Hey Phil

Firm believer that all boards should be grounded directly to the chassis (if chassis mounted obviously) in which I put a copper pad around an unplated hole( to avoid any shredded copper burrs)

The copper pad gets grounded to board chassis.  We then mount the board with a flat washer, lock washer and screw.

If the board is grounded via wire harness, then ensure an unplated hole and mount with a flat washer, lock washer and screw.  Copper pad not required.



MC



-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Nutting, Phil

Sent: Monday, September 30, 2019 3:25 PM

To: [log in to unmask]

Subject: [TN] screws used for mounting circuit boards - so many choices



Hi Technetters,



Over the years I have been involved in discussions about proper hardware to use when mounting circuit boards and if copper should be part of the stackup even if not a ground point.



I'll bet the answer is "it depends".



At one point we used a screw, split lock washer and a flat washer (labor intensive to assemble), then external tooth SEM screws that chewed up the FR-4. So we added a copper pad to act as a washer. SEM screws still chewed up the copper.  With each of these we occasionally had units returned with loose screws.  Enter nylon patch screws with no lock or flat washers but with a copper pad.  No loose screws.



As time goes on there is someone that states the "standard" configuration uses a lock washer and flat washer and no copper.



I know there are SEM screws that incorporate a lock washer (star or split) with a flat washer, so that eliminates the assembly labor.



We make commercial OEM power supplies, no military stuff.  So what do you use and why?



Phil Nutting  |  HVP Senior Development Engineer   |  Excelitas Technologies Corp



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35 Congress St, Salem, MA  01970 USA

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