The 6 Habits of Transparent Via Design Print E-mail
Written by Eric Bogatin, Ph.D.   
Monday, 09 May 2011 21:45

Via performance depends more on stub length than on effective characteristic impedance.

It is remarkable that so tiny a structure as a 0.150" long via can, in some cases, dramatically impact the performance of a 40" long backplane channel. It takes up only 0.1% of the channel, yet sometimes dominates the channel performance.

Then again, it’s possible for a via to be invisible in a channel. It can be so insignificant as to be completely transparent and play no role at all in the channel performance.

Four factors decide where, between these two extremes, your vias will end up: the signal layer transitions between the via, the bandwidth of the signals traveling in the channel, the board stackup features and the via design features.

Electrically, a via is a complex structure whose properties depend very strongly on the precise fringe electric and magnetic field structure in and around the barrel, pads and clearance holes. There are no universally good approximations to estimate the electrical properties of single-ended or differential vias. This means to get an accurate circuit model for a via requires a 3D field solver tool.

However, a via, even as long as 0.064" (and any interconnect), looks like a pretty uniform transmission line, even as high as 10 GHz. Using a 3D field solver, we can extract an effective single-ended or differential characteristic impedance as a rough figure of merit to describe the performance of the via.

In an eight-layer board, with four planes and four signal layers, a through via connecting a signal from the top layer to the bottom layer, with 0.010" drill diameter, 0.020" capture pads, nonfunctional pads on each layer and 0.030" diameter clearance or antipad holes in each plane layer, will have an effective characteristic impedance around 40Ω.

Anything that increases the capacitance, like larger capture pads, or larger drill diameter or more planes, will further decrease via impedance.

For differential vias, another term that is important is the via-to-via pitch. When you have the option to adjust this, it should be as tight as practical to minimize the channel-to-channel crosstalk. In general, typical differential via design rules will result in a differential impedance lower than 100Ω.

While not always an important measure of system performance, the TDR response of a via is a sensitive metric to compare how transparent two different vias are.
The smaller the reflected signal, the more transparent the via.

Consider two vias in a 0.064" thick board: a 40Ω via that connects signals between the top and bottom of the board, and a 50Ω via that connects a signal from the top layer to an adjacent layer, leaving a 0.054" long via stub. Which is going to have worse performance?

As seen in Figure 1, the 50Ω via is not only not transparent, but has even worse performance than the low impedance 40Ω through via. The performance of a via is often more dependent on the length of its stub than on its effective characteristic impedance. For vias to be transparent, the first step is to minimize the stub length, then worry about designing a 50Ω via.



The second step in designing a transparent via is to recognize that, in general, vias tend to be low impedance, so do what you can that is free to raise the impedance of the vias. When these actions are free and do not impact the cost of the design, we call them habits. The six habits to design transparent vias are:

1. Minimize the stub length.
2. Remove all nonfunctional pads.
3. Minimize the size of all capture pads.
4. Use as narrow a drill hole as practical.
5. Use a clearance hole with at least a 0.005" annulus.
6. Add at least one return via as close as practical to the signal via. (This has a minimal impact on impedance, but will help reduce crosstalk between signal vias.)

The impedance of a via will depend not only on the precise via features you end up with, but also on the layer stackup details, and of course, the length of the through and stub sections. The only way of knowing what impedance a via ends up with is with a 3D field solver, but by following these habits, you can improve your vias and give them a better chance of being transparent.

Eric Bogatin, Ph.D., is a consultant and founder of Be The Signal (bethesignal.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column runs periodically.

Last Updated on Monday, 09 May 2011 22:26
 

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