Attaching PCBs to Lego plates

I am working with arduino/raspberry pi modules and trying to secure them to a lego plate. Because of the soldering joints on the bottom, there isn't a flat surface to work with.

I have tried crazy glue which works. I also tried a sculpting clay which attempted to take care of the bumpy surface of the joints, but that didn't hold (the clay had crazy glue on both sides).

Even though the modules still worked, I would imagine that this introduces added resistance to the circuit where it wasn't intended.

It looks like standoffs are the best way to go, even though there is nothing readily available for this purpose.

I'm wondering if there are any other options to consider?

• Can you should your PCB? First thought is to use SMD throughout if you haven't already... – awjlogan Jun 25 '18 at 12:03
• Standoffs would definitely have been my go-to from the start. Another option might be silicone potting, if you don't have to access the board. – DerStrom8 Jun 25 '18 at 12:03
• While probably not great from an ESD perspective, double sided foam tape may work reasonably well in practice for a non-critical project. – Chris Stratton Jun 25 '18 at 15:24
• "I would imagine that this introduces added resistance" I believe you intended to say "reduced resistance"? (as laptop2d's answer points out though, the effect is negligible) – Dave Jun 25 '18 at 20:06
• always build things you can take apart, whatever you do make sure its undo-able. If you use glue only use enough to hold it well, but still be able to break it off if necessary. Standoffs and/or some other physical fastener would probably be prefered. Another idea would be to 3d-print an adapter that can attach to the board, and snap into the lego; basically custom 'lego' standoffs. – esoterik Jun 25 '18 at 21:52

Maybe you can use "Technic pin joiners" as standoffs (this kind of standoff): (some luck and bodging may be involved in getting them to line up)

Not this kind of standoff:

• haha fantastic. yes those joiners would do the trick, but also might be trouble to line up as you suggest. – mrtunes Jun 25 '18 at 14:15
• You could also drill through them and the baseplate dots and use a small screw. – Chris Stratton Jun 25 '18 at 15:23

Since 1963, Lego pieces have been manufactured from a strong, resilient plastic known as acrylonitrile butadiene styrene (ABS).[12][30] As of September 2008, Lego engineers use the NX CAD/CAM/CAE PLM software suite to model the elements. The software allows the parts to be optimized by way of mould flow and stress analysis. Prototype moulds are sometimes built before the design is committed to mass production. The ABS plastic is heated to 232 °C (450 °F) until it reaches a dough-like consistency. It is then injected into the moulds at pressures between 25 and 150 tonnes, and takes approximately 15 seconds to cool. The moulds are permitted a tolerance of up to twenty micrometres, to ensure the bricks remain connected.[33]
Source: Wikipedia

Since most bricks are made primarily out of ABS, they have a high resistivity. Generic ABS has a surface and volume resistivity of $10^{15}$ and $10^{16} cm$ respectively. FR4 (which is what most pcb's are made out of) has a resistivity of $~10^9$ and $~10^{10} cm$ for volume. These are dependent on factors like contamination and humidity.

So glue away, your going to lose less current through ABS than FR4 (both will be in the nA range and below for 10's of volts)

• The question is "How do I glue?". I don't think it is "If I glue, will I have electrical problems". – dim Jun 25 '18 at 18:48
• yeah i was concerned about glueing to the bottom of a pcb board and it causing issues that i'm not aware of. but i wouldn't be glueing the lego directly to it like the response suggests. something like the epoxy putty suggested below would be sitting in between it. – mrtunes Jun 25 '18 at 20:02

Use the mounting holes which already exist.

Looking at a Raspberry Pi, they have four mounting holes.

For the PCB side, either glue one-dot pillars onto the mounting pads, or drill out the holes to take a Lego Technic rod. That part is easy.

You still need a fixing which lines up with the Lego dots underneath. You can get around this by using four 2x2 flat tiles. Once you've got your one-dot pillars fixed to the PCB mounting holes, put those flat tiles onto a base plate in the right places, and glue your one-dot pillars onto these tiles. Since Lego is made from ABS, model aircraft glue (polystyrene cement) works well.

If your beef about the clay is that it didn't work, try epoxy putty.

• ok i'll give this a shot – mrtunes Jun 25 '18 at 20:03
• some epoxy putty can be drilled and tapped when it is cured, you could create some epoxy mounds on your board, dill and tap them, then fasten the board on with screws. – esoterik Jun 25 '18 at 21:54