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38 PCB007 MAGAZINE I MAY 2023 What does an annular ring do? One of the most important aspects of an annular ring is that it keeps a through-hole within the boundaries of a pad. is is cru- cial for solid construction and connectiv- ity between leads, pads, and traces. An annu- lar ring prevents a plated through-hole from shorting to the ground layer as well. On the outer layer of a PCB, the annular ring helps ensure solid construction of the entire board. Planning for properly sized annular rings gives the PCB fabrication process room for dri—within tolerances. In order to plan for sufficiently sized annular rings, these are the recommended pad sizes: Inner and outer layer pads should be at least 18 mil larger than the finish hole size, though via pads only need to be 10 mil larger (typically the non-via holes are drilled larger than the fin- ished hole size, where vias will be drilled at the finished hole size). If the design includes any pad to trace junc- tion minimum requirement or a higher IPC inspection class requirement, that will need to be accounted for in the pad design. For exam- ple, an 18-mil pad and a 2-mil junction should actually have a 20-mil pad. ese sizes should end up creating a 9-mil annular ring target for leads and a 5-mil annular ring for vias to allow for process variation and still meet the require- ments. If you have all the needed numbers from your PCB manufacturer, you can use the fol- lowing equation to calculate your minimum pad sizes. L = a + 2b + c Where: • L is the required via land size • a is the internal diameter of the drill hole • b is the minimum annular ring size • c is the fabrication allowance or allowed dri for the drills What happens when annular rings are too small? It is easy to see why annular ring size can be a problem. Many times, designers want to use the largest through-hole drills to save cost. At the same time, they want to pack their designs tightly so that the required PCB can be smaller. However, larger drill holes mean larger annu- lar ring requirements. Good design requires being aware of the needed trade-offs. at's because an annular ring that is too small can cause manufacturing and quality issues. One of the most common is breakout, when the drill hole crosses over the edge of the pad. Breakout can be a serious issue because it can lead to shorts and poor connection between the lead and the pad. Figure 2: Example of annular ring that is too small.

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