How much attenuation does an EMI Shielding Case need?

Answer: It depends on band and device sensitivity. Consumer electronics meet radiated limits near 40 dB at 1 GHz; military work often asks for 60 dB.

Can the four numbers be trusted without an anechoic chamber?

Answer: Yes. All four are contact measurements of resistance or geometry, so a milliohm meter, feeler gauge and tape measure suffice.

Does conductive paint flake off, and how do I tell?

Answer: It can, especially at latch seats. Run a tape adhesion test, then re-measure sheet resistance; a rise above 50 percent means the coating has failed.

How does an EMI Shielding Case differ from a plain protective case in price?

Answer: Conductive surface, gasket and grounding hardware. At equal size the shielding version costs 1.6 to 2.2 times a plain case.

EMI Shielding Case Selection: Three Lines and Four Numbers

Quick answer: Four numbers decide an EMI Shielding Case: surface resistance, seam gap, overlap width and bond resistance. Each has a pass line, and when any one fails, attenuation falls from 60 dB to below 20 dB, a drop you can measure with a handheld meter before packing.

Four numbers you can measure on the spot
First line: is the conductive surface continuous
Second line: seam overlap and gasket squeeze
Third line: ground path and cable entry
Matching three scenarios
What happens when the order is reversed
FAQ

1. Four numbers you can measure on the spot

Ask one question before ordering: can these four numbers be measured on the spot? If not, the 60 dB on the sample card is typography.

Number	How to check it on site	What failure looks like
Surface resistance under 0.1 ohm per square	Milliohm meter at 5 points, including corners and latch seats	One point jumps to 0.4 ohm
Seam gap under 0.3 mm	Push a 0.3 mm feeler gauge around the lid seam	A 0.5 mm gauge enters 10 mm at a corner
Overlap of 25 mm or more	Tape measure across the gasket contact face	Only 8 mm left at the corner
Bond resistance under 2.5 milliohm	Meter between shell and equipotential bar	Reading multiplies tenfold when the probe moves

2. First line: is the conductive surface continuous

The first line is continuity, not thickness. An EMI Shielding Case needs 50 to 80 microns of conductive paint and roughly 0.05 ohm per square, both easy; coating the whole interior without a miss is hard.

EMI Shielding Case: closed silver body in a grey shelf recess, two cam latches, a recessed metal handle and twin lid ribs

Put one probe inside the lid and the other at the farthest point of the base, and expect under 1 ohm. Repeat at the latch seats and the handle root; any point above 3 ohm is a break. Rotational mouldings have deep cavities and shallow draft, so the gun thins out exactly in the recesses that need continuity most.

Lid and base must both conduct. A coated interior with a bare lid gives the case an open end and drops attenuation to about 20 dB, which inspection rarely catches: a dark lid looks the same either way.

3. Second line: seam overlap and gasket squeeze

The lid seam is the longest joint on the case, and leakage follows length, not width: a 400 mm seam at 0.2 mm can leak more at 1 GHz than a 20 mm hole.

Seam location	Minimum overlap	Gasket squeeze	Common site error
Lid lip	25 mm	30 percent	Gasket cut 15 mm short at the corner
Latch seat perimeter	15 mm	25 percent	Not fully compressed, 0.4 mm gap left
Cable entry flange	20 mm	35 percent	Cable tie only, no 360-degree bond
Hinge mounting face	12 mm	20 percent	Coating trapped between two metal faces

Unbroken overlap matters more than wide overlap. Leave 5 mm extra at each corner; a butt joint that meets exactly opens a 0.1 mm slit after a few thousand lid cycles. Thirty percent squeeze is the target: less loses rebound within twenty cycles, more eats the latch preload.

4. Third line: ground path and cable entry

Grounding is the step buyers leave for last, and the one that undoes everything before it.

EMI Shielding Case: scuffed silver body closed on a workshop timber shelf, four small feet, a coiled black cable, a drill press and window behind

An M5 stainless stud with a serrated washer bites through oxide and paint, bringing bond resistance under 2.5 milliohm. Keep the braid shorter than 30 mm; at 1 GHz a long braid is an antenna that invites interference in.

Cable entry hides the same problem: one signal cable without a 360-degree bond can drag a sound EMI Shielding Case below 20 dB while every panel test passes.

5. Matching three scenarios

The same decibel figure means different things in three settings, so judge by scenario rather than by data sheet:

Bench and lab: 30 MHz to 300 MHz near field, so watch seams and apertures; 25 mm overlap and single-point grounding will do;
Vehicle and transit: -20°C to +55°C plus vibration, so check hinge and gasket compound, keeping squeeze under 35 percent;
Field and coastal: 96 hours of salt fog calls for stainless hardware and potted braid terminations.

If nobody can name the band, do not pay for high attenuation. A plain case already delivers structure and sealing, and money moved to gaskets buys more than another paint layer.

6. What happens when the order is reversed

Quoting first and measuring later is the most expensive sequence. Three quotations differ by 30 percent, the two cheapest arrive with corners above 2 ohm, recoating takes two passes at 45 CNY per unit, and delivery slips 18 days.

Judging appearance before overlap hides the fault in the corners: the sample looks tidy, yet a 0.5 mm feeler gauge slides 10 mm in right where the handle load lands.

Fitting equipment before finding a ground point costs most: with no stud inside, the only option is a 12 mm hole through the wall.

So measure four numbers on the EMI Shielding Case, then discuss price, then finish.

Conclusion

Selecting an EMI Shielding Case comes down to four numbers, three lines and one order. The numbers set the pass lines, the lines set the route, and the order decides whether you spend money first or measure first. A 60 dB EMI Shielding Case with a 0.5 mm slit at one corner behaves exactly like a 20 dB box.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.