Catalog

Recommendations

Info

Conductive Elastomer Gasket Design continued than the 250°F cure, but are well within the specification noted in Technical Bulletin 30. 1091 Primer Because of the sensitivity of surface preparation on certain substrates and the availability of equipment to perform the etching of aluminum prior to the conversion coating, Chomerics has introduced 1091 primer, which is an adhesion promoter for CHO-SHIELD 2000 series coatings. When used in conjunction with an alkaline etch or chemical conversion coating per MIL-C-5541 Class 3, the 1091 primer will provide maximum adhesion when correctly applied. (See Technical Bulletin 31.) This primer is recommended only for the 2000 series coatings on properly treated aluminum and is not recommended for composites. For further assistance on the application of CHO-SHIELD 2000 series coatings on other metallic and non-metallic substrates, contact Chomerics’ Applications Engineering Department. ■ Galvanic Corrosion The most common corrosion concern related to EMI gaskets is galvanic corrosion. For galvanic corrosion to occur, a unique set of conditions must exist: two metals capable of generating a voltage between them (any two unlike metals will do), electrically joined by a current path, and immersed in a fluid capable of dissolving the less noble of the two (an electrolyte). In short, the conditions of a battery must exist. When these conditions do exist, current will flow and the extent of corrosion which will occur will be directly related to the total amount of current the galvanic cell produces. When an EMI gasket is placed between two metal flanges, the first condition is generally satisfied because the flanges will probably not be made of the same metal as the gasket (most flanges are aluminum or steel, and most EMI gaskets contain Monel, silver, tin, etc.). The second condition is satisfied by the inherent conductivity of the EMI gasket. The last condition could be realized when the electronic package is placed in service, where salt spray or atmospheric humidity, if allowed to collect at the flange/gasket interface, can provide the electrolyte for the solution of ions. Many users of EMI gaskets select Monel mesh or Monel wire-filled materials because they are often described as “corrosion-resistant.” Actually, they are only corrosionresistant in the sense that they do not readily oxidize over time, even in the presence of moisture. However, in terms of electrochemical compatibility with aluminum flanges, Monel is extremely active and its use requires extensive edge sealing and flange finish treatment to prevent galvanic corrosion. Most galvanic tables do not include Monel, because it is not a commonly used structural metal. The galvanic table given in MIL-STD-1250 does include Monel, and shows it to have a 0.6 volt potential difference with respect to aluminum – or almost the same as silver. A common misconception is that all silver-bearing conductive elastomers behave galvanically as silver. Experiments designed to show the galvanic effects of silverfilled elastomer gaskets in aluminum flanges have shown less corrosion than predicted. Silver-platedaluminum filled elastomers exhibit the least traces of galvanic corrosion and silver-plated-copper filled elastomers exhibit more. (See Table III). Tables of galvanic potential do not accurately predict the corrosivity of metal-filled conductive elastomers because of the composite nature of these materials. Also, these tables do not measure directly two important aspects of conductive elastomer “corrosion resistance”: 1) the corrosion of the mating metal flange Table III CORROSION POTENTIALS OF VARIOUS METALS AND EMI GASKET MATERIALS (in 5% NaCI at 21°C after 15 minutes of immersion) Material E corr vs. SCE* (Millivolts) Pure Silver –25 Silver-filled elastomer –50 Monel mesh –125 Silver-plated-copper filled elastomer