Caliper Capacitor:
  1. Purpose
  2. Pieces
  3. Soldering

2. Soldering a Capacitor Compatible with a Button Cell

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Soldering the metal ring to the PCB is much more difficult than it would seem at first glance. With the capacitor in place, there isn’t much room to get the soldering tip in. And, a lot of heat is required to get the metal ring up to a temperature where the solder will melt on it.

But, hardest of all, the metal ring needs to be aligned flat and centered directly over the PCB the entire time you’re attempting to solder it in place!

Fortunately, with a little bit of planning and pre-work, a soldering fixture can make this easy.

A wood fixture with a PCB plank holds a difficult-to-solder workpiece in place.

A wood fixture with a PCB plank holds a difficult-to-solder workpiece in place.

Most of the time, I make fixtures out of metal or plastic. But, for soldering, metal would wick away precious heat and plastic would melt. Professionals use more exotic material like graphic or ceramic for soldering fixtures and jigs. But, I didn’t have either of those materials.

Since wood is combustible, normally you wouldn’t think of it as a good material for making soldering fixtures. But, it is cheap, readily-available, doesn’t draw heat away from the workpiece, is easy to machine, and requires quite a bit of heat to actually ignite. Frankly, it takes a lot of effort to even get the wood to char with an electronics-compatible soldering iron (as opposed to a more powerful soldering gun).

I drilled a center hole into the wood with a #4 drill (almost 0.21 inches in diameter). Don’t drill the hole too deeply. You want the top of the ring to stick out so that it can be held firmly flat against the PCB during soldering.

I then drilled holes on the left and right for tiny #0-80 screws. A leftover scrap of PCB material (FR-4) was drilled to hold the copper ring flatly against the PCB during soldering.

Pre-tin the surface to be soldered and then apply some soldering flux.

Pre-tin the surface to be soldered and then apply some soldering flux.

There is a secret to getting a piece of metal to stick to a PCB: clean the metal, “pre-tin” the PCB, and use plenty of flux.

  1. Before placing the ring on the PCB, the capacitor must be already be soldered in place.
  2. Wash the metal ring with soap and water to remove oils and dirt. Let it dry.
  3. The edges of the PCB (where the ring will go) need to be “pre-tinned”. This means heating the edges of the PCB with the soldering iron and applying a thin coating of solder all the way around. This ensures that solder will make contact with the entire underside of the metal ring when everything is re-heated.
  4. Lastly, use plenty of soldering flux around and between the ring and the PCB. During soldering, the flux with remove the oxides from the metal surfaces, allowing for a solid connection between the ring, the solder, and the PCB.

Since everything is prepped and held in place, the only things awkward about the actual soldering are fitting the solder and tip into the limited space, and patiently waiting for the ring to heat up.

Soldering a couple of different locations seemed to be all that was necessary to get each region the ring to melt nicely in place.

A hole in the back of the fixture allows a screw to quickly eject the workpiece for fast cooling on a metal plate.

A hole in the back of the fixture allows a screw to quickly eject the workpiece for cool-down on a metal plate.

The big concern about soldering a relatively large piece of metal to a PCB is that the delicate electronic components may become damaged from too much heat. To avoid this, one normally solders metal items first, or screws them in place with fasteners instead of using solder. Neither solution was possible on this particular project.

At the very least, damage to the capacitor may be avoided by getting the whole thing cooled down as soon as soldering is complete. To do so, a hole was made in the back of the fixture. A screw inserted into the hole ejects the PCB and ring onto a metal plate (scrap piece of aluminum).

Ambient air cooling on a metal plate avoids trying to pull the thing out of the fixture by the edges of the metal ring, potentially leading to ring separation or burnt fingers. This also avoids an attempt at artificial cooling (via a liquid or air canister) that might lead to cracking of the capacitor or soldered bonds due to rapid temperature cycling.

After the capacitor holder has cooled, check the capacitance using a multimeter. If the value is different than expected, the capacitor came loose or became damaged during soldering. (Before I came up with the soldering fixture, my prior attempts to do it by hand resulted in a wrecked capacitor.)

Final Steps

The final steps are to clean up the outer dimensions to meet the specifications or device’s battery compartment. The bottom edges can be rounded using a grinding tool on a Dremel, or even better, in a drill chuck on a lathe with a specially-rounded cutting tool. The final height can be faced on a lathe or cut flat on a milling machine.

So far, by using capacitors in the battery compartments, I’ve not had any more problems with any of my digital calipers attached to external power.