Issue link: https://iconnect007.uberflip.com/i/1307491
NOVEMBER 2020 I DESIGN007 MAGAZINE 63 One thing I was told many years ago was, if we added up all the tolerances, we would never be able to build a PCB. Luckily, many of the tolerances cancel out each other. Teardrops What about the use of teardrops? Teardrops are simply a fillet at the junction where the trace connects to the pad. This is done so that the hole will not break out of the throat where the trace meets the pad causing a discon- nect. If you have enough room on your board design, one way to mitigate the induced drill wander that occurs in a fab environment is to use teardrops. Some examples of various tear- drop styles are shown in Figure 2. Via Types Here, I'll detail a variety of via types, includ- ing (1) through-hole vias, (2) blind vias, (3) buried vias, and (4) stacked and staggered vias (Figure 3). 1. Through-Hole Vias These are simply vias that go from the top to the bottom layer and are through-hole plated. They are used to pass a signal from one side of the board to the other or to make intercon- nects in the case of a multilayer. 2. Blind Vias These are vias that either start from the top or the bottom side and terminate on a given internal layer. They are typically used for where board space is a premium. Examples include blinds 1–2, blinds 1–5, and blinds 3–6 (6 being the bottom layer). Note that you will need a separate NC drill file for each blind via scenario. This can typically be done 2–3 times, but normally no more than 2–3 times from a given side. The limitation is the number of plating cycles the outer layer sees. A 10-layer example would look like top to layer 2, top to layer 3, top to layer 4, bottom layer to layer 9, bottom layer to layer 8, and bottom layer to layer 7. To make more layer connections for 14-, 16-, or 18-layer boards, buried vias can be used in conjunction with the blind vias. Types of Blind Vias Sequential Blind Vias The termination inner layer is processed on the core, leaving the associated outer layer as a copper sheet only to be imaged after lamina- tion. Figure 2: Snowman teardrop (for obvious reasons) on the left; fillet-style teardrop on the right. Figure 3: Types of vias.