PCB007 Magazine

PCB007-Nov2020

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58 PCB007 MAGAZINE I NOVEMBER 2020 Whether you have two layers or 50 layers, it all comes down to how the layers communi- cate. Otherwise, you just have a bunch of two- dimensional layers, and that isn't practical. The practical magic, of course, is plated drilled holes. Through the early years, it was all holes and large analog components with 16–18 lay- ers being state of the art. Standard holes for components were usually 0.060" and spaced 0.100". That was then. Today, the larger holes are left for connectors and hardware, with al- most all componentry evolving to SMT. Now, our plated holes are used for inter- connects or vias. Although some may still ac- cept components, they mainly provide the Z- axis connection between layers. Unlike vias of earlier times, these holes have become ex- tremely small. In fact, some vias don't even go through the board any longer. Some may go from the surface to a layer somewhere in the stack, where others are not visible from ei- ther side. Even though the holes have become smaller, the board thicknesses have not. From a plating engineering standpoint, this has be- come challenging as the aspect ratios have be- come very high. Each layer can be perfectly de- veloped and etched, all to be scuttled by a bad interconnect or via. The entire board ends up being scrapped. This can be costly (Figure 1). Although some would argue that electrical test (ET), what with all the mystical voodoo that goes on, is not a value-added process; in some aspects, they are correct. However, think of it more as an insurance policy. Having a fin- ished board fail at a customer site or CM is the worst thing that can happen. Not only must you deal with the returned product, but you also take one on the nose for delivered quality. It can be difficult to recover from that in this competitive market. How is ET guaranteeing you the peace of mind that what you are ship- ping isn't getting pickled by the barrel? Well, there are couple things we are doing in addition to the standard continuity and isola- tion test. Industry standards specify the mini- mum requirements to which the product must conform. Remember, these are minimums. Higher-reliability products may require more stringent testing. For example, Class 2 Level B products allow the optimization of mid-points during ET, where Class 3 Level C does not. Don't Get Pickled by the Barrel Testing Todd Feature Column by Todd Kolmodin, GARDIEN SERVICES USA Figure 1: Circumferential fracture.

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