Senior Design Part 12: Solder Reflow Sabotage!

2023-07-14 | By Will Siffer

License: Attribution Non-commercial Solder / Desoldering

Solder, the Culprit behind this week's crime!

This week, I thought I wouldn't have much to write about; having a design review next week and our documentation due this week made for a fairly slow and uneventful start to the week. Suddenly, on Wednesday, my project manager came to me with an absolutely stunning issue: The 5V regulator was outputting 3.5V!

Now, if you are reading this, you are probably an EE nerd (just like me) and know that a functioning regulator won't "output" 3.5V. If you have been following this series, then you may remember that neither the V1 nor the V2 boards had issues with the 5V regulator! Yet here we are sitting on the lab bench with a 5V system that has 3.5V on the 5V line.

How is this possible then?

I am so glad you asked! To best break down how this could happen, we must first consider an "ideal system". Below is how the system is supposed to work, with a 5V source turning into a 3.3V source supporting various components. The ICs are represented as resistors in this case for simplicity since we are just looking at the loads on the power rail.

Simplified Power Schematic

As you can see, there is a "connection" to the 3.3 system through the 5V system, but only in that the 5V system creates the power for the 3.3 system. Unless there is an unintentional connection, there is no way to have 3.3V on the 5V line. There is no way to have 3.3V on the 5V rail! Meaning whatever connection is being made must overwhelm the power capacity of the 5V regulator; thus, the search for the short begins!

RAK Module Soldering: The problems all come back

We love using the RAK module with our project, but something that is proving to be quite difficult is the soldering of such a small module. Unfortunately, at the time we were soldering, the EPICS lab was not yet equipped with a hot plate, so we were soldering our RAK with hand-applied solder paste and a hot air gun. This almost always made us apply way too much solder paste and created solder bridges under the module. The proper way to do this kind of solder work would be either by hand or with a stencil and a hot plate. 

Since we had the hot plate available for this week, I decided to be the first to use it in order to desolder the RAK with careful even heat. Sure enough, there was A LOT of solder under that module! Take a look at all of the extra solder on the pads!

Now it's pretty clear we have a huge issue here. But something else I found interesting was the solder I found on the PCB itself and how some solder gathered on the pads of the tiny vias as well!

Well, it looks like we've caught the culprit of this crime! After cleaning up the solder pads and the RAK module, I was able to resolder everything by hand very carefully, and the 5V line was back to a clean 5V! Needless to say, we have learned the importance of using the right tool for the job, and sometimes it really is best to leave some of the harder work to the professionals if you have the ability to. 

With that said, that's it for this week's blog, and I think it is safe to say that as we near the end of the semester, we have learned a lot. Until next time, my name is Will Siffer, and as the Digi-Key ambassador for Purdue University, I hope you learn something new today.

Recommended Reading

• Senior Design Capstone Part 1: What (or Who?) is LoRa?
• Senior Design Part 2: Using Eagle to make a custom PCB
• Senior Design Part 3: Designing a SMD PCB full of sensors
• Senior Design Part 4: Assembling, Testing, and Debugging Circuit Boards!
• Senior Design Part 5: Diagnosing PCB Issues
• Senior Design Part 6: Protecting our Power!
• Senior Design Part 7: Compressing data with Magic? (It's just math!)
• Senior Design Part 8: Squishing Bugs and Finding More
• Senior Design Part 9: Can adding a delay be a good thing?
• Senior Design Part 10: Making Sure the Project is Ready to Pass On
• Senior Design Part 11: Strengthening Integration with Testing

"This week, I was happy to apply some of what I learned throughout the semester to my work on my senior design project. As far as this project has been concerned, it was really all about integration, and it led to most of my challenges being on the packaging side, not the EE side. When we found the issue with the power supply, I had to use my knowledge of circuits to diagnose where the issue could be from before any action was made; isolating systems and comparing with working versions to narrow down where the issue is originating from. This work is something I hope to continue with my new job coming up since I will be faced with new challenges every day. As long as I keep up this habit of lifelong learning, I believe I will continue to be passionate about my job."

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