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Design007-Feb2019

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48 DESIGN007 MAGAZINE I FEBRUARY 2019 Managing split planes? Your CAM tool will not do it for you. We see this almost every day—not exploding PCBs, which 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 spontane- ous combustion offers an immediate, measur- able 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, malfunction- ing boards can make it 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 challeng- ing 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 iso- lated areas using a split plane—an enclosed region on an internal plane layer. But when you have more than one voltage assigned to a power plane layer, your CAM tool does not check whether you have a 12-V via going into a 3-V section of the board. In this situation, the board will fail. 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 Exploding PCBs: Don't Lose Track of Voltage in Your Design Connect the Dots by Bob Tise and Dave Baker SUNSTONE CIRCUITS

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