The Most Effective Decoupling Capacitor Placement Guidelines

July 30, 2020 , in Blog

Is it better to be an expert in one specific area or to be a jack of all trades? Well, maybe not all trades, but multiple ones. Over the last few centuries, since the onset of the Renaissance, until recently, it was more common for scientists, philosophers, medical doctors, and others to strive for a high degree of understanding and practical ability in more than one discipline. It may be argued with some veracity that there was much less to learn back then to firmly establish oneself in a particular field. Even so, there is much to be said for the usefulness of versatility.

Placing decoupling capacitor on PCBA

When it comes to electronic components that populate PCBAs, the most versatile component in terms of how it is used is probably the capacitor. For sure, capacitors are fairly simple components in terms of their construction; although, there have been some notable increases in complexity. These include supercapacitors (or ultracapacitors) that have energy densities far above the more common electrolytic capacitors and graphene supercapacitors that can be made quite small. These advances are exciting and will likely only add to the important roles that capacitors currently play in PCB layout; including the decoupling of signals. Let’s define the most beneficial decoupling capacitor placement guidelines for your PCB layout, but first, let’s discuss the significance of decoupling for your board’s signal and power integrity.

Signal Decoupling on PCBAs

One of the most useful tools in analyzing periodic functions, which are signals that range in amplitude through a fixed cycle at regular intervals, is Fourier Analysis (named for French mathematician Jean Baptiste Joseph, Baron de Fourier). The power of this analysis lies in the fact that it can be used to decompose any analog signal into a series of waveforms with differing constituent frequencies. This isolation allows for the removal or filtering out of signals at unwanted frequencies from the original. This filtering is an example of precise decoupling and the circuits employed for this purpose are composed of some combination of resistance, inductance and/or capacitance.

Due to their inherent energy storage properties, which oppose quick electrical transitions, capacitors can be used for power supply and transient decoupling. PCBAs, especially if they are complex, may have multiple power supply inputs, each of which requires regulation to ensure that the components (typically processors, FPGAs, other ICs, or amplifiers) driven receive the proper voltage level. Decoupling capacitors are used to perform this smoothing function by supplying current to maintain voltage while component outputs transition to the proper level when needed. Capacitors are also used to remove transients that may occur in AC or DC power rails due to the constant switching of logic gates within ICs. The degree to which decoupling capacitors are effective depends upon your choice of type and most importantly on their placement.

The Best Decoupling Capacitor Placement Guidelines for Signal and Power Integrity

As discussed above, decoupling is a form of signal decomposition and isolation based upon frequency. Therefore, awareness of the frequency or range of frequencies to be isolated is necessary to design a decoupling solution. The most general case for PCBAs, which can impact both signal and power integrity, is the separation of AC from DC. By following a good set of decoupling capacitor placement guidelines, as listed below, you can minimize the negative effects on your board’s power distribution network (PDN) and input and output signals.

Effectively Placing Decoupling Capacitors

  • Place capacitor near the signal source

Decoupling capacitors should be placed as close as possible to the source for the signal being decoupled. This means at the pin for ICs and near the connector for input and out signals.

  • Place capacitor in series for input and output signal traces

To remove LF transients from input and output signals, the capacitor should be placed in series with the trace. The capacitor will pass HF, while blocking LF and DC. Additionally, small caps should be used for HF transients and large caps for LF transients.

  • Place capacitor in parallel for power pins and grounds

In contrast to decoupling for input and output signal paths, for power distribution and grounding, the elimination of AC or coupling of DC is the objective. Therefore, the capacitor should be placed in parallel with the signal path.

  • Place capacitor in parallel with a resistor to reduce HF EMI

Decoupling capacitors can also be used in parallel with resistors to remove unwanted HF while passing LF and DC.

  • Place capacitor before the ground plane connection

When using vias to reach the power plane, connect the capacitor to the component pin, then to the via to ensure current flows through the plane.

  • Place capacitor between digital and analog ground pours on the same layer

Decoupling capacitors are also effective for isolating analog and digital returns. This is achieved by connecting a capacitor between AC and digital PCB ground pours.

In conjunction with placement, it is also important to utilize good grounding tips for multisignal boards for the most effective decoupling for power supplies. For IC packages; such as BGAs, it may be necessary to use dogbone routing and vias to employ decoupling capacitors. However, if the power and ground are arranged adjacent to each other on internal balls, you may be able to simply connect directly to the power and ground planes with microvias.

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Capacitors are one of the most versatile components used on PCBAs and one of their most important functions is decoupling. In fact, your board’s signal and power integrity may well depend on how effectively you place decoupling capacitors. At Tempo Automation, the industry leader for fast, precise PCBA prototyping and low volume manufacturing, we can build your boards for the effective placement of these components.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on decoupling capacitor placement guidelines, contact us.

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