SMT007 Magazine

SMT-Nov2016

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36 SMT Magazine • November 2016 by W. Scott Fillebrown LIBRA INDUSTRIES 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 cir- cuit 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 manufac- turer. 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 re- ally have not changed. The most typical are stan- dard 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 signif- icant 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 bare- board. We did just about anything to avoid us- ing them. A couple of the microvia approach- es including laser drilled vias, and via-in-pad made them avoidable a while longer. How- ever, over time, blind and buried vias became unavoidable. While still expensive, their use FEATURE Figure 1: An example of the copper fill process. The Taken-for-granted Via

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