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Losing track of voltage in your design and boards can literally blow up!

Managing split planes—Your CAM tool will not do it for you.

We see this almost every day—not exploding PCBs, that’s pretty rare, but rather problems created by having more than one voltage on a power plane layer. From where we sit, this is one of the more insidious and costly challenges facing PCB designers.

Losing an entire lot of boards to spontaneous combustion offers an immediate, measurable cost, but the less noticeable anomalies can eventually cost even more once you add up production delays and resources directed at solving the problem. Worse yet, malfunctioning boards can make it out into the field and create even bigger headaches.

The severity of the PCB failure seems to have an inverse relationship to the amount of effort required to fix it. When there’s smoke coming out of your PCB, it’s relatively easy to find out why. Digital glitches and signal anomalies are more subtle issues that can take many hours of tedious detective work to solve.

We’ll explain why managing split planes can be so challenging and take a look at some best practices for avoiding this common issue.

Why does this happen?

PCB design tools offer the ability to assign voltage amounts to the entire plane, but that can lead to problems if your design assigns multiple voltages to the same plane. You can divide the plane into separate, electrically isolated areas using a split plane—an enclosed region on an internal plane layer.

But when you’ve got more than one voltage assigned to a power plane layer, your CAM tool does not check whether you have a 12V via going into a 3V section of the board. In this situation, the board will fail.

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Figure 1 Our CAD tool gives a loud warning, but some CAD tools assume that you are aware of this limitation and don't give any warning when you apply multiple nets to a layer.

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Figure 2 Notice +5V & +3KV are both tied automatically to this plane and will not generate any error. It will, however, let ALL of the magic smoke out of your PCB.

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Figure 3 Our CAM tool simply tells you that you have a split plane. It does not analyze the connections to that plane.

Another example is the mixing of digital and analog signals. Analog ground is noisy, and it fluctuates. Digital ground needs to be quiet. Connecting digital and analog planes together probably won’t result in a smoking board. There will, however, be anomalies from the noisy analog ground interfering with signals on digital pins.

If you are relying on manual oversight to discover such issues before production, it’s easy to lose track of individual connections—especially on a complex board design with hundreds of vias.

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Figure 4 Here DGND & AGND are tied to same area of ground plane. This will cause noise on your analog ground; not a lot of fun to troubleshoot. Again, no error will be generated in the design tool!!!

Best practices for managing mixed signal output

For simpler designs, when you need to assign one or more power nets to a layer, it makes sense to apply plane splits and segregate areas containing each voltage. This is an efficient way to distribute power, but it relies on human input to ensure accuracy and gets risky for more complex boards.

We think a better method is to use polygon area fills to make connections. With polygon area fills, when you name each net connections on all layers become visible and vias of different nets will not connect. This enables your CAM tool to perform error checking on your separate area fills. Automatically.

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Figure 5 The drawn polygon assigned to +5V will not connect to any other vias, shorts will be avoided, and +3KV net will show up as open. You will get a warning that U5 is in the wrong space. Notice the guide wires from unconnected pins.

As you design, there are steps you can take to prevent problems with mixed signal output.

  • Partition your PCB with separate digital and analog areas.
    • Make sure digital and analog components are assigned to their respective areas.
    • Never route digital signals through analog territory and vice versa.
    • Straddle analog to digital converters along the border of the two territories.
    • Use a single, solid ground plane.
    • If you have to route a signal trace from one area to the other, place it entirely over the PCB’s ground plane.
  • Keep the power ground and control ground separate for each power supply stage.
  • To keep your digital and analog grounds separate, build in a small impedance path. This will limit power circuit interference and help protect your control signals.
  • Pay attention to routing. For example, you want your analog grounds crossed only by analog lines. This will reduce capacitive coupling on a large ground plane with lines routed above and under it.

Prevention begins with awareness

Knowing what the error checker will and will not catch regarding connections to split planes is the first and most important step to preventing exploding PCBs. We believe that the best method for dealing with complex power plane schemes is one that can rely on automated error checking.

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