Manual Assembly of Modern PCBs Using Low-Cost Tools

TL;DR

I describe the process of assembling a PCB with some somewhat challenging components using only low-cost tools, like a ~10 USD hotplate. The result isn’t perfect, but it’s still very good for a manually assembled board.

Introduction

I think that in electronics, microsoldering has become a necessary skill for professionals and aspiring hobbyists alike. On a professional level, you should be able to make small modifications to a prototype board yourself. As a hobbyist, you need this skill in order to use modern components on your mostly manually assembled PCBs.

I recently worked a lot with the RP2040 and the RP2350B, which are well known to many people as the Pi Pico and Pi Pico 2, respectively. Maybe you’ve considered a project using one of these chips but changed your mind after discovering that they are only available in a QFN package.

With this post, I’m documenting my assembly process to show that soldering these types of components isn’t as hard as you might think, as long as you’re using the right tools. To make this even more accessible, I limited myself to basic equipment and more affordable consumables. The result may not be perfect, but it is far better than just “working.”

Board and equipment

As an example, I’m using one of my recent prototypes for the Open DMG Display, along with the corresponding stencil. The tools are mostly available through sites like AliExpress, eBay, etc.

• USB-C Mini Hotplate – 12.99 EUR incl. VAT (Link)
  • These mini hotplates are really cheap compared to a hot air station. When it comes to board assembly, I would absolutely choose one of these over any hot air station for assembling new boards.
• Low Temperature Sn42Bi58 Solder Paste OL-3016 – 4.07 EUR incl. VAT (Link)
  • This paste is a bit strange. It’s more like a fluid than a paste, but it is still possible to work with.
• Gel Flux OT-338 – 6.39 EUR incl. VAT (Link)
  • Surprisingly good flux.
• Pinecil – 25.99 USD (Link)
  • I have this around for field use and small modifications on the desk.
• TS-BC2 TS100 Iron Tip – ~10 USD
  • The Pinecil comes with a conical tip. Do yourself a favor and get a chisel or beveled tip.
  • Avoid using the really small ones. It might sound counterintuitive, but it’s easier to use a decent tip size.
• KELLYSHUN Solder Wick (came with the paste)
• 0.5mm Solder Wire with High Sn (e.g., Sn99.3Cu0.7)
  • I used my usual high-quality solder wire here, but I don’t think this counts as cheating.
• Bent Tweezers – 1.89 EUR incl. VAT (Link)
  • I also use those as a daily driver
• Metal Spatula or Old Credit Card

Besides the soldering equipment, you will need some sort of magnification. I would not recommend magnifying glasses. If you don’t have a lot of money to spend, use a USB camera with some clearance for soldering. I had one from Andonstar, which did a good job: Link. If you want to invest more, buy a proper stereo microscope.

I expect you to have a suitable USB-C power supply for the hotplate and the Pinecil.

The process

I’m using a hotplate process. In my opinion, this is a much more controlled and much easier process compared to using a cheap hot air station. You don’t have to worry about larger ground planes or uneven heating.

I started with the preparation of the PCB. I like to use other boards of the same height to build a temporary frame that holds the board in place. I used Kapton tape for this, but any type of tape works fine. Before aligning the stencil, I give the board a good wipe with some IPA.

The alignment of the stencil is crucial but doesn’t have to be within µm. Make sure the pads are aligned not only locally but also in other places on the board. Check diagonally at the upper left and the lower right. If your board has fiducials, use those.

After the alignment, tape the stencil at the upper corner to create a hinge. I forgot to take a picture of this, but you can see it in the picture with the solder paste.

Apply the perfect amount of solder paste above the area you want to assemble ;) In this example, I only want to assemble a part of the board that has the critical components that can’t be soldered with an iron. You also have to take into account that the space on the hotplate is limited.

As I mentioned, the solder paste I used was quite liquid. I initially took multiple strokes with the spatula, which resulted in a horrible outcome. The paste just spread under the stencil, and the board was completely covered.

If this happens to you, don’t worry. Clean everything up with some IPA and try again. On my second attempt, I only used one stroke, which worked quite well.

The result isn’t perfect, but you can clearly see the pad shapes even on the small-pitch parts. You shouldn’t worry if there’s a bit too much paste in those places. The paste will distribute itself as long as it’s roughly in the right place.

It is now time to start the assembly process. I recommend getting everything in place before you start assembling.

First, place the parts you can’t solder with an iron. I started with the QFN80 here.

I dropped it a bit misaligned and smeared some of the paste around. Normally, this is not a problem. I corrected the alignment and placed the oscillator, the flash, and the 45-pin FFC connector, followed by the other parts whose pads were covered with paste.

Now the board is ready for soldering using the hotplate. With the low-temperature paste I used, the following temperature profile was applied:

• 60s at 110°C
• 150s at 170°C
• 100s at 220°C
• Switch off for cooldown

Unfortunately, this needs to be done manually with the hotplate. That’s one of the reasons you might consider something like the MHP50 if you do this type of work more often.

I really like watching this process. The solder paste starts to melt, and shortly after, everything aligns itself. If something doesn’t align properly, you can use the tweezers to gently wiggle it until it sits perfectly.

As expected, my result wasn’t perfect but absolutely fine for a hand-assembled board. The only thing that was expected was a lot of bridging on the QFN80 package.

This is not a problem at all. Just use some flux and drag your iron along the sides of the IC.

This is how it looks after some cleanup with IPA.

The only thing left to do is soldering the components that haven’t been covered with the solder paste. Again, you should carefully decide which components you’d like to solder with the hotplate, as the area is limited to the size of the heat bed. The used hotplate allows for a 55mm x 55mm area.

Result and conclusion

The result looks OK overall. It works fine, and I’ve seen much worse. There are some misaligned passives, and a few pads had a little too much paste. I was surprised by the cheap flux—it really did a great job. The hotplate also worked much better than I expected. The solder paste was a bit strange and caused some bridging due to its liquid nature (and me dropping components). Working with the Pinecil feels a bit unusual, to be honest. I missed the round shape of my other soldering iron, especially with the beveled tip.

Overall, there is nothing wrong with the tools and consumables. It’s quite easy to hand assemble modern components with low-cost equipment. Just give it a try.