PCB Design and Fabrication Tips to Prevent Open Circuits During PCB Manufacturing
For a couple years, I lived in a town with a chocolate and candy factory. It was an amazing and terrible time, because you could go to the factory and buy “seconds,” or messed up candies, for about 75% off the normal price. Usually, the mistake was cosmetic, like the chocolate cracking over the caramel, and everything tasted totally fine.
When a PCB manufacturer makes a mistake, sometimes it’s cosmetic, and the board will still function. Something like misaligning the final screen printing probably won’t affect the electrical performance, but a similar misalignment of a solder mask or copper layer could completely ruin your board. Since PCBs are intended to route electricity, most of the significant performance defects are electrical in nature, things like open circuits, shorts, and routing or material failures.
Depending on your source, open circuits constitute about a third of PCB defects, especially in the form of open solder joints. A number of issues can cause open circuits on your board, varying from materials to processing to handling. Here are the most common causes.
If solder paste is applied inconsistently, either with the amount deposited varying, or with some locations being missed entirely, then there won’t be enough to form a solid joint. You might be left with an open circuit, or a joint that’s weak and prone to breakage. Another issue with solder paste is inconsistent reflow temperatures across the surface. If you’ve ever microwaved chocolate, you’ve probably seen hot spots that melt much sooner than the rest. The same kind of variability can happen during solder reflow. If some areas don’t reach reflow temperature and bond completely, the electrical connection won’t form, kind of like leaving unmelted chunks of chocolate in your cocoa or frosting mix.
When solder paste is applied, if the aspect ratio (the aperture width to stencil thickness) is off, you’re more likely to see issues with solder paste deposits. Make sure to verify the layer thickness, especially of your solder mask with your manufacturer.
No one wants to eat contaminated chocolate, and I hope I never had any with a sketchy history. PCB components can get contaminated, too. Environmental contamination can come from a variety of sources, either on the board or in the solder paste. Obvious causes are chemical spills, dust and particulates in the air, and oils from being touched.
Even moisture in the air can lead to accelerated corrosion. Any contamination or corrosion of the pad surface or the component lead can keep the solder joint from bonding correctly. Check for quality controls at your manufacturer and in-house handling to make sure parts stay clean and undamaged.
Gaps and cracks
Gaps caused by surface irregularities can cause areas of the PCB to lose planarity, making the distance between different leads on the same component widely varied, and keeping leads from even making contact with solder paste during reflow. This is most common if you have component warping or solder mask irregularities, but can result from other thermal mismatch issues, problems in the layer stack up (like air bubbles from improper outgassing), or physically mishandling the board.
Sometimes gaps and cracks are severe enough to be visible, but most often you’ll need to use a microscope or X-ray to find issues, especially with smaller packaging on components. Depending on the budget you have for troubleshooting, you may have to stick with identifying the location of the open circuit with electrical testing, and have your manufacturer or a test lab do the final root cause analysis.
Mistakes during fabrication can be time-consuming and costly. You can improve the process by managing your designs and manufacturer information with quality design layout software, like Altium Designer and Altium Vault.
Interested in learning more about how Alitum’s capabilities can help you improve your design and manufacturing process? Talk a PCB design expert here.