Surface mount assembly: It's all about the paste

Sjors de Wit
November 12, 2020
Last updated: Nov 17, 2020

Example of a hand-assembled board

At Jitter, we regularly do in-house PCB assembly for small prototype runs. This is a manual process, involving very tiny components, tweezers, and a lot of patience. Some components are smaller than 1x1mm, or have hundreds of pins at a 0.5mm pitch. How do we succesfully solder these tiny parts?

As you may know, SMD parts are generally not soldered directly with a soldering iron. Instead of soldering pin-by-pin, we apply solder paste to the boad with a stencil. Then the components are placed on top and finally the board is baked in an oven to melt all the solder at once.
This is a very efficient process, but especially fine-pitch chips can still be tricky. In our experience, most problems can be traced back to solder-paste issues.

Why do manual assembly anyways?

For mass production, we work with specialized manufacturing partners. They are equipped with incredibly fast robots that assemble boards fully automatic. So why bother doing it manually?

For small quantities, hand-assembly has a number of advantages:

  • Short lead times: we order bare PCBs + components, and within a week we can already build a first prototype board.
  • Flexibility: when assembling your own board, it is easy to make last-minute BOM adjustments or fix small issues
  • Learning: if we come across manufacturability issues, we can immediately adapt the design and learn from it

The right amount of solder on the right place

If you have some experience with a soldering iron, you may recognize this: too much solder and it all becomes one big blob. For SMD assembly, things are similar: you need exactly the right amount of paste at exactly the right place. Too much paste and everything shorts together.


The best way to apply solder plaste is with a stencil, which should be nicely aligned with the PCB. Misplace the stencil by just 0.2mm, and the paste is already way off: some pads are less than 0.3mm wide! If you’re lucky, you might get away with some misalignment (surface tension of the solder tends to fix small shorts). But for professional solder quality and high success rates, the alignment needs to be perfect.

For one-offs, I might just tape a stencil to the desk. It takes a bit of trial and error, but eventually you’ll get acceptable alignment.

For even better results, I have built a custom alignment tool.The tool consists of a big block of aluminium with a precisely machined grid of alignment holes. Each hole accepts 4.0mm dowel pins; these pins can effectively be positioned at any position on a 10mm grid. We design the PCB and stencil with a few holes that match this grid, leading to perfect alignment. We can even do double-sided assembly: after soldering components to the bottom side, the PCB still fits on our alignment tool.

Stencil design

The last time I had a short-circuit issue was with a very fine-pitch chip. Even with perfect alignment, the yield was still very low: over 50% of the boards had a short! In the end, the fix was to order a new stencil. Slightly smaller openings for the tricky footprint completely eliminated the problem: yield for the next batch was almost 100%.

Reducing the solder paste volume by slightly reducing stencil openings is actually a very common industry practice. As a bonus, it makes alignment of the stencil a bit more forgiving: even if you are slightly out of alignment, the paste will still end up on the solder pads.

Squeegee force

A problem I’ve had for longer that I’m willing to admit: even with perfect alignment, sometimes the solder paste print was blurry, with bits of paste shorting out fine-pitched pads. I had always assumed this was due to movement when lifting the stencil, but it turns out there is another cause: too much force on the squeegee. Don’t push too hard! Under high pressure, the paste will sneak into even the tiniest gap between the stencil and the board.

PCB Assembly: solder paste is a big factor

We have gained a lot of experience over the years, and I’ve become more and more convinced that solder paste issues are the biggest factor in succesfull production. Our strategy for PCB Assembly is simple: the solder paste must be perfect. If not, clean the board and try again. Since we have implemented this rule, almost all boards we assemble work on the first attempt. If the solder paste is good, the board will work.

Collage: our stencil tool

Want to know more?

If you have any questions about designing or assembling your own PCB, feel free to send me an e-mail at