Assembly of the uCurrent Gold: reflow soldering!

Bottom view of the assembled uCurrent Gold

In this post, I am going to discuss an exciting new skill that I learned this past weekend: reflow soldering. I used this technique to assemble a uCurrent Gold device. I previously talked about the uCurrent Gold by Dave Jones from EEVBlog in this post and the design improvements I added to the PCB. Some time passed and I now (finally) have all the tools and components to assemble this little device.

Reflow soldering is a soldering process that allows many components to be soldered to a PCB in one go. Contrary to using solder wire and a soldering iron, this process works by dispensing solder paste onto a PCB, placing components on the PCB and subsequently reflowing the PCB in an oven to create the mechanical and electrical connection between the PCB pads and component leads. I’ve been assembling ClockSquared Mini prototypes using hand soldering techniques and to be fair, it is quite annoying and difficult to get right. Reflow soldering makes the assembly process easier, quicker and less prone to errors.

A solder paste stencil aligned over solder pads
Aligning the gold-plated solder pads with the stencil apertures

The first step in the process is to align the SMT stencil with the PCB’s solder pads. Many prototype PCB manufacturers offer stencil manufacturing services too, so these can be ordered together with the PCBs. Once the stencil is positioned correctly, I secure it in place using tape. I noticed how the stencil contains a lot of scratching. Perhaps the factory used some sort of abrasive tool to deburr the apertures? I’m not sure if burrs are formed in the cutting process.

A PCB with solder paste applied to the gold-plated pads
Solder paste applied to the SMT pads

Solder paste is subsequently applied to the stencil. Using a squeegee, the solder paste is distributed over the stencil and finds its way into the apertures. When all apertures are filled with solder paste, the stencil is lifted from the PCB. As you can see in the image above, solder paste bridges have formed on some pads. This is no problem: later on, when the solder paste liquifies, surface tension will break the bridges.

A PCB with surface-mount components placed on it, but not yet reflowed
Components are placed onto the paste-covered PCB pads
A PCB with surface-mount components placed on it, but not yet reflowed
The shunt resistors. R1, R2 and R9 are the mA, nA and uA range shunts, respectively

This next step requires a steady hand and diligence since all SMT components must now be placed onto their correct pads in the correct orientation. I used a pair of miniature tweezers to do this. Some components are slightly misaligned, but this is again solved later by our big friend surface tension.

A reflow oven setup with a Fluke multimeter as a thermometer
Reflow oven and a fluke 87V as a thermometer
A uCurrent Gold and a temperature probe in a reflow oven
The temperature probe that is connected to the Fluke is positioned above the PCB

The actual reflow process now begins. The PCB with all SMT components assembled is carefully placed into an oven, together with a K-type temperature probe, because the €30 oven’s thermostat can obviously not be trusted. The probe is just flapping in the breeze slightly above the PCB. In front of me is a piece of paper that shows the recommended reflow temperature over time. I simply follow this trajectory using a stopwatch, the Fluke and constant adjustments of the temperature control on the oven. When peak temperature is reached, I open the oven door to quickly ramp down the temperature. No advanced oven modifications necessary! I’ll probably automate this process in the future since it is quite straightforward to implement and widely documented by other DIY reflowers who have gone before me.

Close-up of an assembled and reflowed PCB
PCB after taking it out of the reflow oven

The picture above is the result of my first PCB reflow cycle and I’m super happy with the results. The solder joints look good, there are no bridged pads, and the components seem to have endured the heat quite well. We continue!

Through-hole solder joints for a sliding switch
Soldering the through-hole components

The reflow process of course only took care of the SMT components, but this device contains some through-hole components, too. In particular, the switches must be soldered and these come with some pretty beefy pins. No worries, these are soldered to the PCB using the regular soldering iron.

Top view of the assembled uCurrent Gold
Fully assembled top view
Bottom view of the assembled uCurrent Gold
Fully assembled bottom view

Lastly, the gold-plated binding posts are assembled. These just use a bolt-nut connection. I’m really happy with the quality of the end product.

A Fluke multimeter connected to a uCurrent Gold device for functional verification
Functional verification using a test current

Does it work, though? It turns out that it does. I set up a test current using a fixed-voltage source and a known resistance. The image above shows the test on the uA range, and the multimeter reads the expected value. Perfect! Another great tool in my toolbox, and a new skill picked up along the way.

You can find the Mouser BOM here. I used the original BOM as a guideline. Though not all parts are exactly the same ones as mentioned in the original BOM, almost all parts that I use have identical specifications. The only difference is that R9 has a tolerance of 0.1% instead of 0.05%. As always, feel free to leave a comment if you have any questions about this project.

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