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Design007-July2019

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12 DESIGN007 MAGAZINE I JULY 2019 of 20 mils, this would result in a total pad di- ameter of 34 mils and allow a max board thick- ness of 120 mils. Via size is important, but location is para- mount. If a via is located close to a solder pad, a myriad of problems may arise, the foremost being the issue of solder wicking. As the via heats up, it pulls solder from the solder pad, through the via, and onto the other side of the board, leaving the pad either solder-deficient or completely solder-free. The larger the via, the more solder will likely wick away, mak- ing it less likely that you will have a solid me- chanical and electrical joint. Fortunately, this concern can be fixed by any of three no-cost methods. Providing a solder mask between the lead and the via creates a barrier to the movement of the solder. This is a simple yet effective meth- od, though it does have its drawbacks. Due to the minimum width required for solder mask, this may require the via to be moved even far- ther from the lead. The distances required may seem minimal (in the 2–5-mil range). When space is at a premium or the board is carrying high-frequency signals, this may have a pro- found effect on your design. However, when these aren't issues, this is a great way to avoid solder wicking concerns. If there is no space to move the via and you need to minimize the via size, it is possible to use an encroached or tented via. By masking the via pad, you save space and also make it possible to silkscreen over the via. However, this makes it impossible to use the via as a test point as the copper will no longer be accessi- ble. At this point, you need to decide whether an encroached or tented via is best. A tented via is completely sealed and will create a bet- ter surface for silkscreening as well as a bet- ter barrier against contamination. This barrier works both ways, though. If a via is tented on both sides of the board, contamination can fill the void during fabrica- tion. At elevated temperatures—such as when the PCB is being reflowed or wave soldered— the contamination can outgas and destroy the via and thus the board. When you tent a via, make certain you only do so on one side. An encroached via eliminates this issue by keep- ing the hole itself open and also has the added benefit versus a tented via of being possible no matter the via size. While tented vias need to be small enough for the solder mask to bridge the drill hole, the encroached via only covers the annular ring and can be as big or small as needed. Filled vias are also an option; they provide increased strength and electrical/thermal con- ductivity while protecting the via from solder wicking and contamination. The main draw- back to filled vias is that they can add signifi- cant cost to the board. But the other methods should have no impact on cost whatsoever. Every design has its own requirements and constraints. However, when possible, utilize these tips by using the largest vias and the appropriate aspect ratio while thoughtfully choosing the solder masking style that suits your needs. This will help reduce the overall lifetime costs by increasing the reliability of your products. DESIGN007 Greg Ziraldo is director of operations for Advanced Assembly LLC. Illustration 3.

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