EMC & Thermal Management
S6305 (Ni/Gr-Si) and CHO-SEAL 6452 (Ni/Gr- EPDM) lost 39 mg and 44 mg respectively. The big caveat here is that CHO-SEAL 6503 (Ni/Al-Fl), CHO-SEAL 6460 (Ni/Al-EPDM), CHO- SEAL 1298 (Ag/Al-Fl) and CHO-SEAL 1285 (Ag/ Al-Si) exhibited more gasket swelling, leading to changes in dimensions (thickness) and volume resistivity. Thickness change for these elastomers ranged from 8-21%, while volume resistivity change was in the region of 2000- 3000 mOHM-cm. To measure volume resistivity, Parker Chomerics used a four-point pressure probe connected to a digital ohmmeter with a range of 10-4 to 104 ohms (Ω). It was possible to calculate the change in volume resistivity by multiplying the measured resistance by the area of conductive elastomer and dividing the result by the measured thickness.
In contrast, although CHO-SEAL S6305 (Ni/Gr-Si) and CHO-SEAL 6452 (Ni/Gr-EPDM) showed the highest average coupon weight loss, they demonstrated the lowest change in thickness (a negligible loss of 1-2%) and volume resistivity (circa 30-300 mOHM-cm). As a very important point, in terms of gasket type, form-in-place (FIP) gaskets outperformed the conductive elastomers after salt fog exposure on ENIG. Within the FIP gaskets tested, CHOFORM 5560 (Ni/Al) outperformed CHOFORM 5575 (Ag/Al) in volume resistivity and gasket dimensional change.
cleaning and drying in accordance with the test method, followed by evaluation in the company’s laboratory.
The results
The corrosivity of conductive elastomer materials is simple to calculate as it is proportional to the weight loss of sample coupons after testing. Parker Chomerics tested three fixtures for its CHO-SEAL elastomers and two for CHOFORM elastomers, with the average of each taken for the final results. CHO-SEAL 6503 (Ni/Al-Fl), CHO-SEAL 6460 (Ni/Al-EPDM), CHO-SEAL 1298 (Ag/Al-Fl), and CHO-SEAL 1285 (Ag/Al-Si) outperformed CHO- SEAL S6305 (Ni/Gr-Si) and CHO-SEAL 6452 (Ni/Gr-EPDM) in ENIG coupon weight loss. In fact, the former four elastomers exhibited no weight loss whatsoever, while CHO-SEAL
Also worth reporting is that there were no significant differences concerning the use of nickel-aluminium or silver-aluminium fillers, regardless of whether the elastomer binder was silicone, fluorosilicone or EPDM.
Conclusion
As the test results show, numerous electrically conductive elastomeric gaskets are available that demonstrate excellent resistance to corrosion when in contact with ENIG. However, the trade-off is often greater change in thickness and volume resistivity in comparison with elastomers demonstrating slightly lower resistance to corrosion. Material selection is therefore application dependent, where it is possible to strike a balance for individual PCB types.
Ultimately, selecting the right gasket requires knowledge of both electrical and mechanical requirements. Shear forces, environmental effects, compression set, and application method are just some of the important factors influencing the choice of gasket for a particular application. Furthermore, materials must be cost effective and ensure equipment and system compliance with military and commercial EMI/EMC test requirements, as well as environmental tests.
www.parker.com/chomerics
www.cieonline.co.uk. Components in Electronics June 2023 25
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