Printed Circuit Board Design For Repair And Rework

“Unplanned obsolescence” can happen to a PCB so the board designer has to provide a few hooks to give the project a second chance.

Once upon a time, car stereos were almost universally interchangeable so you could get a new, improved one off the shelf from your local specialty store. Upgrading car sound systems was my ‘side hustle’ as a teenager. There were no how-to videos back then but there are now.

That’s because it’s necessary! For my car, it starts with vents and the display screen at the top of the center stack and works through the HVAC control module and then you can pull the info-tainment system out. You have hope it all clicks back into place afterwards. Who’s got time for that? (Life in the auto repair trade.)

Figure 1. Image Credit: Mura Car Accessories (MCA) - Reworking a car stereo’s main audio board for faulty amplifiers. The repair kit is accompanied by a video that walks the user through the cuts and jumpers along with a new daughter card required to do the upgrade.

No user serviceable parts inside? Give Me A Break!

The head unit is tough to get to but the PCBs within the chassis use active components in quad-flat-pack (QFP) packages rather than ball grid array (BGA) packages which are more difficult, but certainly not impossible, to solder or desolder. The QFP’s perimeter pins are simply more accessible for the do-it-yourselfer. Functionally, these circuits provide entertainment to the driver; unrelated to safety so a Class 2 PCB should be fine. Still, the BGA package is avoided as it doesn’t fit the high reliability mindset of the auto industry. The QFP devices allow for more robust Class 3 circuit boards if perhaps the voice navigation aspect is considered a vital system and integrated with the audio.

The maker's movement of recent years put a lot of people in the manufacturer’s seat. The shared spaces typically include hand tools, 3D printers and metal working equipment as well as know-how spread out among the makers. The members are the hands-on people who create new products so, of course, they would also be in the repair business to some extent. Sometimes, the end users of a product will figure out a new way to use it. Paying attention to these communities will help product developers decide what to do next.

Modularity Matters If There Are Components That Are Prone To Failure

On a PCB level, it may be useful to divide the project into different circuit boards. The part most likely to fail deserves its own circuit board that snaps in and out of the box. If the problem is that a part is overly receptive to outside interference, I like the idea of going 3D and putting a little card up on stand-offs or using stacking connectors if there are numerous interconnects. Ribbon cables can be a shortcut for connecting a series of boards without going to flex circuits.

A socket can make replacing a chip much easier but they are limited to IC replacement. Sometimes, an entire daughter card makes more sense. A wireless chip or a microphone array come to mind since they usually have a specific location within the chassis. The material stack-up can be dialed in for the kind of work expected from the part. Anything from ceramic to polyimide can be used for a small PCB that acts more as a module with a tighter focus.

You Can’t Fix What You Can’t See

The devices on a printed circuit board assembly have gotten smaller with each generation. We have to work with the parts that can be sourced in production quantity for a period of time. That usually means the kind with little to no part marking, especially among the passive components.

This makes it more important for us to have clear assembly diagrams and schematics. These can be annotated with rework instructions as necessary to create an engineering change order or a field service bulletin as necessary. The ECAD workstation can be just the tool for finding the right location for a controlled depth incision to cut a buried trace without affecting others.

How The Information Age Helps The Circuit Board Repair Industry

Back at the beginning of 2020, a recruiter at a certain fruit company contacted me to find out if I would be interested in joining the team responsible for the Apple watches. I had done my share of tiny boards so I gave it a shot. One of the things the recruiter advised me to do was to look at teardown videos of their watches. I did that but you only get one shot and it wasn’t my best day so someone else got that job.

Still, if you’re going to go work for a consumer hardware company, I’d consider checking out the ifixit channel on YouTube. They, or perhaps another group, have likely created a video where they disassemble an item and talk about what makes it tick. The ifixit team will also give it a repairability score. Just type in your product type and the word “teardown” and you’re likely to find something.

Figure 2. Image Credit: MCA - A rather low-tech solution to a problem allows the savvy user an opportunity to extend the life of their equipment.

Figure 3. Image Credit: MCA - Giving the user a leg up with clear labels for the patch. Cutting the traces is brutal but effective.

When I applied to work for the Chrome hardware team, I binged watched the videos that came up after searching for “laptop repair”. That was very useful for understanding what I’d be getting into. Short answer: flexes. Long answer: flexes for the battery, the keyboard, the touchscreen display, a haptics enabled touch pad, the camera, microphone, USB-A, USB-C, GPS, NFC and WiFi/BT to name a few.

Everything came apart and could be updated as required. We also had a version that included LTE as this laptop was destined to run Android apps. It was a giant phone in disguise. The Pixel laptop series was a powerhouse for software developers. We were in the trenches for a while. I eventually followed the lead engineer to two more companies after Google. It was that kind of effort.

There were a lot of laptop repair videos but one of the common things was that to repair a certain failure, you just had to find and replace the right field effect transistor (FET). Those so-called FETs are the local switch mode power supplies for various devices and the most likely cause of failures. Getting to the main logic board in the first place is a chore requiring a number of little prying tools and unusual screwdrivers. From a layout perspective, just keep those little power supplies near their load, do a solid layout of the output inductor and they should be alright.

Figure 4. Image Credit: MCA - Maybe not pretty but, in under eight minutes, the video takes you from removing the PCB to all buttoned up with restored audio. You can literally choose from a handful of instructional videos for this one installation. That is one of the keys to the reparability movement.

Design For Repair Tips

• Fan-out the through-hole connectors on the secondary side of the PCB. This makes it easier to reassign a connector pin to a different function by cutting a trace without removing the connector.
• The same placement rules for pick and place machines will also help with rework. Taller components like extra room and all components should have access to the solder joints; not always practical but it helps if you can manage it.
• Label the I/O ports wherever possible. The etch or even solder mask layer can play a role in component identification and orientation.
• EMI shielding should be the two-piece type where the lid can be pried off without desoldering.
• Gratuitous test points, as long as they do not affect impedance requirements, are a reworker's best friend for adding jumper wires. Incorporating test points on a transmission line may require using a larger gap between the transmission line and the reference plane. This allows a wider line; ideally matching the line width to the pad width of any series elements and allowing test points without a bulge.

In closing, give your customers some credit for wanting to be able to update and repair things when they break down. The know-how is out there to be learned so anyone with a few tools can tackle basic repairs. At the end of the day, it sure is nice to be able to get a new audio board for my vehicle rather than having to shop for a new car. Getting the industry to provide replacement parts seems to be the sticking point as this repair path is from a third party. “No user serviceable blah, blah, blah.” Meanwhile, you can enable the improvement capability of your products for a greener planet. Be the change.