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Connect the Dots: The Split Planes Challenge

PCBs don't explode every day, but if your design calls for more than one voltage on a power plane layer, spontaneous combustion is just one of the many problems this tricky challenge for PCB designers can cause.

It may seem counterintuitive, but when a PCB fails spectacularly, the cost is less because it’s both immediate and easier to measure. If you make a small crater the first time you turn on the first production device, you lose an entire lot of boards and the time spent on designing and prototyping. When less noticeable or intermittent issues occur, they can eventually cost far more depending on how long they remain undiscovered.

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.

Split planes are enclosed regions on an internal plane layer, and problems with them can be among the most difficult design flaws to ferret out. The further a board makes it through the development cycle without someone noticing, the more it costs. Production delays and resources directed at solving the problem can be quite costly, but the worst-case scenario involves malfunctioning boards making it out into the field. Those events create warranty claims, product recalls, damaged reputations, and lost jobs.

Here we will explain why managing split planes can be so challenging and examine some best practices for avoiding this common issue.

Why Does This Happen?
There often exists a gap between what a design tool allows the designer to do with split planes and how difficult it can be to recognize issues with the design.

Your PCB design tool will 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. Problems are avoided if you divide the plane into separate, electrically isolated areas using a split plane.

Just because different voltage amounts are “isolated” doesn’t mean you’re out of the woods. When more than one voltage is assigned to a power plane layer, your CAM tool does not check whether the voltage of your via matches the section of the board it’s going into. If you have a 12V via going into a 3V section of the board, it will fail.

Mixing digital and analog signals can also create headaches. An analog ground is noisy, and it fluctuates, while digital grounds are quieter and steadier. Connecting digital and analog planes together probably won’t result in a board failure that trips the smoke detector, but there will be performance 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 is easy to lose track of individual connections—especially on a complex board design with hundreds of vias.

How Do You Best Manage Mixed-Signal Output?
When you need to assign one or more power nets to a layer on a simpler design, applying plane splits that segregate areas containing each voltage is a simple and efficient way to distribute power.

Warning! This method relies on the human eye to ensure accuracy and gets risky for more complex board designs.

We think a better method is to use polygon area fills to make connections. With polygon area fills, when you name every net connection on each layer, they become visible. Vias of different nets will not connect. The enhanced visibility makes it so your CAM tool can perform error checking on your separate area fills and automatically recognize mismatched voltages.

How Do You Prevent Mixed-Signal Problems?
Digital and analog signals don’t play well together. Here are some actions you can take to prevent problems with mixed-signal output.

• 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. 
• 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 must route a signal trace from one area to the other, place it entirely over the PCB’s ground plane
  • 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.

A Designer’s Best Friends: Vigilance and Awareness 
Designing with voltage top of mind is vital, as is knowing what the design tool’s error checker will and will not catch regarding connections to split planes. This is how you avoid burning boards and doing costly rework. If you build your designs such that you can rely on automated error checking, we believe that is the best method for dealing with complex power plane schemes.

This column originally appeared in the August 2021 issue of Design007 Magazine.