Editorial Feature

Electromagnetic Interference Shielding With Perforated Metals

Jul 16 2002

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EMI/RFI shielding can be achieved by using perforated metal. The perforated pattern that is best suited for this task was established by testing 16 samples of perforated metal at nine frequency levels and by evaluating their performance.

Data related to the samples, the frequencies used, and the test results are shown in the tables below. The results have been represented as dB of shielding attenuation and as a percentage of attenuation.

Table 1. (Part A) EMI/RFI shielding for various perforated metal materials

Hole Diameter
(inches)		 60° Centre Spacing Open Area
(%)		 Thickness
(inches)		 Material Sample Size
(inches)
0.040 0.055 48.00 0.022 Alum 8x8
0.062 0.093 40.00 0.027 C.R. Steel 24x24
0.062 0.125 23.00 0.025 Alum 8x8
0.072 0.100 47.00 0.038 C.R. Steel 8x8
0.075 0.125 32.70 0.049 C.R. Steel 8x8
0.078 0.109 46.10 0.027 C.R. Steel 24x24
0.079 0.115 42.80 0.036 C.R. Steel 8x8
0.100 0.187 25.90 0.040 Alum 8x8
0.125 0.187 40.30 0.030 Stainless 24x24
0.125 0.187 40.30 0.030 Alum 24x24
0.125 0.187 40.30 0.060 Alum 24x24
0.125 0.187 40.30 0.125 Alum 24x24
0.156 0.187 63.00 0.057 C.R. Steel 24x24
0.156 0.250 35.40 0.057 C.R. Steel 24x24
0.187 0.250 51.00 0.027 C.R. Steel 24x24
0.187 0.250 51.00 0.057 C.R. Steel 24x24

Table 1. (Part B) EMI/RFI shielding for various perforated metal materials

Shielding Effectiveness – Attenuation in dB
MHz GHz
30 100 300 1 2 4 6 8 10
70 70 80 56 55 48 57 35 48
61 67 66 56 45 48 46 44 33
92 84 90 68 65 60 67 35 45
64 70 90 62 55 52 40 33 37
72 70 90 72 68 63 68 43 48
66 75 72 63 55 54 45 41 39
60 64 88 66 60 56 62 33 40
66 70 77 69 62 62 67 41 48
58 62 57 49 40 37 34 31 26
62 63 61 51 44 38 34 31 28
71 72 69 58 49 48 39 35 34
85 84 84 73 68 84 73 63 51
57 58 59 48 40 35 31 29 26
59 64 61 53 43 43 41 36 31
52 52 49 37 31 24 21 19 18
55 56 55 46 36 33 30 27 24

Table 1. (Part C) EMI/RFI shielding for various perforated metal materials

Shielding Effectiveness - % Attenuation
MHz GHz
30 100 300 1 2 4 6 8 10
99.97 99.97 99.99 99.84 99.82 99.60 99.86 98.22 99.60
99.91 99.96 99.95 99.84 99.44 99.60 99.50 99.37 97.76
100.0 99.99 99.97 99.96 99.94 99.90 99.96 98.22 99.44
99.94 99.97 99.97 99.92 99.82 99.75 99.00 97.76 98.59
99.98 99.97 99.97 99.98 99.96 99.93 99.96 99.29 99.60
99.95 99.98 99.98 99.93 99.82 99.80 99.44 99.11 98.88
99.90 99.94 100.0 99.95 99.90 99.84 99.92 97.76 99.00
99.95 99.97 99.99 99.92 99.92 99.92 99.96 99.11 99.60
99.87 99.92 99.86 99.65 99.00 98.59 98.01 97.18 94.99
99.92 99.93 99.91 99.72 99.37 98.74 98.01 97.18 96.02
99.97 99.98 99.86 99.87 99.65 99.60 98.88 98.22 98.01
99.99 99.99 99.99 99.98 99.96 99.99 99.98 99.93 99.72
99.86 99.87 99.89 99.60 99.00 98.22 97.16 96.45 94.99
99.89 99.94 99.91 99.78 99.29 99.29 99.11 98.42 97.18
99.75 99.75 99.65 99.59 97.18 93.69 91.09 88.78 88.78
99.82 99.84 99.82 99.50 99.42 97.76 96.84 95.53 93.69

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The shielding tests were carried out by Dash, Straus & Goodhue, Inc., testing laboratories and are made available courtesy of the Industrial Perforators Association.

Shielding Test Results

The test results showed that a shielding effectiveness of 40 dB provides 99.000% attenuation of the electromagnetic (EMI/RFI) radiation, while a shielding effectiveness of 92 dB, which is the maximum value found in the tests, provides an attenuation of 99.997%.

In a vast number of the applications, a shielding effectiveness of 40 dB is the proposed minimum. Most of the samples delivered a very effective shielding up to frequencies of 7 GHz. Beyond 7 GHz, although the shielding effectiveness of certain samples dropped below 99.000%, the majority of the samples were seen to easily retain shielding effectiveness of over 95% even at a maximum frequency level of 10 GHz.

Conclusion

In conclusion, it is obvious that designers have the opportunity to select from several perforated patterns to realize their design requirements.

Contact surfaces between two components are the single largest leakage source. In case an electrical connection is not securely sealed, the leakage along the interface can be more than that through the structure.

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