Silver-plated-aluminum filled elastomer –190 –200 Copper –244 Nickel –250 Tin-plated Beryllium-copper –440 Tin-plated copper-clad steel mesh –440 Aluminum* (1100) –730 Silver-plated-aluminum filled elastomer (die-cut edge) –740 *Standard Calamel Electrode. Aluminum Alloys approximately –700 to –840 mV vs. SCE in 3% NaCl. Mansfield, F. and Kenkel, J.V., “Laboratory Studies of Galvanic Corrosion of Aluminum Alloys,” Galvanic and Pitting Corrosion – Field and Lab Studies , ASTM STP 576, 1976, pp. 20-47. and 2) the retention of conductivity by the elastomer after exposure to a corrosive environment. Instead of using a table of galvanic potentials, the corrosion caused by different conductive elastomers was determined directly by measuring the weight loss of an aluminum coupon in contact with the conductive elastomer (after exposure to a salt fog environment). The electrical stability of the elastomer was determined by measuring its resistance before and after exposure. Figure 14a describes the test fixture that was used. Figure 14b shows the aluminum weight loss results for several different silver-filled conductive elastomers. The aluminum weight loss shows a two order of magnitude difference between the least corrosive (1298 silver-plated-aluminum) and most corrosive (1215 silver-platedcopper) filled elastomers. For silvercontaining elastomers, the filler 200 US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817
Non-Conductive Sealing Gasket Volume Resistivity (mohm-cm) 150 100 50 0 Non-Conductive Sealing Gasket Upper Delrin Block Aluminum Alloy Coupon Weight Loss (mg) Figure 14a Test Fixture Lower Delrin Block 300 50 Conductive Gasket 19 CHO-SEAL 1298 22 1.25" Dia. 1.50" Dia. 1.75" Dia. 1/4-20 CRES Fastener 167 1.00" 1.00" 2.2 0 1298 1287 1285 S6304 1350 1224 1215 CHO-SEAL Material (for composition, see Specifications Table, pgs. 32-34.) Weight Loss (mg) 40 30 20 10 0 Figure 14b Average Weight Loss of CHO-SEAL Elastomers 2.2 CHO-SEAL 1298 25.5 Nickel Powder Filled Nickel Powder Filled 37 10.3 Nickel Fiber Filled Nickel Fiber Filled 237 Figure 14c Weight Loss of 6061-T6 Aluminum Coupons in Contact 35.2 with Conductive Elastomers During 168 hr. Salt Fog Carbon Filled Figure 14d Volume Resistivity (mohm-cm) of Conductive Elastomers Before and After 168 hr. Salt Fog Exposure 1052.2 200 Before After 445.1 7.3 7.3 102.3 84.8 Carbon Filled 281 132.4 126.7 substrate that the silver is plated on is the single most important factor in determining the corrosion caused by the conductive elastomer. Figure 14c shows the weight loss results for nickel and carbon-filled elastomers compared to 1298. The nickel-filled materials are actually more corrosive than the silverplated-aluminum filled elastomers. The carbon-filled materials are extremely corrosive. Figure 14d compares the electrical stability of several conductive elastomers before and after salt fog exposure. In general, silver-containing elastomers are more electrically stable in a salt fog environment than nickel-containing elastomers. Design Guides for Corrosion Control The foregoing discussion is not intended to suggest that corrosion should be of no concern when flanges are sealed with silver-bearing conductive elastomers. Rather, corrosion control by and large presents the same problem whether the gasket is silver-filled, Monel wirefilled, or tin-plated. Furthermore, the designer must understand the factors which promote galvanic activity and strive to keep them at safe levels. By “safe”, it should be recognized that some corrosion is likely to occur (and may be generally tolerable) at the outer (unsealed) edges of a flange after long-term exposure to salt-fog environments. This is especially true if proper attention has not been given to flange materials and finishes. The objective should be control of corrosion within acceptable limits. The key to corrosion control in flanges sealed with EMI gaskets