PLCC 'plug' backing spring

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Badwolf
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PLCC 'plug' backing spring

Post by Badwolf » Thu Jan 13, 2022 12:17 pm

Based on work with the experimental STE TF536 adapter which plugs in to the empty PLCC68 CPU socket:

Image


I've developed a 3D-printable spring-loaded backing plate to provide support for the pins which are pressed inwards by the PLCC socket.

The logic behind this is to reduce the tolerance required in the 3D printing process.

A solid plug would probably be more stable in the long term, but 3D printer accuracy has to be high of the order of a tenth of a millimetre in the X and Y directions. My 3D printer isn't up to that.


Image


The principle of operation is you print this slightly longer (say about 1-2mm) than it needs to be and it has enough flex to allow it to clip into place providing outward pressure to opposite edges of the PLCC plug.

A second version fits across the remaining axis. The pressure plates are the full height of the pins and the spring mechanism is half height, to allow crossing.

To use:

This model is configured for PLCC68 specifically, but if you have Fusion 360, the parameters may be changed for any size PLCC.
  • Set the pressure pad width to be very slightly greater than the length of one row of pins (say pin strip length + 0.5mm). This cannot be too large otherwise it will clash with its neighbours.
  • Set the length (distance pad to pad) to be 1-2mm longer than the distance between opposite pin strips. This means you have to compress the spring to fit and and this is what applies the pressure. If you need more pressure, make this distance longer.
  • Set the height to be just shy or equal to the pin height, excluding the black plastic depth-control sheath.
  • 3D print two copies.
  • Load the bottom spring into your PLCC 'plug' first, making sure the pad applies pressure to the pins, no the plastic sheath. Pinch the middle of the spring or insert one pad and apply even force to the other to make a fit. The first spring should lie with the spring furthest down to allow room for the second.
  • Repeat this procedure in the other axis, with the spring inverted this time.
  • Profit!
STL and Fusion360 models attached.

BW
Attachments
PLCC Spring v2.f3d.zip
(69.91 KiB) Downloaded 3 times
PLCC_Spring.stl
(46.18 KiB) Downloaded 3 times
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rubber_jonnie
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Re: PLCC 'plug' backing spring

Post by rubber_jonnie » Thu Jan 13, 2022 2:22 pm

Very cool Idea, nice one, I'd be interested in how well it works :)
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JezC
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Re: PLCC 'plug' backing spring

Post by JezC » Thu Jan 13, 2022 2:40 pm

rubber_jonnie wrote:
Thu Jan 13, 2022 2:22 pm
Very cool Idea, nice one, I'd be interested in how well it works :)
Me too! No 3D printer at home & not sure if I'm allowed access to the one at work.

Once I get the Dual TOS adapters fitted to the first of my STE's (and working!) I'll be looking at the STE relocator board as the next task on the STE's (though I have waaay too many other retro tasks on the other Atari's to make that an urgent problem to overcome!)

:sigh: :roll:

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Re: PLCC 'plug' backing spring

Post by Badwolf » Fri Jan 14, 2022 9:05 pm

Quick installation example vid for you.



BW
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Re: PLCC 'plug' backing spring

Post by exxos » Sun Jan 16, 2022 12:44 pm

How tough is it though ? Without anything supporting the pins, the pressure on them over time bends them inwards. After a while the board pops out of the socket because of it. I've always gone for solid or hollow squares and have to be a precision fit to make sure there is no "flex" in the square.
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Re: PLCC 'plug' backing spring

Post by Badwolf » Sun Jan 16, 2022 3:59 pm

exxos wrote:
Sun Jan 16, 2022 12:44 pm
How tough is it though ? Without anything supporting the pins, the pressure on them over time bends them inwards. After a while the board pops out of the socket because of it. I've always gone for solid or hollow squares and have to be a precision fit to make sure there is no "flex" in the square.
This one is quite bendy, but that's kind of what I'm testing out.

My theory is that we only have to apply enough pressure to match what the pins on the socket are exerting. These are, approximately, spring-like so the amount of force applied inwards is roughly proportional to the displacement. With a spring on one side and another on the other, there'll be a point of equilibrium as one releases its pressure as the pins bend and one increases it.

