PCB007 Magazine

PCB-Nov2016

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12 The PCB Magazine • November 2016 In simple terms, a circuit board is but a com- position of things designed to be connected and not. The number of layers and number of con- nections make no real difference as the geometry shapes the landscape. The via is but one tool that helps in building the wonder that is a printed cir- cuit board, and in this column, we'll discuss the via in relation to the variety of types available for use as well as some of the testing that can be per- formed on them to ensure their reliability. A small—sometimes very small—yet impor- tant part of the circuit board landscape, the via, by definition, is simply the means by which lay- ers of the board can be interconnected. What dif- ferentiates the via from a plated through-hole is simply the fact that nothing (i.e., a component lead) will get inserted and possibly soldered into the structure. Further, vias don't have to extend from one side of the board to the other, although they certainly can as a through via. Blind vias start at the surface on one side of the board but don't extend to the other side, while buried vias (Figure 1) are completely encapsu- lated within the board with no end extending to either surface of the board. From there, IPC has defined seven types of vias. From IPC-T-50M [1] , Terms and Definitions for Interconnecting and Packaging Electronic Circuits, these types are identified and described as follows: • Type I—Tented: A via with a mask ma- terial applied bridging over the via wherein no additional materials are in the hole • Type II—Tented and Covered: A Type I via with a secondary covering of mask material applied over the tented via • Type III—Plugged: A via with material applied allowing partial penetration into the via (Figure 2) Vias for Dummies FEATURE COLUMN: LET'S TALK TESTING by Keith M. Sellers NTS-BALTIMORE Figure 1: Buried via. Figure 2: Type III via, plugged.

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