FR4 mechanical properties. - designing in some small flex.

[gophert]

Has anyone made slits in FR4 so close together that it would flex/spring? I mean, flex in-plane with the board (while staying within the board's x-y plane). Essentially like a snake moving on the ground. Whether it was intentional or not, I would be interested to hear your story and see the board design or an approximated sketch of a board design. I don't want it to flex easily, some force is good but I don't want to risk cracking FR4 (unless it can be preplanned and crack along some well-placed panelization-like mouse-bite holes.

Also, anything where a part of a board or a panelized set of boards may have had unexpected movement/sag in the z-direction during pick and place.

I know the physical properties are published but, before I get to that point (I'm not an ME), I would like to get a feel for how much movement is possible.

I have a lot of empty space on my board for strain-relief cuts so that is not a problem, I just need to get a feel for how many and how close. In short, I need a fairly long skinny board to be compressed in the long direction by about 1% for super easy/fastener-free mounting. Ideally, it will only be spring and released once so I am not concerned with material fatigue.

also, we realize this is an uncommon and, therefore a likely off-label use of FR4 but, hey, Why not push the envelope. It really simplifies (removes) a step in the supply chain if we can clip this in after the device is assembled.

 

[Visitor]

I wonder if something like this would work? It confines the flexing of the board to the ends. I'm not sure if the gap between the ends of the tab is useful or not. The neck-down at the center ends of the tab would allow inserting a screwdriver to release the board were it ever necessary.

 

[gophert]

That's exactly the kind of photo I'm looking for. Is that a rendering or a real part - I left my glasses in the basement and too lazy to get them tonight. (if you drew that up for me, special thank you!).
That is the basic design idea I had - But I need to know if those tabs can be pushed all the way in to the main part of the board without cracking the FR4.

 

[Visitor]

Just a sketch. If you put a similar arrangement on each end, the total deformation would be shared between the ends.

I think the construction of FR4 would be amenable to some flexing. I have some circuit boards with slots that could be sacrificed to get an idea of flexibility, but I'm away from home for another week or two. If you're not in a big hurry, I'll give it a test when I get home.

 

[Visitor]

I did a quick&dirty test with this circuit board I had. The slot is about an inch long and an eighth of an inch wide. When I deflected the thin piece about half the thickness of the slot, it bounced back when released.

When deflected the entire width of the slot, it was permanently deformed as shown in the picture.

The board is 1.6mm thick, with a copper plane on both sides. If you'd like some actual dimensions, I'll dig out the board file and take a look.

 

[gophert]

Ok, so an 1/16" radius (1/8" diameter) with about 1/8" joint thickness can handle about 3° to 3.5° deflection - assuming I remembered how to do trig.

Thank you very much!

now I just have to read the board maker's rules on slots and routing board edge limitations.

 

[Visitor]

It might be worthwhile to make a run of cheap (unpopulated, no footprints even) boards with a couple designs to test the concept but it looks like a great idea.

 

[gophert]

Even though the FR4 collapsed, it looks like copper is in tact. I thought there might be buckling/tearing of the copper.