THT soldering fixture from 3D model

Ever since I started using 3D printing for PCB fixtures and jigs, I’ve imagined an auto-generated soldering fixture for through-hole parts. In my mind it would “simply” be a matter of generating the model for the assembled PCB, subtracting that from a block and voila, you have an inverse of the assembly to hold all the THT parts upside down while they’re soldered to the underside of the PCB. Wrap the magic in a little KiCad plugin and we’re done.

Well I had occasion to give it a go. I have a PCB with top-side components only and a good mixture of SMD and THT.

I didn’t find much prior art, though I struggled to come up with good search terms. This is the best I found: Creating 3D model of a PCB holder for soldering

After some experimenting with the STEP format and exploring some options in FreeCAD, I imported the model into Fusion360 and tried out the plan:

A block and an inverted PCB. Now subtract.

Ah… I got exactly what I asked for, which is not nearly what I wanted. All my lovely models of course, have accurate internal voids, which don’t get subtracted. What I’d really like to do is press the model into the block and subtract anything that touches. Again, my search for solutions to this task came up empty, so I tried manually lowering the model 0.5mm at a time into the block and repeatedly subtracting. But my models were too clever for that, with lots of features finer than 0.5mm that get left behind.

And still, this perfect inverse concept has other issues - I need some wiggle room around each component to accommodate tolerances, and if just one component model is wrong I run the risk of the whole fixture not fitting because it fouls on one feature.

So I dialled my aspirations right back and started with a MVP. The job of THT soldering gets a lot easier if each component is simply held up against the PCB. Alignment is still an issue, but at least the component will sit flat against the PCB.

I hid the PCB itself, manually drew rectangles around each of the same-height THT components, and then pulled each of them up to kiss the closest face of the component.

I’m a long way from my auto-generated, component cradling nirvana, but at least from here I can experiment, demonstrate and find folks who know better!

Speaking of, I realise now thinking of this as a KiCad thing might have been limiting. I really value the community here, but if there’s a more appropriate forum I’m all ears.


Before starting, the question always arises what number of such boards do you need to collect? As far as I see on your board very long distances between the inputs and outputs, which allows the socket to move to the left and right… With serial production, this is not permissible and most likely will have to be redone… There is a large number of nests of different shapes with different fixation, another solution is to place the nests closely in one row…

I recently collected 10 boards with a similar configuration, it took me one working day (6 hours) without equipment

For through hole soldering, I’ve often used a piece of 25+mm foam plastic. Conforms to most shapes.


In the previous post that you linked there is this post by maui (the creator of the KiCad StepUp Workbench for FreeCAD)

I did a small test with one of the KiCad’s sample projects

Normal KSU import:

Using the bounding boxes options (ksu Tools->Edit preferences->kicadStepUpGui->Bounding Boxes set to ‘ALL’):

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Ah yes. My interest in PCB population techniques is firmly in the low volume production space. For one or two you can make do with just about any cumbersome process. For 50+ you give it to the factory to wave solder or selective solder and be done with it. I’m always interested in the 5 to 49 range. I think it’s a very interesting place to play, because for 5 off you need a process that is efficient to initiate and for 49 you need a process that is efficient to execute. Getting both right is the sweet spot.

I really like this. Means you can line everything up the way you want, then hold it all in place while you solder. Plus you can tune as you go. I guess with a few rubber bands you could allay concerns about it springing apart while you work. Definitely on the fabrication end of the fabrication vs production spectrum since it relies on the handiwork of the operator rather than dictating the outcome (to be fair, my MVP is no better), but it also scores really well on the effort vs reward balance!

That’s lovely. I’d dismissed the bounding box approach because it wont work well if the top face of the component is not flat. So the “impression” or playdough idea was the mental model I was running with. But this is great food for thought.

I’ve considered doing this, but doesn’t the soldering melt a bit of the foam, leaving a bit on the THT leads? Maybe I need some silicone foam, if such a thing exists.

I think the foam is placed in the component side to keep the components in place by “pushing” them.

Yes, but heat is transmitted by the component leads and makes the component hot to touch so would melt foam.

Here is a picture of what I used for ages.
A soldering frame, able to hold two rows of pcb.
First you place the empty pcbs in the fixture. Then populate lower parts. Next place the lid with the foam and lock the lid.
Now flip the pcb-frame alltogehter with the lid.
The base stays in place meanwhile. Solder.
Flip back, remove lid, populate higher parts. Repeat till youre done.
And here‘s what is mindblowing:
Use the same gear for your next, different project!


Your concernes are right:
You need to replace the the foam every 5-10 years.


I’m not worried about the foam. Foam is cheap. I’m concerned bits might melt and be left on the component.

For the foam solutions, how do go controlling for the slack in the through holes? I’m often surprised how easy it is for parts to look askew without a guide, particularly if they’re next to each other.

I guess this orientation is easy enough to control for:


But what about this one:


Or perhaps more commonly, this scenario (excuse the protoboard, it’s the best photo I could find)?

il_570xN.3742539220_8cjp copy

Not often, and usually only longish THT resistor leads that I have dwelt too long with the iron. When cold, plastic easily scratches off with a fingernail.

The image of the boards posted by @m852 seems as though they could be stuff and soldered in one hit, but I’d probably leave the green connectors to do separately. Likewise the images by @Heath_Raftery .
Critical stuff like those connectors would also be done last.

Make a tack at one point then flip the board and do the whole row… All devices that are do not give you information in what position to be a narrow component… You don’t see him… If we are talking about different height of components and width, then faster than sticking in one place you will not make and holes cannot be made with tension there should be delta… Ultimately your speed depends on your experience there are several soldering techniques that will help you a lot


If you need to keep stopping to let the Iron heat catch up, then the Tip is too fine, the Temp. is too low or the solder is too large a diameter for the pad size.

The concept of making a Mold can be used to make a PCB Holding Fixture. I’ve made several Molds and extract the process for Posting this…

The Details of various Tweaks/Process somewhat depends on the Models but, in general, it’s made a bit easier using StepUp’s BoundBox Tool.

Once the experience is somewhat dialed-in, the next few steps can be a few click’s away.

I did Not bother to:
• Fuss with eliminating/delete/fixing the Obvious Need-to-Delete items but, you get the Idea…
• Expand (Scale) the PCB/Parts as would be needed to avoid interference fits (usually 2 or 3 Percent of original size nor did I consider Taper/Drafting. (I definitely do that for Molds…)

• Load the PCB/Parts
• Create a Base Part to contain the above
• Boolean Subtract (the Order of Selection is important)
• Create an aperture to expose the guts (or slice the base…)

Add Features to the Base as needed for a good tool/fixture…

Some screenshots…

I have one that a previous employer gave me that is for industrial/production use that is very nice. I believe the intent is to merely hole the most difficult and/or heaviest components until tacked, check the alignment/placement and then complete the soldering. It’s pretty much the way I’ve always done boards anyway whether it was rubber bands or any other method except for a breadboard. I mean I like your idea of the custom 3D printed alignment tool but I think anything will have advantages and disadvantages and a generic tool/method might be better overall in the long run.

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I was going to try the same method to cut rectangles in kapton tape using a laser. I gave up. I may revive the project now.

I only used such a soldering frame two times. But, if the foam melts away, it won’t the next time. :slight_smile:
You could try permanent backing paper (PTFE foil) It withstands 260 °C. Cover the foam with it.
If “backing paper” (translated from German) makes no sense to you, ask your wife what type of paper she used to bake the cacke you got yesterday. It is used to lay out the baking form, so the dough doesn’t stick to the form.
You might need some cutouts.


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