The Taken-for-granted Via

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Vias are interesting little creatures. They are so simple, yet they can be so complex at the same time. Designed properly, they complete a circuit. However, a poorly designed via can be a reliability nightmare. The bottom line is that, in many ways, they are the unsung hero to a circuit board, much like an offensive line is to a football team.

Historically, we have tried to minimize their use to save money at the bareboard manufacturer. While this is still common practice, there are times extra vias are a good, useful addition to the board. In today's world, standard vias have very little impact on the costing structure. Complex vias, on the other hand, can add cost and potentially decrease the reliability—clearly not the bargain you wanted.

So what are the different types of vias, and why are they complex? First, the types of vias really have not changed. The most typical are standard vias, which are drilled from one side of the board to the other. Then you have your blind via that can be seen only from one side of the board. Finally, you have your “crazy uncle” via that we call the buried via. He is the one we hide in the middle of the board so no one can see him! While each of these vias has been around for a long time, space constraints have caused significant design approach changes to support them through the manufacturing process.

The largest growth area for vias is blind and buried vias. Many of us remember the days when using a blind or buried via was considered a death nail in the cost structure of our bareboard. We did just about anything to avoid using them. A couple of the micro via approaches including laser drilled vias, and via in pad made them avoidable a while longer. However, over time, blind and buried vias became unavoidable. While still expensive, their use typically is not optional due to space requirements 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 properly. However, they also can have their drawbacks. The best recommendation is to become best friends with your board manufacturer and rely on their experience to make recommendations for the most reliable and cost-effective approach for your application. Clearly, blind and buried vias still are a more expensive approach and should be avoided if at all possible. Now, regarding 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 hopefully is a short stay for your board, having additional vias is not just a short-term gain. The biggest gain for having additional vias is for flying probe test. Flying probe is well on its way to being the standard for testing assembled boards in North America. With the advent of dual-sided testers, you have the potential to reach 95% test coverage. Unlike bed-of-nails testers that requires a 30–40 mil test point, flying probe requires 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.

To read this entire article, which appeared in the November 2016 issue of SMT Magazine, click here.



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