The key is to get the correct balance so that an equilibrium is found before we lose electrical contact.

The initial force the plates apply is controllable in my design by two configurable variables: the length of the spring and the thickness of the curved section.

I suspect mine is a little too weak at the moment, but it's a nice simple print without much accuracy required so one can always run off some longer or stiffer ones if needed.

It could be the wrong route to go down, but I prefer to try this instead of trying to get hyper-accurate prints, I think.

A commenter on YouTube suggested laser-cut plywood may actually be a better option again. I don't dislike that idea at all.

BW
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Re: PLCC 'plug' backing spring

Post by exxos » Sun Jan 16, 2022 6:58 pm

Badwolf wrote:
Sun Jan 16, 2022 3:59 pm
The key is to get the correct balance so that an equilibrium is found before we lose electrical contact.
A spring pressing against a spring. Possible, like you say is the balance is right. But there is also a possible problem that the print could strain and weaken over time.
Badwolf wrote:
Sun Jan 16, 2022 3:59 pm
A commenter on YouTube suggested laser-cut plywood may actually be a better option again. I don't dislike that idea at all.
I guess it's having a laser as a option... I'm not sure if plywood would would hold up well with the tight tolerances involved. Ply doesn't tend to have much strength on its edges. You only need a tiny fraction of a mm to chip away and you have a bad contact on a pin.

In fact now I think about it, the hollow squares I did didn't hold up well. The square has to be a good fit to work, and IIRC the sides of the square being just a line basically would tend to bend inwards causing the center pins to have contact issues. You might run into the same problem.

3D printing needs tight tolerances which is why I'm faffing about so much with my printer. When I want 25mm its gotta do 25mm. Problem becomes calibration of XYZ plus first layer has to be spot on. If its fraction too low, it bulges the first layer and you end up with 25.1mm .. I hate printers soooo much :lol: :roll:
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Re: PLCC 'plug' backing spring

Post by Badwolf » Sun Jan 16, 2022 7:02 pm

exxos wrote:
Sun Jan 16, 2022 6:58 pm
In fact now I think about it, the hollow squares I did didn't hold up well. The square has to be a good fit to work, and IIRC the sides of the square being just a line basically would tend to bend inwards causing the center pins to have contact issues. You might run into the same problem.
Interestingly the commercial (Winslow) options are hollow with just a little bit of filleting at the base.
3D printing needs tight tollerences which is why I'm faffing about so much with my printer. When I want 25mm its gotta do 25mm. Problem becomes calibration of XYZ plus first layer has to be spot on. If its fraction to low, it buldges the first layer and you end up with 25.1mm .. I hate printers soooo much :lol: :roll:
You could taper the bottom so it squishing out wouldn't affect things, if that's the PCB end rather than that which supports the 'dangling end' of the pins, it wouldn't matter.

BW
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Re: PLCC 'plug' backing spring

Post by exxos » Sun Jan 16, 2022 7:16 pm

Badwolf wrote:
Sun Jan 16, 2022 7:02 pm
Interestingly the commercial (Winslow) options are hollow with just a little bit of filleting at the base.
Just had a quick look. Seems they put supports in the middle to avoid bending I assume.

product_4392&sz200x200&cp1&tn&ql&fm&bo&bc&sgecba79b934&ft1566207510&WinslowW9306.png
product_4392&sz200x200&cp1&tn&ql&fm&bo&bc&sgecba79b934&ft1566207510&WinslowW9306.png (70.22 KiB) Viewed 122 times
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Re: PLCC 'plug' backing spring

Post by Badwolf » Sun Jan 16, 2022 7:32 pm

exxos wrote:
Sun Jan 16, 2022 7:16 pm
Badwolf wrote:
Sun Jan 16, 2022 7:02 pm
Interestingly the commercial (Winslow) options are hollow with just a little bit of filleting at the base.
Just had a quick look. Seems they put supports in the middle to avoid bending I assume.
Sorry, yes. That's what I meant. Wrong term.

I iz not an mechanicist engineerer. :)

BW
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