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Design007-Nov2020

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64 DESIGN007 MAGAZINE I NOVEMBER 2020 Controlled-Depth or Back-Drilled Vias All the inners are processed as a normal multilayer and then laminated as normal. The connection to the inner blind layer is done with controlled-depth mechanical drilling. The drilling can drill partially into the core between layers but must not connect to the layer past the blind termination layer. Laser Blind Microvias Use of a laser, either Nd:YAG (neodymium- doped yttrium-aluminum-garnet) or Nd:YLF (yttrium-lithium-fluorine), can only go through very thin substrates. An infrared laser can inherently penetrate deeper but is not able to remove copper with the longer wavelength they emit. Laser Microvias These are typically used for high-density interconnection (HDI) designs. Due to the physical shape of a laser microvia, the depth of a given microvia is typically two or less con- secutive layers deep due to the copper plating constraints of having to remove the ablated ash produced by the laser. They can be stacked or staggered, and both are additive processes. Microvias are used for higher functionality in less space, such as cellphones or tablets. 3. Buried Vias A via is either mechanically or laser drilled between inner layers and does not extend to the surface layers (such as blind vias). They are drilled and filled (either laser or mechani- cally drilled). They are usually filled during the lamination process by the prepreg. Uses of Various Vias Via-in-Pad and VIPPO With the extensive use of fine-pitch devices and smaller PCBs came the advent of via-in-pad structures. Via-in-pad is literally a via inside of a pad. It is first drilled, plated, or flash plated, filled with either epoxy or copper epoxy, and planarized so the surface is made flat for the assembly process. The advantage of this tech- nology is tighter, more closely packed compo- nent placement, enhanced thermal manage- ment, and elimination of parasitic inductance and capacitance as these reduce the signal path lengths. Via-in-pad plated over (VIPPO) is basically the same as via-in-pad with the exception that it is associated with an SMT pad, not a normal pad, such as one for a blind via. Additionally, VIPPO is also used where they will also back- drill (controlled depth drill) out the excess metal from the hole beneath the termination to an internal layer. Thermal Vias These dissipate heat from one side of the board to the opposite side of the board and are typically placed directly below (or as close as possible) heating elements or components that generate a lot of heat. PCBs are more conduc- tive across the board than they are through the dielectric. If traces are only present on the outer layers, then most of the heat is carried sideways (hori- zontally), and the internal core planes may be cooler. This adds thermal stitching vias con- necting surface features to the internal planes and creates more conductivity that dissipates heat to the core, reducing the overall tempera- tures more effectively. Stitching Vias Via stitching uses ground coupling. The most common use for stitching vias in a plane is to ensure short return paths for signals or to help maintain a constant ground. As soon as any current starts to flow, it will cause a voltage across the copper through which it is flowing, serving to both spread the cur- rent out but also cause the ground to bounce around. Via stitching can be an effective and low-effort way to more tightly couple ground across the PCB. Shielding Vias or Via Fences After reading a lot of literature on shielding vias, I will paraphrase the information I found. Via fences, also known as "picket fences," are structures to improve isolation between com- ponents that would otherwise be coupled by

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