Heat Inserts on PCBs

The LEGOS project at the institute contains a number of PCBs that need to be mounted to some 3D-printed parts, and the PCB always forms the bottom of the assembly.

Sure, this is an easy task to solve with mounting holes, but then you’d have the screw head or a nut sticking out the bottom, ruining the slick design and maybe providing a tempting piece for users to fiddle with. Also, one of the entities in this project contains 8 M4 threaded rods that need to be mounted on top of the PCB.

For this purpose, we have so far used Würth SMD screw terminals. These are basically some round pieces of metal with a screw terminal, that can be soldered onto a PCB and provide a way to screw something to the PCB. I believe, they are actually intended as an attachment point for some high amperage wires with ring terminals, but they serve our purpose as well. So far so good, but there is a drawback with this approach: The terminals are fucking expensive! Seriously, they cost 2.50 € per piece, so 18 € total for a PCB which otherwise contains fairly cheap components.

Heat inserts to the rescue.

I don’t exactly recall the genesis, but a friend of mine and I had an idea on how to reduce these costs down to a few cents: Simply use brass heat-inserts instead. Heat-inserts not only occupy one of the top-places on my whish-I’d-known-this-earlier list when used as intended, but because they’re made of brass they can be soldered without any issues. Oh, and they cost basically nothing - you can get them for ~0.05€ per piece on AliExpress.

Heat inserts are super cheap and can be soldered easily

Evaluation

I had some scrap PCBs lying around, and coincidentally also one with a plated hole that was the perfect footprint for a M4 insert. So as you can see, we have tried that part, a piece of prototype board, and a normal PCB with a heat sink pad for a microcontroller.

The test pieces

So just add some solder paste, put it on a heat plate and -voilà- that worked pretty well. But does it hold?

The soldered heat-inserts hold surprisingly well.

So, that works surprisingly well. A little thrilled, we tried to push our luck even further:

The soldered heat-inserts carrying a water bottle of about 700 grams.

We were completely blown away on how well this worked. And this was just a plain copper pad on the PCB. The insert that was sunk in the hole worked equally well (I forgot to take a photo). With enough force, we eventually broke off the insert, but this is definitely strong enough any normal applications. BTW, the prototype board did delaminate before the insert snapped off:

The prototype board delaminated before the heat insert snapped off

Summary

So TLDR: Heat-inserts are a cheap and solid way of attaching screws to a PCB. I’d be curious to hear if you can use this technique in a project, feel free to drop me a mail if you do! 😀