Circuit boards are just as vulnerable as any part of an HVAC system. They can fail for any number of reasons over time. Everything dies eventually. Now, the question is, you can repair just about anything with enough work. You can remake the windings in a shorted motor or alternator with enough patience. If we throw money, time, and talent at a circuit board, how do we repair it?
The contents of this post are for informational purposes only. We want to highlight the work that goes into repair. Have a professional repair your circuit boards. We are not responsible for any consequences of you doing your own repairs.
Some Basic Concepts
All the parts on a circuit board are interconnected in a very specific way. Connecting them any other well will make them short out and do nothing useful. For example, imagine we have a little computer chip. It has 8 little legs, called Pins. The first pin on this chip will turn a screen on and off. The next leg will control a relay for the furnace. The next two are for a button. There’s two more for a temperature sensor. Lastly, two pins are for power and ground.
We need to wire this chip into all these components somehow. On the one hand, we could plug in wires or solder wires between everything, but that will quickly become a rat’s nest of chaos. Just soldering components together like that is called Dead Bugging (it can look like a dead bug), and it’s a little less than professional or even safe most of the time.
This is where the circuit board comes in. A circuit board is essentially a piece of plastic, with copper traces on it. These copper traces act like wires. We can connect components together by attaching them to these copper traces. An engineer will have looked at all the parts, worked out where to put them on the board, and designed a way for all of them to be connected together and function.
When we build a board, we essentially put the parts on it and use solder to physically and electrically connect the components together. If we can put a part on the board, then it must also be possible to take it off the board, if we’re very careful.
Step 1: Diagnosis
In principal we’re going to find the bad part, take if off and put a good part in its place. The first obstacle however is figuring out what broke. In a simple board, such as those used in a washing machine, maybe there’s 20-100 little components and just knowing what’s wrong with the washer will tell you what you need to change. On more sophisticated equipment however, such as a laptop, you have tens of thousands of tiny parts to keep an eye on. That is: Good Luck.
Repairs on the more complicated scale require a process of testing, measuring, and elimination. Let’s consider a thermostat. The screen doesn’t work. It flickers up random nonsense and it’s essentially useless, but it does still run the heat. Pushing the buttons causes it to adjust the set point for the temperature and sometimes causes the screen to flicker more wildly.
We open the thermostat up, and we have a few things we need to check. Is the cable for the screen broken or damaged. Is the chip that drives the screen dead. And is everything between the screen and that chip working right? The screen could be dead too, but first we’re going to test everything first, because it’s not easy to test the screen aside from outright replacing it. And certain failures on the board could kill the new screen.
A simple volt-meter is all it takes to test the cable. You do a continuity test. We use the sensing probes and see what pins on the cable are connected to each other. The volt meter will beep if 2 pins are connected. You just compare both sides of the cable and if you find a trace that’s connected to more than one output or that isn’t connected at all, you’ve found your culprit.
There are other components involved such as capacitors and resistors. We can measure these with volt meter, but often times if we suspect they’ve failed, it takes just minutes to replace them and see if anything has changed.
The display driver chip though, that’s more complicated. We’ll need documentation about how it works just to get started. And we’ll need an oscilloscope, which will show us the wave form of the chip’s output. If the chip is outputting signals that don’t match the documentation and the input for the screen, then the chip needs to be replaced.
If nothing else is amiss, then it’s probably the screen that needs to be replaced.
Step 2: Extraction
Replacing the screen is probably just a matter of unplugging it’s cable, so that’s a no-brainer. Replacing anything else however, depends on how the board’s built. There are multiple styles of board construction, the current leading type being Surface Mounting. We’ll focus on that, but you can google how to work on a through-hole board or other designs.
The idea with surface mount boards is that we can have machines do most of the work during manufacturing. We make the board, put solder on all the contact pads for the components, put all the parts on those pads, and bake the board in a special oven (note: NEVER bake a circuit board in an oven used for food, you’ll expose all the future food in that oven to toxic chemicals that will kill you in a slow and painful death). The oven will heat the solder to the point that it melts, the components fall into the liquid solder, and are then attached to the board. It saves time, space, and money compared to older methods.
To work on these boards, you’ll need a few tools:
- Soldering Iron
- Hot Air Gun/Station
- Microscope (these parts are TINY)
- Solder Braid/Solder Wick
- Heat Shielding
- A ventilated area, solder can contain Lead.
Suppose we’re going to remove that chip with 8 legs from earlier. We need to first put some shielding material on the board so we don’t accidentally remove or even melt another component. We’ll cut out a square in our shield over the area we’re working on so that region will still get hot. Make sure to note the chip’s orientation. The new part must go on in exactly the same direction.
We’d set the hot air gun to be hot enough to melt the solder. This temperature can vary by solder formulation. It’s essential to slowly heat the component up, don’t thrust the hot air gun into the board, hold it a fair distance away and circle around the chip. Use tweezers to see if the part has come lose.
Eventually, the chip will pop up from the gentlest of prods and you can remove it from the board. Afterwards, you’ll use some solder wick and the soldering iron to remove the old solder from the board. It’s best to start with fresh, new solder. Just press the wick on the pads and apply heat with the soldering iron, working the wick around gently.
Once the pads are clean it’s time to put down new solder. You can hold the iron to the pad and hold solder down, or some tend to put a bit of solder on the iron, then apply it to the pad. Make sure no pads are connected by solder (they’ll be shorted). You can use Flux to make it easier to apply the solder. It will want to draw itself to the pads.
Almost done. Put down your new chip, making sure it matches the orientation of the old one. Make sure the heat shield is in place. Break out the flux again, bathe the area in flux, and turn on the hot air. Repeat the process from earlier used to remove the chip, essentially orbit it with heat and eventually it’ll fall into the solder. Let everything cool before testing the repair.
Step 3: Testing the Board
There are a couple ways to go about testing your repairs. You can start a fresh batch of probing with the oscilloscope and/or volt meter. Sometimes it’s faster and safe enough to just reassemble everything and turn it on to see what happens. If you’re lucky, it works. If you’re not, then you either repaired the wrong thing or broke the board during the repair process.
It’s always best to leave the repair work to your professional, especially with circuit boards. It takes years of experience to reliably repair circuit boards. And on top of that, it takes a few thousand dollars in tools. A good, electronics repair professional might charge a few hundred dollars for the work, but if it’s for a board you can’t find new, it’ll save you tens of thousands in the long run.