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10 DESIGN007 MAGAZINE I JULY 2019 Feature by Greg Ziraldo ADVANCED ASSEMBLY When considering the long-term reliability of a PCB, you must take into account any vias on your board. While an invaluable and essen- tial part of board design, vias introduce weak- nesses and affect solderability. This article will discuss vias, the potential concerns that are in- troduced into your board through their imple- mentation, and how to minimize those con- cerns 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 ther- mal conductivity. While space is always a con- sideration 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 chang- es in its processing or end working environ- ment, the varying coefficient of thermal ex- pansion (CTE) between the laminate and the copper can cause issues. PCBs are constrained through structural latticework to limit hori- zontal expansion but can expand and contract significantly in the vertical direction. As cop- per 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 Illustration 1. Illustration 2.

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