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I want to solder like this:

mintyPi RPI Zero Soldering

And here's the image source: https://sudomod.com/mintypi-custom-parts-guide/

Basically, I want to solder something with GPIO holes onto another board without using GPIO pins headers to save space, exactly how the Raspberry Pi is soldered onto the mintyPi shown on the image provided, so I am here to ask for some tips, because it does seem like a challenging thing to do.

Does solder flux help in any way? Any other techniques or tips and tricks that I can use to improve conductivity or stability? I have never done this before I am pretty stressed out.

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This is a bit dodgy. You're depending on the solder mask for insulation which officially is a no-no, and mostly on the pads for strength. It should work most of the time though if you get the boards into intimate contact and feed some thin solder into the aligned hole pair Using a flux pen on both halves before aligning them would indeed help, and I suggest using solder thin enough to actually feed into the holes. You can't inspect the joint but you can see if both holes are filled with solder. If the other board just has pads (no holes) all you can do is keep the boards in intimate contact (don't allow a gap) during soldering and hope.

Worst case you should be able to get the assembly apart again if necessary using hot air (controlled temperature, not a paint stripper heat gun) or a reflow oven, if you have access to one.

Not sure it will work in your particular case, but for reliability, and assuming holes on both boards, I'd prefer to put a piece of polyimide (Kapton) tape on one of the boards and use thin bare wires (for example, cut-off 1N4148 diode leads) between them, then cut the wires off almost flush, but you won't get quite as flat an assembly that way and the tape and adhesive itself adds a few thousandths of an inch.

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  • \$\begingroup\$ "You're depending on the solder mask for insulation which officially is a no-no" Why is that? Surely it will be decent insulation for common low voltage, low current applications? I can understand why it would be a no-no when dealing with 230VAC and the like. \$\endgroup\$
    – Lundin
    Aug 8, 2023 at 7:25
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    \$\begingroup\$ @Lundin probably because defects are allowed in the solder mask and its easily damaged. I've seen field failures from a designer doing this (component metal part over solder mask over unrelated trace- 5V circuit). In this case it's probably okay with two layers of mask, but I thought it was worth mentioning, especially if there's something that isn't obvious going on with the boards. \$\endgroup\$ Aug 8, 2023 at 10:54
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Sure, here are some tips:

  • Does the red PCB have holes you are trying to align to? You could use some trimmed through-hole header pins to align the holes in both boards. For that matter, you could use normal header pins and just trim off the excess later.

  • Applying flux absolutely helps because it removes oxidation and helps prepare the surface for soldering.

  • Use a quality soldering iron with a good tip. If the solder doesn't flow as you apply it, the iron may not be keeping up with the thermal demand, making a job like this very difficult.

  • Find some scrap perfboard (or 'solderable breadboard') and practice first! If you're not experienced with soldering, you should definitely try with some scrap first.

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  • \$\begingroup\$ The red PCB, in this case the mintypi, does not actually have holes, I am supposed to align the Raspberry Pi Zero onto some solder pads on the red PCB \$\endgroup\$
    – Jazz Handy
    Aug 3, 2023 at 3:01
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This is a bad idea, but there are similar solutions with low (not no) profile. In-pad spring pins like this:
Harwin P70-7000045
and arrays (headers) like these:
Rectangular connectors - Spring contact | Digikey

As a one-off, thru-pads can be soldered to SMT pads, with some preparation. Set up so that you can inspect joints (preferably x-ray, but continuity at the very least), and bolt or glue the boards together to minimize stress at the joint. I would not recommend it in production.

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From what I read on the link the other board only has pads, no holes.

So the instructions call for you to solder a hole to a pad through the hole, forming something like a BGA connection. geiting enough heat and enough flux to the pad seems tricky making the whole setup somewhat janky.

I think all the needed pads are duplicated on the exposed part of the board, so a possible more reliable solution would be to run fine insulated wires from the holes to the exposed duplicate pads.

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That whole tutorial is quite a hack job. The part about attaching a crystal to two wires to create a nasty "EMI generator" is particularly fishy; I would question if these people have the slightest clue about EMC.

As seen in other pictures in that same tutorial, the end result of the soldering specifically is not pretty and a lot of the joints are probably cold. This will by no means be reliable over time, especially not if the device is regularly exposed to moist air, which one has to assume will happen.

In order to do a proper solder joint, the two metal surfaces must touch with no space between them. It is really as simple as that - you can never do a proper joint by "bridging" two surfaces with solder.

With two PCB next to each other, that is never certain, because the space between the pads on the lower board and the holes on the top board is unknown. In the worst case the solder will only wet on the hole and then just touch the pad below by chance. Oxidation or shock/vibration will break that connection sooner or later.

In order to carry out this job with some manner of reliability, you will need a hot air soldering gun, solder paste, soak the whole thing in flux and then do x-ray inspection. Basically flux & (re)heat the part over and over at a professional assembly shop until you happen to get it right, which isn't ideal at all (and expensive).

Unfortunately the problem seems to be the PCB design, it ought to have a header strip on the bottom PCB and then you could solder the pins to the holes with more confidence and easier inspection. A suitable low-profile connector like maybe a "bottom entry" socket/header might have worked, or some manner of "mezzanine" connector.

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  • \$\begingroup\$ Counterpoint: BGA packages. The difference is, a BGA package is small and fully supported; two oddly sized PCBs will inevitably have some stress between them, and leverage coming from a distance. PCB-PCB mounting isn't strictly problematic (there are LGA and castellated modules in common use, and BGAs' interposer could be considered a PCB itself), but doing it arbitrarily is a problem. \$\endgroup\$ Aug 8, 2023 at 8:51
  • \$\begingroup\$ @TimWilliams It's called BGA since it is supposed to have protruding metal objects in the form of balls and not just flat pads. And you can't really solder those reliably either without x-ray inspection. As everyone who has ever worked with BGA knows, it has always been a pretty horrible technology - one to avoid and only use as last resort. QFP is so much more reliable. \$\endgroup\$
    – Lundin
    Aug 8, 2023 at 9:05

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