Transfer Impedance Test Method. & Conclusion

Transcription

1 Part 3 of 3: Transfer Impedance Test Method & Conclusion Educational Seminar 3-1 by George M. Kunkel Spira Manufacturing Corporation Transfer Impedance Test Method per SAE, ARP-1705, Rev. A EMI gaskets provide an electromagnetic (EM) bond between conductive members of an enclosure to produce a Faraday Cage (Shielded Enclosure.) Transfer Impedance testing measures the EM bond of a gasket as a function of the structural material and finish to be used to construct a shielded enclosure under design. Transfer Impedance testing is used by the EMP engineering community to grade EMI gaskets for protecting electronic weapon systems from the EMP effects of a nuclear explosion. The Transfer Impedance test method tests for the following: 3-2 Shielding Quality of a gasket (QC test) Shielding Quality of a gasket after being subjected to storage and transportation environments. Shielding Quality of gasketed joints. Shielding Quality of gasketed joints after being subjected to the various environments the system will be subjected to. 1

2 Transfer Impedance Theory 3-3 Electromagnetic Leakage via Seams and Gasketed Joints in Shielded Enclosures occurs primarily as a result of currents which cross the seam. Such crossing causes a voltage to appear on the far side of the seam. Electromagnetic Leakage via the seam is directly proportional to this (transfer) voltage. In Shielding Theory the seam is characterized in terms of its Transfer Impedance as follows: Z T = V / J S Z T = Transfer Impedance of Seam (Ohm-meters) V = Transfer Voltage (Voltage across Seam) J S = Density of Current which crosses the Seam (A/m) Shielding Quality vs. Shielding Effectiveness Manufacturers of EMI Gaskets advertise the Shielding Effectiveness of EMI Gaskets What is of concern to the Design Engineer is the actual shielding obtained from the gasketed joint under test. Transfer Impedance Test data provides the design engineer with the transfer impedance of a gasketed joint. The transfer impedance test data can then be converted to a measurement called Shielding Quality

3 Shielding Analysis Using Transfer Impedance Test Data J S = Current due to Wave Impinged on Barrier (A/m) H I e = Voltage across Gasket = J S Z T Z T = Transfer Impedance of Seam or Gasketed Joint (Ω-m) E T 2e/l l (V/m) ΠR (meters) H T E T λ /(377*2ΠR) (A/m) R < λ/2π 3-5 H T = E T / 377 (A/m) R λ/2π Shielding Quality vs. Shielding Effectiveness of E Field Shielding Quality = Z W /Z T where Z W = Impedance of Incident Wave Z T = Transfer Impedance of Gasket Given Incident Wave = 377 V/m plane wave E = 377 V/m H = 1 A/m Z T = 10-3 Ω-m e = 10-3 V l 2 R = 1.0 m E T = 10-3 V/m Shielding Effectiveness of E field SE E = 377/10-3 = 3.77x Shielding Quality = 377/10-3 = 3.77x10 5 SQ SE R = 1.0m 3

4 Transfer Impedance Test Fixture 3-7 I I = Input Current (Amps) L G = Length of Gasket (m) V I = Input Voltage (dbm) V O = Output Voltage (dbm) Input power (from 50 Ω source) comes into the Input connector and is terminated into a 50 Ω resistor that makes contact with the contact plate. The Input Current (I I ) associated with the power flows through the gasket under test and returns to the input source via the base plate. The voltage drop (Output Voltage V O ) is measured by a 50 Ω receiver attached to the output connector. V O Transfer Impedance = Z T = L G (V I / 50) Shielding Quality Test Data Shielding Quality of Tin Plated Gasket against Tin Plated Surfaces, as a function of the force of the gasket against the joint surface. 2.5 Pounds / Inch 6 Pounds / Inch 13 Pounds / Inch Shielding Quality of Tin Plated Gasket against Nickel Plated Surfaces, as a function of the force of the gasket against the joint surface. 2.5 Pounds / Inch 6 Pounds / Inch 13 Pounds / Inch 3-8 Spira-Shield Gaskets were used for Testing. 8 4

5 Shielding Quality Test Data Shielding Quality of Stainless Steel Gasket against Tin Plated Surfaces, as a function of the force of the gasket against the joint surface. 2.5, 8 & 11 Pounds / Inch Shielding Quality of Stainless Steel Gasket against Nickel Plated Surfaces, as a function of the force of the gasket against the joint surface. 2.5 Pounds / Inch 8 & 11 Pounds / Inch 3-9 Spira s Quick-Shield Gaskets were used for Testing. Shielding Quality Test Data Nickel Plated Tin Plated EMI Gaskets against Plated Aluminum Joint Surfaces Chem Film Plated Tin Plated Chem Film Plated Tin Plated Stainless Steel EMI Gaskets against Plated Aluminum Joint Surfaces Nickel Plated 3-10 Spira-Shield & Quick-Shield Gaskets were used. 5

