Decoupling Capacitor Placement in PCB Layout

Key Takeaways

• Learn about what a decoupling capacitor is.

• Understand why decoupling capacitor placement matters.

• Find out how to place decoupling capacitors correctly.

A decoupling capacitor stabilizes voltage for ICs.

I’m a true Ikea fan at heart. Still, I couldn’t understand why I ordered a dozen of modern-looking vases from the retailer’s online store. These vases looked great on the picture, but I had trouble fitting them in my home. Despite trying out a few locations, the vases stuck out like a sore thumb. 

Perhaps my lack of sense in interior design is to blame as I continued to struggle with the vases’ placement. Eventually, I gave up and returned the vases. Thankfully, I have no such issues with decoupling capacitor placement, or I would have bigger issues than getting criticism for my poor interior design sense. 

What is a Decoupling Capacitor?

In the strictest sense, there isn’t a specific component that’s defined as a decoupling capacitor. Rather, the term decoupling capacitor refers to the function of a capacitor in an electronic circuit. A decoupling capacitor is one that stabilizes the voltage on the power supply plane. 

In any design that involves semiconductor ICs, you’ll always need decoupling capacitors. That’s because the voltage supplied to the components is far from ideal. Unlike the perfect horizontal line depicted in theory, voltage readings in real-life applications tend to fluctuate even if you’ve got the cleanest power supply.

The decoupling functions as a reservoir and acts in two ways to stabilize the voltage. When the voltage increases above the rated value, the decoupling capacitor absorbs the excessive charges. Meanwhile, the decoupling capacitor releases the charges when the voltage drops to ensure the supply is stable. 

Often, you’ll need at least two coupling capacitors of different values to stabilize the voltage supplied to a component’s VDD pin. A capacitor in the range of 10 uF acts as a larger buffer to smoothen low-frequency fluctuations. High-frequency changes in voltage is dealt with a smaller capacitor, typically around 100 nF. 

The absence of decoupling capacitors results in unstable microcontroller operations.

Why Decoupling Capacitor Placement Matters?

Ever wonder what would happen if you skipped decoupling capacitors in your design? I’ve done that for curiosity and here are a few signs to look out for. 

An onboard microcontroller will have trouble operating as the voltage fluctuation can send it into brownout mode where it gets reset. Any attempts to get reliable ADC conversion will be futile, as the analog voltage supply is anything but stable. If you’re to send a PCB that has no decoupling capacitors installed to the field, you’ll have weirder problems due to the greater electrical noises.

So, does placing a few decoupling capacitors on the PCB solve the problems? 

It depends.

The placement of decoupling capacitors is crucial to mitigating voltage fluctuations. If you’re not placing the capacitors at the right place, the effect will be minimal at best. In some cases, the wrong placement of decoupling capacitors can be a problem by itself, as it could pick up EMI coupled onto the copper trace.

Where To Place Decoupling Capacitors?

Unlike finding the best spot for a modern vase, placing decoupling capacitors is easier.

The golden rule of decoupling capacitor placement is to minimize the distance between the component’s voltage pin and the capacitor. This means you’ll need to place the decoupling capacitor as close as possible to the IC’s pin. If you’re designing a multilayer PCB, place the capacitor beneath the component’s pad. On a single-layer design, the capacitor is placed near to the pin and routed with a short trace. 

Place decoupling capacitors close to voltage pins.

As mentioned, you’ll need a 10uF and a 100 nF capacitor to stabilize against low and high-frequency fluctuations. The 100 nF capacitor should be placed closest to the voltage pin followed by the 10 uF capacitor. Repeat the process for as many VDD pin on the IC. There are some cases where the lack of space prevents the 1 decoupling capacitor per pin principle. In such instances, you’ll still need a minimum of 1 decoupling capacitor per component.

If you’re still doubtful of the needs of decoupling capacitors in design, try working with one in your next layout. OrCAD PCB Editor, with its reliable and strong layout capabilities, will help you understand the difference of including decoupling capacitors in the circuit.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.