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24 The PCB Design Magazine • November 2016 pitch. Understand that without a design to see the via image data, it is difficult for a fabricator to properly understand what pitch means to the customer. The pitch is the distance from the center of a given entity to the center of an adjacent entity. This can mean different things based on pad/mount size. Let me give you an SMT pad as an example: A .5mm pitch means .0197" between the centers of two BGA pads or surface mounts. This seems like a very reasonable distance, giv- en today's circuit board geometries. However, what if the designed surface mount pad width is .015" wide? This would mean a .5mm pitch would leave only .0047" edge to edge between the mounts. This gives you little or no room to route a trace between mounts at that width and pitch. Clearly as the pitch between parts decreases based on today's shrinking chip footprints, the associated surface mount or BGA pad widths need to decrease as well to be able to route even a .003" or .004" trace between them. Blind and Buried Vias What exactly is a blind via? And what is a buried via? Typically used when real estate becomes an issue and you have a finite amount of space to get all the interconnects done, a blind via goes from a given surface layer (top or bottom, or both) down to a specific layer. As an example, a blind via scenario on an 8-layer board may be blind vias on layers 1-2 and 7-8 and through hole. This is common for folks who want the best of both analog and digital in their designs; they may, for instance, have blinds on layers 1-2 and 7-8 in this scenario as Rogers materi- als to take advantage of very specific Dk and Df numbers for co-planar waveguides and then have all their less critical signals internally on a standard hi-temp FR-4 type. A buried via is a via on a layer set internal- ly that does not go to the surface. An example would be a 10-layer board with buried vias on layers 3-8. This scenario would typically be constructed as a core cap, with core material between layers 1-2, 3-4,5-6,7-8 and 9-10. The layers 3-4,5-6 and 7-8 would then be laminated together forming the buried drill scenario. They would then be drilled as 3-8, imaged, plated, stripped, etc., and then laminated to the 1-2 and 9-10 in a final lamination process. The part is then through- hole drilled and finished as a standard multi- layer. The advantage of blind and buried vias is that of space. When all desired interconnects are exhausted by through-hole, you look at going to blind or buried vias to open routing space. What do you need to know about them from a fabrication standpoint? It's straightfor- ward, really. Provide a separate NC drill file for each blind and buried via scenario. Optimally, provide sep- arate drill drawings for the blinds or at least an additional table showing all drill scenarios in your drill drawing showing any blind or buried vias. Remember than on a plane or split plane used as a blind termination layer, you must add pads to facilitate through-hole plating of the blind and/or buried section. Many blind via scenarios assume some per- centage of epoxy fill through the lamination process. To avoid surface dimpling from blind vias, many times customers specify an addition- al epoxy fill after lamination to maintain good surface flatness. Which brings me to... When should you specify via fill, and what type should you specify? Why should you fill a given via with either epoxy or metal epoxy? First, let's talk about epoxy vs. a metal epoxy and why you would choose one over the other. What is the obvious difference between the two? Well, the partially metalized epoxy fill is conductive. So a via that is designed with a thermal application in mind (for instance, to disperse heat from one side to the other) could benefit from metal epoxy, as opposed to epoxy fill that is non-conductive. Remembering that the via itself will have plating in its barrel so there is still continuity regardless of whether it is filled with conductive or non- conductive type epoxy paste. HEY, THEY'RE JUST VIAS—OR ARE THEY?

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