is proper design of the flange and gasket (and, of course, proper selection of the gasket material). A properly designed interface requires a moisture-sealing gasket whose thickness, shape and compressiondeflection characteristics allow it to fill all gaps caused by uneven or unflat flanges, surface irregularities, bowing between fasteners and tolerance buildups. If the gasket is designed and applied correctly, it will exclude moisture and inhibit corrosion on the flange faces and inside the package. Bare aluminum and magnesium, as well as iridited aluminum and magnesium, can be protected by properly designed conductive elastomer gaskets. It is important to note that magnesium is the least noble structural metal commonly used, and a silver-filled elastomer in contact with magnesium would theoretically produce an unacceptable couple. Some specific design suggestions for proper corrosion control at EMI flanges are: 1. Select silver-plated-aluminum filled elastomers for best overall sealing and corrosion protection. CHO-SEAL 1298 material offers more corrosion resistance than any other silver-filled elastomer (see Figure 15, next page). 2. For aircraft applications, consider “seal-to-seal” designs, with same gasket material applied to both flange surfaces (see Figure 16). Non-conductive sealant ,,, ,,, 1287 ,,, ,, CHO-SEAL Non-conductive sealant Figure 16 “Seal-to-seal” design incorporating CHO-SEAL ® 1287 conductive silver-aluminum fluorosilicone gaskets on both mating flange surfaces. Gaskets are bonded and edge sealed to prevent moisture from entering the gasket/ flange area. 3. To prevent corrosion on outside edges exposed to severe corrosive environments, use dual conductive/ non-conductive gaskets (see page 55) or allow the non-conductive protective paint (normally applied to outside surfaces) to intrude slightly under the gasket (see Figure 17). US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 201
Page 1 and 2:
Chomerics is the world’s largest
Page 3 and 4:
Contents COST-EFFECTIVE SOLUTIONS F
Page 5 and 6:
Cost-Effective Solutions for Major
Page 7 and 8:
As Lead Supplier, Chomerics... Prov
Page 9 and 10:
Shielding Solutions for Wireless Co
Page 11 and 12:
SHIELDING SOLUTIONS FOR WIRELESS CO
Page 13 and 14:
Table 2 Table 1 *Copies of CHO-TM-T
Page 15 and 16:
stable platform for direct, high pr
Page 17 and 18:
SHIELDING SOLUTIONS FOR WIRELESS CO
Page 19 and 20:
gaskets will typically save 60% or
Page 21 and 22:
Table 1 TYPICAL PROPERTIES OF CHO-F
Page 23 and 24:
Optimizing the design of Cho-Form S
Page 25 and 26:
Optimizing the design of Cho-Form S
Page 27 and 28:
Conductive Elastomer Gaskets SECTIO
Page 29 and 30:
Material Selection Chomerics’ arr
Page 31 and 32:
Table 2 continued Equipment Materia
Page 33 and 34:
Table 3 continued Test Procedure CH
Page 35 and 36:
Conductive Elastomer Extrusions...
Page 37 and 38:
Bonded gaskets — Similar and diss
Page 39 and 40:
Table 2 EXTRUSION MANUFACTURING GUI
Page 41 and 42:
Standard Extrusion Sizes Table 7 ,,
Page 43 and 44:
Table 8 continued HOLLOW O-STRIPS C
Page 45 and 46:
Table 11 continued Chomerics P/N* M
Page 47 and 48:
C Table 15 A Chomerics P/N MIL P/N:
Page 49 and 50:
19-09-12023-XXXX 19-08-12057-XXXX 1
Page 51 and 52:
19-09-15245-XXXX 19-09-15486-XXXX 1
Page 53 and 54:
19-05-E328-XXXX 19-08-F676-XXXX 19-
Page 55 and 56:
CONDUCTIVE ELASTOMERS Co-Extruded S
Page 57 and 58:
19-18-15351-XXXX 19-18-M391-XXXX 19
Page 59 and 60:
Table 2 STANDARD SHEET STOCK SIZE A
Page 61 and 62:
,,,,,,,,,,,, ,,,,,,,,,,,, ,,,,,,,,,
Page 63 and 64:
,, ,,,,, ,,,,, ,,,, CONDUCTIVE ELAS
Page 65 and 66:
Table 3 continued Chomerics P/N* MI
Page 67 and 68:
CONDUCTIVE ELASTOMERS Waveguide Gas
Page 69 and 70:
Table 2 continued Frequency Range (
Page 71 and 72:
,,,, Table 4 Note: Raised portion w
Page 73 and 74:
temperatures to 125°C, and CHO-SEA
Page 75 and 76:
Jam Nut EMI Seals MIL-C-38999, MIL-
Page 77 and 78:
