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EMI Shielding Caulk Cures Quickly and Is Paintable Continued from page 57
than legacy EMI shielding caulks and do not require a polysulfide sealant coating to be paintable.
Materials being considered for EMI shielding caulk were
mounted on panels and evaluat- ed in a test chamber. This one passed all the tests.
For improved shielding and cor-
rosion resistance, it is necessary to use conductive fillers. Fillers with silver-plated particles provide good EMI shielding while improving upon the corrosion resistance of fillers with copper-based particles. Three conductive particles were considered for Cho-Bond 1019: silver-plated alu- minum, nickel-plated aluminum, and nickel-coated graphite. The shielding effectiveness (SE) of a material is determined by its electrical conduc- tivity and magnetic permeability, and silver-plated and nickel-plated particles both provide good SE. Since the galvanic corrosion resist- ance of a material is related to its similarity to the material it is con- tacting, such as an aluminum elec- tronic enclosure, aluminum-based particles were preferred, but graph - ite-based particles would also be con- sidered a viable option.
Effective EMI Shielding Caulk Combining a high-performance
polymer with a high-performance conductive filler does not necessarily ensure a high-performance final product. Other factors must be con- sidered. First, the "resin demand" of the filler will dictate the minimum resin-to-filler ratio that will provide an effective EMI shielding caulk. If this ratio is too high, proper shield- ing won’t be achieved. Careful selec- tion of a filler-polymer combination is necessary since the resin demand is a function of the size and morphol- ogy of a particle and the liquid prop- erties of the uncured polymer. Second, the binding properties of the polymer resin will dictate the degree to which the conductive filler parti- cles are held together in a compound. If this is not adequate, shielding per- formance will suffer and corrosion resistance may decrease. An important binding property
is "shrinkage." When a liquid polymer cures, there is a decrease in over all volume as a result of the tightening of the filler-containing matrix, which maximizes the impact of the filler’s inherent conductive properties. When polymers lose their tight-
ly bound matrix, whether due to environmental exposure or changing physical loads, the final product fails to retain its desired performance lev- els.
Several trial formulations were
subjected to initial SE testing for comparison. During this testing, it was determined that PTE experi- enced resin-demand issues when mixed with nickel-plated aluminum. Consequently, the associated formu- lation was omitted from further test-
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ing. The final four trial formulations subjected (along with the control compounds) to the full battery of shielding and environmental testing included silver-plated aluminum (Ag/Al) filled PTE (Cho-Bond 1019), silver-plated aluminum (Ag/Al) filled PU, nickel-plated aluminum (Ni/Al) filled PU, and nickel-coated graphite (Ni/C) filled PTE. Control materials were two currently available alterna- tives offered by other EMI shielding manufacturers: silver-plated copper (Ag/Cu) filled silicone and nickel (Ni) filled PTE. The full battery of tests includ-
ed environmental testing (ET), shielding effectiveness (SE), and physical testing. The environmental tests measured the effect of thermal cycling on adhesion (ASTM D3359)
and flexibility of the epoxy-painted compound and the effect of salt fog (ASTM B117, 1000 hours total) on the compounds and aluminum pan-
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els with and without the protective paint. The SE tests (IAW modified IEEE-STD-299) measured the effect of thermal cycling and salt fog (ASTM B117, 1000 hours total) on the compounds’ capability to shield against EMI. Finally, the physical tests measured the cured compounds’ capability to adhere to the aluminum panels and also verified adequate workability of the uncured com- pounds for application purposes.
A variety of different compounds were tested for their EMI shield- ing capabilities compared to the Ag/Cu control which showed areas of poor adhesion to the “protected” structure.
Prepping Test Panels Each ET panel was comprised of
two 12.0 x 4.0 x 0.03125-in. panels riveted together with a 1-in. overlap to create a 12.0 x 7.0 x 0.0625-in. test panel. These panels had the com- pounds applied to them on both sur-
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