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When considering the long-term reliability of a PCB, you must take into account any vias on your board. While an invaluable and essential part of board design, vias introduce weaknesses and affect solderability. This article will discuss vias, the potential concerns that are introduced into your board through their implementation, and how to minimize those concerns to acceptable levels.
The first rule for via design is simple—bigger is better. Larger vias have greater mechanical strength as well as greater electrical and thermal conductivity. While space is always a consideration when it comes to PCB design, vias should have a minimum drill width of 20 mils with an annular ring of 7 mils and a minimum aspect ratio of 6:1. For many boards, this may be an unachievable goal; however, the basic premise of “bigger is better” stands true.
When a PCB is exposed to thermal changes in its processing or end working environment, the varying coefficient of thermal expansion (CTE) between the laminate and the copper can cause issues. PCBs are constrained through structural latticework to limit horizontal expansion but can expand and contract significantly in the vertical direction. As copper expands and contracts at slightly less than one-fourth of the rate of FR-4 laminate, vias are being pulled apart every time the board is heated. If the board is too thick and the copper in the via too thin, then the board will expand too much, and the copper will break, tearing the via apart. In the previous example, to get the appropriate aspect ratio with a drill width of 20 mils, this would result in a total pad diameter of 34 mils and allow a max board thickness of 120 mils.
Via size is important, but location is paramount. 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, making it less likely that you will have a solid mechanical 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 method, though it does have its drawbacks. Due to the minimum width required for solder mask, this may require the via to be moved even farther 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 profound 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 accessible. 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 better surface for silkscreening as well as a better barrier against contamination. This barrier works both ways, though.
To read the full article, which appeared in the July 2019 issue of Design007 Magazine, click here.