CONDUCTIVE ELASTOMERS Molded-In-Pla
Page 79 and 80:
Table 1 ELASTOMER SPECIFICATIONS FO
Page 81 and 82:
Load, lb./inch Load, lb./inch 20 10
Page 83 and 84:
Stress Relaxation As important as C
Page 85 and 86:
Heat Aging The primary aging mechan
Page 87 and 88:
CONDUCTIVE ELASTOMERS Part Number I
Page 89 and 90:
19-06-11223-XXXX . . . . . . .Hollo
Page 91 and 92:
SOFT-SHIELD® Low Closure Force, Fo
Page 93 and 94:
LOW CLOSURE FORCE, FOAM CORE EMI GA
Page 95 and 96:
ORDERING INFORMATION Referring to T
Page 97 and 98:
Table 6 KNIFE-EDGE STRIPS OTHER SOF
Page 99 and 100:
Table 1 TYPICAL PROPERTIES FOR SOFT
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Metal EMI Gaskets, Clip-on Gaskets
Page 107 and 108:
METAL EMI GASKETS MESH STRIP All M
Page 109 and 110:
METAL EMI GASKETS MESH STRIP with
Page 111 and 112:
METAL EMI GASKETS SPRINGMESH ® Hig
Page 113 and 114:
Fabricated COMBO GASKETS: Select th
Page 115 and 116:
METAL EMI GASKETS SHIELDMESH Compr
Page 117 and 118:
Table 2 SINGLE EMI/EMP FRAME GASKET
Page 119 and 120:
Width Table 2 POLASHEET COMPOSITE G
Page 121 and 122:
METAL EMI GASKETS METALASTIC ® & P
Page 123 and 124:
METAL EMI GASKETS METALKLIP ® Clip
Page 125 and 126:
SHIELDING EFFECTIVENESS Figures 8-1
Page 127 and 128:
TWIST SERIES FOLD OVER PART NO. 81-
Page 129 and 130:
RIVET & TRACK PITCH .190 (4.83) PAR
Page 131 and 132:
METAL EMI GASKETS SPRING-LINE ® St
Page 133 and 134:
Conductive Compounds Adhesives Seal
Page 135 and 136:
CONDUCTIVE COMPOUNDS CHO-BOND ® Co
Page 137 and 138:
Table 1 continued SPECIFICATIONS AN
Page 139 and 140:
Silicones and Flexible Polyisobutyl
Page 141 and 142:
• CHO-SHIELD 4900 silver-filled a
Page 143 and 144:
Engineered Laminates & Grounding Pr
Page 145 and 146:
Ordering Procedure CHO-MASK II tape
Page 147 and 148:
Table 1 Property Test Method Typica
Page 149 and 150: LAMINATES & GROUNDING PRODUCTS EMI
Page 151 and 152: Table 1 SPECIFICATIONS Property Tes
Page 153 and 154: LAMINATES & GROUNDING PRODUCTS CHO-
Page 155 and 156: SPRING-LINE ® “Pick and Stick”
Page 157 and 158: EMI Shielded Vents SECTION CONTENTS
Page 159 and 160: Customization Options • Alternati
Page 161 and 162: EMI SHIELDED VENTS CHO-CELL Shield
Page 163 and 164: Round SHIELD CELL and OMNI CELL Pan
Page 165 and 166: EMI SHIELDED VENTS SLIMVENT Shield
Page 167 and 168: EMI SHIELDED VENTS Common Parts Tab
Page 169 and 170: EMI Shielded Windows & Contrast Enh
Page 171 and 172: Substrate and Surface Treatment Lig
Page 173 and 174: WIN-SHIELD Windows EMI Shielding Pe
Page 175 and 176: Cable Shielding SECTION CONTENTS PA
Page 177 and 178: BEADS ON LEADS Impedance-Frequency
Page 179 and 180: Table 5 continued Dimensions - Inch
Page 181 and 182: FLAT SPLIT CABLE CORE Impedance-Fre
Page 183 and 184: CABLE SHIELDING CHO-DROP ® EMI Abs
Page 185 and 186: Figure 2 Table 2 STRAIGHT LENGTHS i
Page 187 and 188: Connector Boots and Cable Transitio
Page 189 and 190: CABLE SHIELDING CHO-JAC ® Flat Cab
Page 191 and 192: EMI Shielding Theory & Gasket Desig
Page 193 and 194: E i Current on front wall due to re
Page 195 and 196: Figure 7 Magnetic Field Reflection
Page 197 and 198: E = D 2 G ,,,, ,,,, ,,,, ,,,, G D =
Page 199: Table II Group Material Groupings*
Page 203 and 204: enclosure, or by using an insert si
Page 205 and 206: Gasket Mounting Choices Our various
Page 207 and 208: Actual deflection vs. distance betw
Page 209 and 210: percentage increase required to sat
Page 211 and 212: Figure 28d Section B-B from Figure
Page 213 and 214: Formulas (see definition of terms a
Page 215 and 216: in the unit price of the gasket. Bo
Page 217 and 218: Wire Mesh EMI Gasket Selection Guid
Page 219 and 220: Relative Conductivity: Conductivity
Page 221 and 222: Global Compliance Testing Page 222
Page 223 and 224: TEST SERVICES FCC, CSA, CISPR, VCCI
Page 225: THERM-A-GAP TM Thermally Conductive
show all

Catalog

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?