Applying Solder Paste for PCB Prototyping

During prototyping, I have the habit of assembling a few PCBs myself. This has several benefits, such as:

• it's possible to only place part of the components, for easier, step-by-step, testing
• when ordering components through an automated process from your BOM, it's a great end-to-end test of this BOM.
• design for manufacturability : If I have to assemble the prototypes myself, it forces me to think about that the during the design.

I order the PCBs from Aisler.net with a stencil for applying the solder paste. However this was always the most tricky part of the process:

• you need a perfect alignment of the stencil with the PCB
• the stencil needs to be perfectly flat on the PCB during application of the solder paste. If not, the solder paste spreads out and may bridge several pads...

Euro-Circuits has a tool for this, but at 1500 euro, I find it a bit too expensive, given that I don't need this daily.

So I used to 3d-print a jig which holds the PCB and tape the stencil to it.

Then I found this tool on AliExpress.

At ~75 euro, it is really affordable, and so I decided to give it a try.

After inspection of the tool, I noticed a few shortcomings:

1. the stencil was not perfectly flat on the PCB. In fact this should not come as a surprise as there are different types of PCB materials, with different thickness.
2. the stencil was not flat, because the screws fixing it push directly on the thin stencil
3. several different tools are needed (screwdriver, allen keys, ..) to use and adjust the setup, which is cumbersome during setup and alignment of stencil and PCB.

But these were all fairly easy to correct with a few 3d-printed parts, and by replacing some screws with other parts.

Getting the stencil perfectly flat on the PCB

The tool comes with some parts to fix the PCB on it. But when I used them, I noticed still about 1mm of space between the PCB and the stencil. So I reverse engineered this part in Fusion360 and then 3D-printed an improved version. I've made the design flexible, so I can mount different types of PCBs on it:

• holding it in the 1.8mm routed slots
• holding it in M2, M3 or larget mounting holes,
• holding it by the corners

An example is shown below with the parts printed in Prusa Orange. The source files are available on printables.com.

Getting the stencil flat in the tool

When fixing the stencil (usually only 0.1mm thick metal sheet) with the provided screws, it pinches the stencil. But also here it was possible to design an extra wedge to insert above the stencil

Manually adjusting it all.

I replace all screws with equivalent bolts which can be adjusted by hand. This allows to quickly adjust the position of the PCB, the tension on the stencil etc, without needing screwdrivers or allen keys.

The proof of the pudding is in the eating.

One of the PCBs I was prototyping has a Broadcom ADPS-9253 RGB sensor on it, and it has really small pads : 0.25 mm * 0.3 mm. In the picture below you see this footprint below a 0603 capacitor footprint.. I think the application of the solder paste is perfect. Another great tool in my toolbox!

Happy Prototyping!