Via Filling or Via plugging: is it Tomato or tomato? Don’t Get Caught Out…

Some years ago, one of our printed circuit board engineers worked around the clock to complete a painful board redesign. The high-value project had already been through several unplanned re-spins and was now critically overdue. So, it was decided to order half the quantity of the PCBs using special quick-turnaround ordering with a high-quality supplier in Europe to speed things up. The testing should have been done with the series supplier's boards, but we had no choice and only the remaining quantity would come from the actual series plant in Asia, six to seven weeks into the future! Three days later, the expensive quick-turn boards arrived but there was an issue: it was of course the via filling. The expensive boards arrived with 100% metal-filled vias. This now rendered the testing completely obsolete as the boards would perform differently in several ways – thermally, electrically, mechanically, and so on.  

For those of you who do not know, via plugging refers to the process of covering or filling vias with a non-conductive material. This is done to prevent the flow of solder and storage of contaminants in the via, which can produce migration, during the PCB manufacturing process. Via filling, on the other hand, involves filling the via holes with a conductive material, usually copper, to establish electrical connections between different layers of the PCB. Via plugging is simpler and cost-effective compared to via filling, and it allows for greater design flexibility. Via filling requires additional steps (such as electroplating or solder filling) to deposit the conductive material inside the via holes, which increases its cost. The presence of conductive material within the vias requires careful routing and consideration to avoid signal interference or short circuits. Moreover, it can impact signal integrity, especially for high-speed or high-frequency signals. The conductive via can introduce impedance changes or cause signal reflections, which need to be carefully managed during the PCB layout and design process. 

Long story short, this issue was not found in the PCBs that came from Asia 8 weeks later. The same documentation had been used for both orders, where it was clearly written on the drawings that all vias were to be plugged, not filled. It was an unfortunate mistake, and the experience taught us to take extra care when stating the via specs to the manufacturer. In general, it is a good practice to make sure that the people you work with use the same terminology, as our jobs involve interacting with cultures around the globe. The decision to use via plugging or via filling depends on the specific requirements of the PCB design. There are several reasons to pick one or the other, but since their end goal is different, you might see designs that implement both.  

Do you know of any other rotten tomatoes out there that need to be squashed before anyone else repeats avoidable mistakes? Please share any technical term or PCB knowhow which you may have come across over the years and help all of us in this forum to not repeat the same mistakes.