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MAY 2023 I PCB007 MAGAZINE 39 Too-small annular rings can also cause lon- gevity issues for a PCB. While the PCBs may work great at first, the small annular ring could cause a failure somewhere down the line. Usually, though, when annular rings are too small, the PCB will encounter shorts or just simply won't work. ere have been instances where small annular rings caused an "unin- tended thermal fuse situation"—that's right, the PCB caught on fire. How common are undersized annular rings in PCB manufacturing? Ideally, a PCB design will be reviewed for issues like undersized annular rings before it hits the manufacturing floor. is review helps ensure the PCB can be created within IPC spec- ifications. IPC-A-600 covers the acceptability of printed boards, while IPC-6012E governs "rigid printed boards." ey both define dif- ferent classifications for minimum annular ring requirements. For example, in IPC-A-600, the different classes define requirements as follows: • Class 3: No breakout, though holes are not centered in the pads; the annular ring mea- sures 2 mil or more. • Class 2: 90° breakout or less, with the con- ductor junction measuring 2 mil or more. • Class 1: 180° breakout or less, as long as the conductor junction is not reduced by more than 30%. Note that only Class 3 requires some amount of leover annular ring. A quality-focused manufacturer will do everything it can to meet that requirement. Unfortunately, undersized or absent annu- lar rings are very common in PCB designs. We sometimes see as many as 30% of jobs in the queue put on hold for annular ring issues. is can delay production for hours while the designer and manufacturer work to resolve the issue. You can avoid annular ring problems in your PCB designs by carefully considering the drill size and pad sizes you want to use. is can cer- tainly affect cost, but it may be worth the dif- ference. Note that your PCB design soware probably has configuration settings for mini- mum annular ring widths. Make sure these set- tings match IPC-A-600 Class 3, and you should be ready to go. Another option you can use to avoid annu- lar ring issues and reduce the chance of break- out is to add teardrops to your pads and traces. is can be a very effective method for reduc- ing annular ring issues. It's important to remember that annular rings play a crucial role in PCB functional- ity and reliability. Poorly planned and under- sized annular rings are a leading cause of PCB manufacturing delay. By realizing these impor- tant facts and by planning ahead, you can keep annular rings from derailing your PCB manu- facturing plans, budgets, and timelines. PCB007 Matt Stevenson is vice president at Sunstone Circuits. Download The Printed Circuit Designer's Guide to... Designing for Reality by Matt Stevenson. Kevin Beattie is a production manager at Sunstone. Karla Thompson is a CAM manager at Sunstone. To read past columns, click here.

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