Printed Circuit Board Via's

Dictionary of Engineering terms
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Editor note; All the PWB terms are listed below. The alphabetic links above will switch to the main glossary, but just below are definitions related to Printed Wiring Boards.

Lexicon of PCB Terms
'A' to 'Ge', 'Gr' to 'O', 'P' to 'R', 'S', 'T' to 'Z'

Via's in High Speed Design

Via definition. A plated-thru hole in a printed wiring board. A via may exist between one or more adjacent board layers, or through the entire board. A blind via runs vertically between the top [or bottom] side and through one or more adjacent board layers, but not through the entire board. A buried via runs between one or more adjacent internal layers, but does not run to the top or bottom layer.

Vias in a printed wiring board
PWB Vias
Types of PWB Vias
Via's in a PCB

The graphic above shows the three different types of via's used in a Printed Circuit Board [PCB]. The one to the left is a normal via which extends from the top side of the PCB to the reverse side of the PCB, connecting to any internal layer as required. The center via is called a buried via because the via does not extend to the surface of the PCB. The right most via's are called blind via's because they extend to only one surface (primary / secondary) of a multilayer printed wiring board, with the other end terminating to an internal plane or land.

Any via style may be used with high speed traces, but the choice of via type depends on which layer the high speed traces reside on. The basic issue, for fast edge rate signals, to watch for is that the via only translates between the two layers being used and does not extend past them. Once the trace or via extends past the layer being used by the trace it becomes a transmission line stub. Also refer to How to Terminate a Trace. Once the stub is formed, unwanted reflections will occur because of the impedance mis-match.

Via Stub. The portion of the via that is not in series with the signal. When a signal pin does not traverse the entire via, or when the signal layer using the via is not the bottom layer of the PWB. Two methods may be used to over come a via stub; Counter Boring, and Blind Vias, in both cases the via is terminated at its destination and not allowed to continue to the other side of the PWB. A via stub only applies to a trace to trace connection and differs from a thermal via.

Example; Layers 3 and layer 5 contain the traces to connect, but the via extends past both layers. This produces two small transmission line stubs.

Thermal Via definition: A via in the Printed Wiring Board [PWB] to effectively transfer heat from the top copper layer of the Printed Circuit Board [PCB] to the inner or bottom copper layers. The thermal vias should make their connection to the internal ground plane with a complete connection around the entire circumference of the plated through hole. Do not use a thermal relief web or spoke connection which impedes the conduction path into the inner copper layer. A thermal via can still be used to carry signals, but the via would need to terminate at the layer being used otherwise the via becomes a stub.

Also refer to PWB Design for information related to PCBs, or PWB Vendors.

Note the terms PCB [Printed Circuit Board] and PWB [Printed Wiring Board] are used interchangeably in the text. However the term Printed Circuit Assembly [PCA] always refers to a printed wiring board with the components attached.

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