The Effectivess of Perforated Metals for EMI Shielding


The following information is from the Hendrick Manufacturing Co. Click on logo above for their home page.

Perforated metal is being used to enclose electrical devices to attenuate the EMI/RFI radiation they emit and to ventilate them at the same time. Many questions have been asked about which perforated pattern should be used to satisfy both of these design requirements and the shielding effectiveness of various perforated patterns and materials.

To answer these questions, the I.P.A. contracted with Intertek Testing Services, testing laboratories in Boxborough, MA to evaluate the shielding effectiveness of 16 perforation and material combinations at 9 frequency levels. The results of these tests with descriptions of the test samples and the frequencies tested are shown in the charts that follow. Results have been expressed in dB of Shielding Attenuation and in % of Attenuation. Details of the tests are available from Hendrick Manufacturing Co.

The test results show that a Shielding Effectiveness of 40 dB provides 99.000% attenuation of the electromagnetic (EMI/RFI) radiation while a Shielding Effectiveness of 92 dB, the highest Shielding Effectiveness found in the tests, provides 99.997% attenuation. A Shielding Effectiveness of 40 dB is the targeted minimum in most applications.

Very effective shielding was provided by most samples up to frequencies of 7 GHz. Above that frequency, some of the samples dropped below 99.000% effective, but most samples stayed comfortably above 95% effective even at the highest frequency level of 10 GHz. The obvious conclusion to be reached is that there are many perforated patterns that designers can choose from to meet their design requirements.

The largest single source of leakage is along contact surfaces between two parts. If a tightly sealed electrical connection is not made, the leakage through the interface can be greater than through the structure.

Shielding Effectiveness: Attenuation in dB

Hole Dia. 60 Center Spacing % Open Area Thick Mat'l Sample Size 30MHz 100MHz 300MHz 1 GHz 2 GHz 4 GHz 6 GHz 8 GHz 10 GHz
.040 .055 48.00 .022 Alum. 8"x8" 70 70 80 56 55 48 57 35 48
.062 .093 40.00 .027 C.R.St'l 24"x24" 61 67 66 56 45 48 46 44 33
.062 .125 23.00 .025 Alum. 8"x8" 92 84 90 68 65 60 67 35 45
.072 .100 47.00 .038 C.R.St'l 8"x8" 64 70 90 62 55 52 40 33 37
.075 .125 32.70 .049 C.R.St'l 8"x8" 72 70 90 72 68 63 68 43 48
.078 .109 46.10 .027 C.R.St'l 24"x24" 66 75 72 63 55 54 45 41 39
.079 .115 42.80 .036 C.R.St'l 8"x8" 60 64 88 66 60 56 62 33 40
.100 .187 25.90 .040 Alum. 8"x8" 66 70 77 69 62 62 67 41 48
.125 .187 40.30 .030 Stainless 24"x24" 58 62 57 49 40 37 34 31 26
.125 .187 40.30 .030 Alum. 24"x24" 62 63 61 51 44 38 34 31 28
.125 .187 40.30 .060 Alum. 24"x24" 71 72 69 58 49 48 39 35 34
.125 .187 40.30 .125 Alum. 24"x24" 85 84 84 73 68 84 73 63 51
.156 .187 63.00 .057 C.R.St'l 24"x24" 57 58 59 48 40 35 31 29 26
.156 .250 35.40 .057 C.R.St'l 24"x24" 59 64 61 53 43 43 41 36 31
.187 .250 51.00 .027 C.R.St'l 24"x24" 52 52 49 37 31 24 21 19 18
.187 .250 51.00 .057 C.R.St'l 24"x24" 55 56 55 46 36 33 30 27 24

Shielding Effectiveness; % Attenuation

Hole Dia. 60 Center Spacing % Open Area Thick Mat'l Sample Size 30 MHz 100 MHz 300 MHz 1 GHz 2 GHz 4 GHz 6 GHz 8 GHz 10 GHz
.040 .055 48.00 .022 Alum. 8"x8" 99.97 99.97 99.99 99.84 99.82 99.60 99.86 98.22 99.60
.062 .093 40.00 .027 C.R.St'l 24"x24" 99.91 99.96 99.95 99.84 99.44 99.60 99.50 99.37 97.76
.062 .125 23.00 .025 Alum. 8"x8" 100.0 99.99 99.97 99.96 99.94 99.90 99.96 98.22 99.44
.072 .100 47.00 .038 C.R.St'l 8"x8" 99.94 99.97 99.97 99.92 99.82 99.75 99.00 97.76 98.59
.075 .125 32.70 .049 C.R.St'l 8"x8" 99.98 99.97 99.97 99.98 99.96 99.93 99.96 99.29 99.60
.078 .109 46.10 .027 C.R.St'l 24"x24" 99.95 99.98 99.98 99.93 99.82 99.80 99.44 99.11 98.88
.079 .115 42.80 .036 C.R.St'l 8"x8" 99.90 99.94 100.0 99.95 99.90 99.84 99.92 97.76 99.00
.100 .187 25.90 .040 Alum. 8"x8" 99.95 99.97 99.99 99.92 99.92 99.92 99.96 99.11 99.60
.125 .187 40.30 .030 Stainless 24"x24" 99.87 99.92 99.86 99.65 99.00 99.59 98.01 97.18 94.99
.125 .187 40.30 .030 Alum. 24"x24" 99.92 99.93 99.91 99.72 99.37 98.74 98.01 97.18 96.02
.125 .187 40.30 .060 Alum. 24"x24" 99.97 99.98 99.86 99.87 99.65 99.60 98.88 98.22 98.01
.125 .187 40.30 .125 Alum. 24"x24" 99.99 99.99 99.99 99.98 99.96 99.99 99.98 99.93 99.72
.156 .187 63.00 .057 C.R.St'l 24"x24" 99.86 99.87 99.89 99.60 99.00 98.22 97.16 96.45 94.99
.156 .250 35.40 .057 C.R.St'l 24"x24" 99.89 99.94 99.91 99.78 99.29 99.29 99.11 98.42 97.18
.187 .250 51.00 .027 C.R.St'l 24"x24" 99.75 99.75 99.65 98.59 97.18 93.69 91.09 88.78 88.78
.187 .250 51.00 .057 C.R.St'l 24"x24" 99.82 99.84 99.82 99.50 99.42 97.76 96.84 95.53 93.69

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