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26 The PCB Design Magazine • November 2016 Years ago, before epoxy and conductive ep- oxy were more prevalent, fabricators attempted to fill vias with mask material. This presented a problem at fabrication for BGA applications where you have a clearance for the via on one side but no clearance on the opposing side. With today's polymer plastic masks, this forms a plastic "cup" which does not allow the solu- tion to move freely through the via when met- allized and sometimes results in oxidation, cre- ating electrical anomalies. Today, for those same vias, or for via-in-pad, we can fill them with epoxy, planarize them (which is to say, make them flat) and flash plate over them, making the whole conversation about the absence of a clearance on one side or the other a moot point. So, where else might I choose to fill a via? Some applications for filled vias are under chips and have stitching vias under them to dissipate heat. Here again, the desire is to have a large met- al land area on one side directly under the chip and nothing on the opposing side. This is an- other great opportunity for an epoxy or even conductive epoxy fill. See Figure 1. In some applications, you may choose a con- ductive filled via for thermal applications to dis- sipate heat, such as a Faraday shield (Figure 2). In others, you may be more concerned with EMI and fill vias in the region of a ground strap (Figure 3). And yet in other applications you may choose to stitch the vias on either side of an impedance controlled structure (Figure 4). Via-in-Pad Lastly, let's talk board geometry and when to epoxy fill via-in-pad applications, specifically where you do not have room to dog bone off a SMT and have a via in the SMT pad or partially in the SMT pad. These are also perfect situations where epoxy fill is advised. This way the via is plated, filled, made flat and encapsulated with more plating, the fact one side is exposed based on via-in-pad and the opposite side is walled off with mask is of no consequence for flatness, oxidation or dimples. Laser and Stacked Via Structures So, what happens when even you still do not have enough room for all the routing you may need, even when using blind or buried vias? You consider using laser drills or stacked vias. The obvious benefit for a laser drill is size. If, based on geometries, you are relegated to a sub Figure 1: Vias stitched in a metal land. Figure 2: A Faraday shield with stitching vias. HEY, THEY'RE JUST VIAS—OR ARE THEY?