The Impact of Via Structures on Multi-Gigabit Signal Transmission


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Abstract
This article briefly introduces various via structures on the printed circuit board (PCB) for layer transition purposes. It also investigates the impact of these via structures on multi-gigabit transmission by analysing time domain reflectometry (TDR), differential insertion loss (Sdd21), and eye diagrams.

Introduction
The via is an essential structure used to make an electrical connection between the layers in multilayered PCBs. Connecting multiple layers of the board makes it possible to reduce the dimensions of the PCB. The via structures available in the PCB industry include through-hole vias and microvias, as depicted in Figure 1.

A through-hole via goes straight through the PCB from top to the bottom and can be applied to connect all the layers of a PCB. It is the most common via and is easiest to construct. A microvia, on the other hand, connects the outermost layer of a PCB to an inner layer, but is not visible on the other side of the board. Microvias are classified as staggered or stacked. With stacked microvias, the via barrels are constructed on top of each other in distinct layers. Staggered microvias, however, are scattered in various layers, and each via barrel is linked with a short horizontal conductive bar.

There are two major advantages of selecting microvias over through-hole vias. First, the microvia has a smaller dimension and aspect ratio, which facilitates the miniaturization of the PCB. Secondly, the dangling stub in a through-hole via’s layer transition causes degradation to the signal transmission due to the inverse proportional relationship between the quarter wave resonance frequency and the via stub length.

This article outlines our two-stage study of the impact of through-hole vias, staggered microvias, and stacked microvias on multi-gigabit transmission. In the first stage, the three-dimensional models of these via structures were constructed, followed by the extraction of the s-parameter using Keysight EMPro. In the second stage, the s-parameter files of these via structures are imported to Keysight ADS to analyse the time domain reflectometry (TDR), differential insertion loss (Sdd21), and eye diagram. The analysis and results are explained in the following section of this article.

To read this entire article, which appeared in the August 2021 issue of Design007 Magazine, click here.

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