth of the wavelength. The table below gives the wavelength and the practical aperture limit at common frequencies, which sets the maximum gap and the maximum finger pitch.
| Frequency | Wavelength (free space) | Keep apertures below (~λ/20) |
|---|---|---|
| 100 MHz | 3.0 m | ~150 mm |
| 500 MHz | 600 mm | ~30 mm |
| 1 GHz | 300 mm | ~15 mm |
| 3 GHz | 100 mm | ~5 mm |
| 6 GHz | 50 mm | ~2.5 mm |
Read this as a maximum gap, not a target. Real designs keep apertures and finger pitch well below the limit because gasket contact resistance and seam impedance add further loss. Finger length also matters: longer fingers bend to accommodate imperfect mating surfaces and larger tolerance stacks, while shorter fingers hold higher force for the same deflection. A common pattern is finger pitch around 3–8 mm for gigahertz-class enclosures, with each finger deflecting 0.3–1.0 mm in service.
Design Rules: Force, Compression and Assembly
Finger gaskets are springs, so the rules from clip design apply. Keep working deflection at 50–70% of the elastic range so force stays stable and the finger does not relax. Set the free height higher than the fully compressed clearance so the gasket always presses against the mating surface after tolerance stack-up — a gasket that barely touches when the enclosure is closed will open under thermal or mechanical movement. Provide over-travel clearance so the fingers cannot be flattened during assembly, which would destroy their force and their contact resistance.
Pay attention to the mating surface. A gasket finger presses against bare or plated metal; if that surface is anodized or painted, it is an insulator and the gasket has nothing to conduct into. Shielding seams need bare, conductive mating surfaces, often a masked or masked-off zone on the housing. Assembly force is set by finger count and stiffness, so a design that needs more fingers for pitch reasons will also need more closing force — balance the two. Our guidance on stamped shields and enclosures covers the mating-surface and assembly side in more detail.
Manufacturing and Inspection
Stamped finger gaskets run well in progressive dies: pierce the finger pattern, blank the strip, form the fingers and coin the contact faces. The formed features hold about ±0.05 mm, which keeps finger height — and therefore force — consistent across a run. The critical quality parameters are finger height and force, plating thickness, and the absence of burrs or cracks at the finger roots, where fatigue begins. We check plating thickness by XRF and can run deflection and contact-resistance checks on request. Grounding-specific parts follow the same rules; see stamped grounding clips for the contact side. Send the drawing with frequency, available compression and force budget, and we return an indicative tooling and unit price within 12 working hours.
Frequently Asked Questions
Q: How small should finger pitch be for EMI shielding?
A: Keep finger pitch and any aperture well below one twentieth of the wavelength of the highest frequency you must block. At 1 GHz that limit is about 15 mm, at 3 GHz about 5 mm. Real designs use 3–8 mm pitch for gigahertz-class enclosures to keep margin for contact resistance and seam impedance.
Q: Which material is best for a stamped EMI gasket?
A: Beryllium copper C17200 gives the best spring force and good conductivity for high-cycle finger gaskets. Phosphor bronze is a lower-cost alternative with adequate conductivity. Stainless steel suits rigid shield cans where no plating is wanted, though its conductivity is lower than copper alloys.
Q: Does the gasket plating affect shielding?
A: Yes. Plating sets contact resistance and stability. Tin is the standard for a few milliohms and good solderability; gold gives the lowest and most stable resistance for signal-critical parts; nickel trades higher resistance for temperature and wear resistance. Keep the plating ductile so it survives finger bending.
Q: Why does my shield lose effectiveness at higher frequencies?
A: Because a fixed aperture becomes electrically larger as frequency rises, so slots and gaps that were harmless at low frequency start radiating. Reduce aperture size and finger pitch, lower contact resistance, and make sure seam mating surfaces are bare metal rather than anodized or painted.
Q: Can stamped gaskets survive many assembly cycles?
A: Yes, if the fingers operate at 50–70% of their elastic range and the bend radii are generous. Running near the elastic limit causes stress relaxation and force loss; a sharp radius causes fatigue cracks. We hold formed features to about ±0.05 mm to keep finger force consistent.
Related Resources
- EMI shielding contacts guide: materials, plating and grounding for shielded enclosures.
- Stamping services: progressive-die stamping of finger gaskets, shields and contacts in Dongguan.
- About BQUQ: an ISO9001-certified source factory running stamping, CNC, springs and heat sinks under one roof.
- Contact us: send your drawing and get an indicative quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
