Metallic screws, nuts and fasteners seem like a common way of attaching PCBs to enclosures. Are they, especially stainless steel, also a CDM or direct metal to metal discharge risk?
For instance, stainless steel fasteners may not stay connected to a magnetic tipped driver, and could be more likely to fall onto a PCB during assembly. I have memories of bags of coins and screws being shaken causing ElectroStatic Discharges caught on an antenna during a symposium. A stainless steel screw, standoff or nut dropped on a PCB seems likely to generate similar damaging discharges. Additionally, it doesn’t seem like well grounded tools, personnel, or dissipative worksurfaces would have much of a mitigation affect.
Would zinc plated or galvanized fasteners or some kind of plastic fastener be preferred, as the fasteners could be less likely to be dropped on a PCB or create direct contact discharges, or are dropped fasteners on a PCB not likely to cause ESD damage on a production line assembling PCBs in an enclosure?
Hello Scott,
Thank your for asking your question. While there is concern about a charged isolated conductor, which a screw or fastener is, what makes the charged isolated conductor an ESD threat is the capacity for stored charge. Thus, the larger the isolated conductor, the more likely it is to discharge a significant ESD pulse. However, based on the size of a typical screw used to fasten a PCB board (several millimeters long), the capacitance might be several femtofarads at most. This is hardly sufficient to pose a significant threat to any component on the board. Furthermore, the boards will have their own ESD protection as well. Second, the only access that one has to a full built-up board that is being assembled into an electronic product is to directly connect the electrical connectors for the board. For these, the greater risk is direct contact by personnel or hand tools during assembly. A screw driver with an insulating handle, for example, will have much more capacitance than a screw. It is also capacitively coupled to the technician holding it.
As for other metals used for fasteners, zinc plated or galvanized fasteners have negligible differences in conductivity to stainless steel. Also, my understanding is that metal fasteners can act as grounding connections for the PCB board. Where this is the case, it would not make sense to change out metal fasteners for plastic or any other insulator.
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great point that a fastener has small capacitance compared to a tool/technician. Thanks!