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EMC & Thermal Management


Assessing the corrosivity of EMI gasket materials in contact with ENIG


By David Inman, applications engineering manager, Scott Casper, applications engineering and Gary Brown, R&D - Parker Hannifin, Chomerics Division


O


ver the years, Parker Chomerics has invented numerous electrically conductive elastomeric gaskets to address progressive requirements


within the electromagnetic interference (EMI) and electromagnetic compatibility (EMC) marketplace. As a result of these efforts, the company receives regular questions about the corrosivity of these materials when in contact with electroless nickel/immersion gold (ENIG), a popular surface finish for printed circuit boards (PCBs). In response, Parker Chomerics is revealing the results of a comprehensive series of corrosion tests, where the company evaluated the effects of conductive elastomer and form-in-place (FIP) EMI gasket materials in contact with ENIG.


Electrically conductive elastomeric gaskets comprise a homogenous dispersion of particles, process modifiers and additives within a silicone or fluorosilicone resin binder system. These binders are popular due to their ease of processing, excellent compression-set performance across a wide temperature range and ability to compress at low closure forces. EPDM (ethylene propylene diene monomer) based resin binder systems are also common, largely in applications exposed to harsh chemicals. Whichever the material, the resulting gasket offers both EMI shielding and a degree of environmental protection, particularly when featuring filler particle technologies based on nickel-plated graphite, silver-plated aluminium, and nickel-plated aluminium.


The challenge ENIG is one of the plating processes/surface finishes used as an interface between (EMI) shielding gaskets and PCBs. Comprising an electroless nickel layer capped with a thin layer of immersion gold, ENIG is a multi-functional PCB surface finish used for soldering, wire bonding, and general electrical conductivity.


To test the corrosivity of EMI gasket materials in contact with ENIG, Parker Chomerics put forward its CHO-SEAL electrically conductive elastomer and


24 June 2023


CHOFORM form-in-place gasket materials, which have been proven to provide the best corrosion resistance (as per the company’s CHO-TM100 test method). These materials also provide the highest degree of shielding effectiveness after long-term ageing tests of any EMI shielding elastomer gasket material. The galvanic corrosion tests focused on a range of gaskets featuring different combinations of conductive filler and elastomer binder.


Components in Electronics


The tests


Based on CHO-TM100, Parker Chomerics set out to determine (in a quantitative manner) the corrosivity of the conductive elastomers toward aluminium alloys after exposure to a salt fog environment.


The company developed a special test fixture that holds a conductive elastomer (measuring 28.6 mm in diameter) and an aluminium coupon plate (simulating an aluminium mating flange) in compressive


contact between two cylindrical Delrin blocks. Compressive force came courtesy of a central stainless-steel bolt featuring a non-conductive (rubber) environmental seal at each end to prevent fluid from penetrating into the middle of the fixture.


To ensure a comprehensive test, the neutral salt fog exposure for this evaluation was in accordance with ASTM B117 for a duration of 500 hours. After the test, Parker Chomerics performed sample disassembly,


www.cieonline.co.uk.uk


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