I decided to design a PCB heating pad to save some soldering time. I don’t have the space for a reflow oven and I thought it would be easier than my current hot-air setup. Miniature soldering hot-plates seem too expensive so I figured I could make one for myself. For this project, I used Altium Designer as its interactive length tool makes designing the traces easier. In this article we will walk through designing the board schematic and testing it as a hot-plate soldering pad.
Designing the Board
We aren’t using any components for this prototype, so don’t need to add a new schematic, all we need to add is a PCB layout file.
Select the “Mechanical 1” layer to draw the board outline.
Now we can use the “Rectangle” Tool to draw our business-card size board outline.
As the business cards are 81x59mm we can use the rectangle properties to create the desired size.
Then let’s switch bak to the top copper layer and set the line width for the traces to be 1mm.
First I will place 2 rectangular copper pours on the left-hand side of the card for the power and ground output.
Next we draw a line between the two copper pours.
Now we can click on the interactive length tuning tool and drag along the line that I just placed.
It gives us a box with an accordion pattern inside of it.
If we extend this box out to the ends of the board, we get our accordion pattern element trace. Hopefully this will give us enough resistance to create a high-enough temperature without damaging the traces or the board itself.
One issue that popped up was I had to move the trace at the bottom as there wasn’t enough clearance from the bottom edge.
There also was an artifact that I noticed at the top of the board that was created when I expanded the accordion trace to the top edge, past the horizontal part of my initial trace, but It ended up not being an issue.
Now w select the top solder mask so that the top layer traces can be protected by the solder mask.
Put a fill over the pads that you would like to keep exposed and all of the other traces will be covered with the soldermask.
Then we can add some text to the top silkscreen with the “Top Overlay” layer. We edit the text in a textbox and choose the text height from the properties tab.
Here is the final design as seen in Altium. We can see all of our layers, the green being the copper traces, the red is the power and ground pours, the yellow is the top silkscreen, the purple outline is the board dimensions.
Sending the Design Out
One of the first things that we have to do before sending this board out to production is to check the design rules and make sure that our board is manufacturable. When I run the design rules, the only rules that are flagged are the width constraints, which is expected as I changed the width to 1mm, so we’re good to go!
Now, under the file menu export the design as a Gerber file from ‘Fabrication Outputs’
In the Gerber setup select the units as mm.
As we are only making a 1 layer board, we can select the mechanical layer for the board dimensions, the top layer for the copper traces, the solder mask layer to cover the copper traces, and the overlay layer for the silkscreen
When we go into the “Project Outputs for __ ” folder in your project directory you can see all of the manufacturing files that were generated. Select all of the CAMtastic files and sent them to a zip folder.
Then we upload the zip folder to the JLCPCB website. Here we select the board dimensions, the board type, and the number of layers. We can also see an image of what the completed board will look like in the top of the window. These come from the gerber viewer that is built into the website.
the grand total for the 5 boards and 8-12 day shipping is under $4. 70c per board is not bad at all for saving me a few minutes on my soldering.
Testing
Two weeks later, the boards arrived and I was able to test them out. I used some machine screws in each corner to act as some spacers to keep the board off of the silicone mat. Then I used some Kapton tape and a thermocouple to measure the temperature of the surface of the boards with my multimeter.
I was able to get it up to 200C which was enough for my low-temp solder paste and I used the boards as a simple hot plate to desolder an MSP430 chip. I was careful with the current as the sources that I could find lists the maximum current for a 1mm trace as between 2 and 3 amps, but the soldermask melting temperature is much more than the 200C that I was shooting for so I wasn’t to worried about melting anything. It turned out that setting the power supply to 10V and 3.5A was enough to do the trick.
Finally it was ready to test the soldering. Here is some shaky video of desoldering an MSP430 chip that has a solder bridge filmed under the microscope.
I also then used it to solder a small SMD LED onto a copper-clad blank. I scratched off the copper with a carbide scribe tool to break the connections between the different nodes in the circuit, add the solder paste and components, then soldered it with the hot plate.
Conclusion
Overall this simple design worked exactly as expected. I’m not sure how many heating cycles this will be able to take, but I have 5 of them so they should be able to last a while. To make this design a bit smarter, we could record the time response of the heating pad to make a simple model and see if it has a fast enough thermal response to perform a reflow profile. I hope that this helps give people ideas for some of their own designs!