6 Shielding Quality Test Data Chem Film Plated Stainless Steel EMI Gaskets against Plated Aluminum Joint Surfaces Tin Plated Nickel Plated (repeated) Stainless Steel EMI Gaskets against Nickel Plated Joint Surfaces Flange Mounted Groove Mounted 3-11 Spira s Quick-Shield Gaskets were used for Testing. Effects of Moisture Soak on Shielding Quality of Gasketed Joint Shielding Quality of Tin-Lead plated EMI Gaskets using various joint surfaces before and after being subjected to 336 hours of Moisture Soak. Nickel plated aluminum joint surfaces measured a loss in Shielding Quality of approximately 25 db. 1. Tin Plated Aluminum Joint Surface 2. Chemical Film Plated Aluminum Joint Surface 3. Nickel Plated Aluminum Joint Surface Chemical film plated aluminum joint surfaces measured a loss in Shielding Quality of approximately 15 db. Tin plated aluminum joint surfaces had no loss in Shielding Quality. 2a 3b 3a 2b 1a 1b 3-12 Spira-Shield Tin-Lead Plated Gaskets were used for Testing. 1G 6

7 Shielding Quality after 672 Hours of Salt Fog Exposure Shielding Quality of a Stainless Steel Gasket against Chemical Film Plated Aluminum and Stainless Steel Joint Surfaces after 672 hours of Salt Fog Exposure. The poor level of shielding is due to a build-up of aluminum oxide in the joint surface due to galvanic corrosion. Shielding Quality (db) Stainless Steel Gasket Frequency (Hz) 3-13 Shielding Quality after 672 Hours of Salt Fog Exposure Shielding Quality of Stainless Steel Gasket against Nickel Plated Aluminum and Stainless Steel Joint Surfaces before and after being exposed to 672 hours of Salt Fog Exposure The shielding after exposure was sufficient for the specific application. Shielding Quality (db) Recorded Data After Exposure Recorded Data Before Exposure MHz Frequency (Hz) 1 GHz 7

8 Loss of Conductivity Due to Storage Environment This test data illustrates a loss of conductivity of various gaskets as a result of being stored on a shelf in an office exposed to normal temperature, moisture and pressure environments. New Aged 1 Year (stored on shelf) The silver filled elastomeric gasket experiences a significant loss of conductivity after 1 year. This was caused by sulfur in the environment penetrating the elastomeric binder and coating the silver particles with a non-conductive silver sulfide. Stainless Steel, Tin Plated, and Monel materials had less conductivity loss. Shielding Quality (SQ) = 20 log (377/Z T ) 3-15 Transfer Impedance Test Data of EMI Gaskets Against Chemical Film Plated Aluminum Summary & Conclusions 1. Requirements for EMI Shielding Gasket Selection & Use EMI gaskets and gasketed joint surfaces should last the life of a system in the intended environment, and be cost effective. The test method used to grade the gaskets should be able to test for the applicable environmental conditions. Material selection, quality and compatibility is imperative for a long lasting system. Fastener spacing can be an area for reducing costs if the compression force of the gasket is controlled. All of Spira s gaskets are available in standard, moderate, and low compression force to assist in the most effective and cost saving design

9 Summary & Conclusions 2. Shielding Effectiveness Test Methods The EMI gasket must meet and/or exceed the shielding requirements of the application. Depending on the test method used, the information provided by most manufacturers is inflated MIL-STD-285 Test Method Designed by manufacturers of EMI gaskets as a promotional tool Gave inflated shielding data with no correlation to real world shielding levels MIL-DTL-83528C Test Method Also designed by manufacturer of EMI gaskets for promotional purposes Data is more inflated than the results of MIL-STD-285 Used my most EMI gasket manufacturers, inaccurately grading their products SAE, ARP 1173 Test Method Developed by EMC design engineers to provide applicable real world data Not used by manufacturers due to low shielding test results Summary & Conclusions 3. Transfer Impedance Test Method (SAE-ARP-1705 Rev. A) Created by EMP design engineers to test the EM bond the gasket will provide a system in the actual application. Tests for a loss in EM bond from environmental conditions that will be experienced in real life. The Transfer Impedance Data is converted to Shielding Quality values which provide an accurate measurement of the shielding that can be achieved in real applications. The information provided can guide a design engineer in selecting the best cost effective shielding design that will last the life of the system Spira Manufacturing Corporation uses only SAE-ARP-1705 Rev. A to supply our customers with the most accurate and useful information. 9

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