Issue link: https://iconnect007.uberflip.com/i/745476
38 SMT Magazine • November 2016 typically is not optional due to space require- ments or signal integrity issues. In the past, blind and buried vias required very expensive processes through the board manufacturer, like sequential lamination. However, today there are many approaches to accomplishing blind and buried vias, which have helped reduce the cost of their implantation. A few of these processes include control depth drill, flip drill, stacked via and staggered vias. Each of these methods has their place, and can be reliable when used prop- erly. However, they also can have their draw- backs. The best recommendation is to become best friends with your board manufacturer and rely on their experience to make recommenda- tions for the most reliable and cost-effective ap- proach for your application. Clearly, blind and buried vias still are a more expensive approach and should be avoided if at all possible. Now, re- garding their older brother, standard through- the-board vias, could more be better? When it comes to a standard via, there are times when more can be better. So often we see a via as another hole in the board; however, many times they can be used for more than a conduit from one layer to another. In the R&D process, they can be used for a simple task like probing to diagnose issues or as a point in the circuit to solder a blue wire for engineering changes. While the engineering lab hopeful- ly is a short stay for your board, having addi- tional vias is not just a short-term gain. The big- gest gain for having additional vias is for flying probe test. Flying probe is well on its way to be- ing the standard for testing assembled boards in North America. With the advent of dual-sid- ed testers, you have the potential to reach 95% test coverage. Unlike bed-of-nails testers that re- quires a 30–40 mil test point, flying probe re- quires a simple via with a 20 mil pad and a 10 mil hole. This approach gives the high-mix, low- to medium-volume manufacturer a viable test approach that supports every company's desire for high quality. The key to taking advan- tage of these approaches is not to treat them like your crazy uncle, the buried via. You need to be proud of your older brother, the stan- dard via, and not cover them with soldermask. Where the design allows, access is the key to be- ing able to use the via for probing. Just like the offensive line of your favorite football team, vias get little attention and are taken for granted. But, just as a quarterback is in trouble without a solid offensive line, a circuit board is nothing without a good lineup of vias. It may be time to review your use and design ap- proach for using vias. Are they reliable? Are you getting everything you can out of them? SMT W. Scott Fillebrown is the CTO of Libra Industries. To reach him, click here. THE TAKEN-FOR-GRANTED VIA Figure 2: Epoxy fill with copper vias. Figure 3: Resin fill—stacked vias. Figure 4: Resin fill—offset vias.