Hi Wayne-

I see. The SEMS you use is a split ring PLUS a flat washer. I never had
luck with those split rings--when I went through a heavy vibration profile,
they actually contributed to backing out the fasteners! I stopped calling
them "lock washers" after that. Their purpose is to keep the threads under
relatively constant tension. In that situation they guarantee that if the
threads can move at all, they will back out! Yes, they have their place in
keeping fasteners loaded during extreme temperature events, where varying
TCEs would could precipitate a gap in the interface. But the price is a
tendency to "unlock" the fasteners. Probably the best design is to analyze
the CTE situation and decide whether they are necessary, then make darn
sure you have something in the system to prevent the screws from backing
out. The nylon patch sounds like a good solution for that.

Thanks for the discussion--I've learned a bunch!

Wayne Thayer

On Wed, Sep 12, 2018 at 12:12 PM Wayne Showers <[log in to unmask]>
wrote:

> Wayne,
> To clarify this, firstly, my full comment is necessary: "I flag designs
> that apply a lock washer directly to the PCB."
>
> A flat washer between the lock washer and PCB is recommended as lock
> washers tend to tear up the PCB as the copper pad, if present, is generally
> only 2 maybe 3 mils thick and tends to get torn to varying degrees roughly
> proportional to the type of lock washer and the applied torque.  And, if
> there is no pad, the lock washer often tears the FR4.
> The other reason for wanting a flat washer is to more evenly distribute
> the torque load as lock washers, especially split lock washer apply a high
> percentage of the load to the split.
>
> I am old school, but my preferred stack-up from PCB surface out is FLAT
> (washer)- LOCK (washer)- NUT.  If using SEMS, then this stackup is FLAT
> (washer)- LOCK (washer)- SCREW (head).
>