PCB Design
If you’ve ever worked on a printed circuit board, then there’s one type of capacitor that’s probably in almost every single location on the board. It’s called the decoupling capacitor, and its job in life is to absorb (or more correctly shunt to ground) any electrical noise that may be on the power supply pin of an active device. This prevents the device from spewing noise into the entire board, and disrupting other devices.
Capacitors are very useful components, but there is a very specific way that they must be used for effective pcb decoupling. They need to be positioned in close proximity to the IC power pins, and they need to have a low connection inductance. It’s not enough to simply throw a 0.1 mF ceramic capacitor in the same spot for each of the ICs on the board.
Integrated circuits are designed to operate at specific voltage levels, and any deviation from this can cause erratic behavior or even complete failure. To counteract this, decoupling capacitors store and release energy to compensate for these power fluctuations. With the design approved, the manufacturing process kicks into high gear. It starts with the fabrication of the bare PCB, where a thin layer of copper is laminated onto the substrate material. Next comes the etching process, where excess copper is removed, leaving behind the desired traces and pathways. This step demands a high level of precision to ensure the integrity of the connections.
The Role of Decoupling Capacitors in PCB Design
The size and positioning of these capacitors is crucial for the success of decoupling, as they must be large enough to handle IC voltage spikes but small enough to minimize connection inductance. The selection process should take into account IC sensitivity to power supply variations, along with the target frequency range for the capacitor.
Inductance is also dependent on the distance from the IC to the capacitor. This is where the bulk decoupling capacitors come in: these should be placed as close to the power and ground plane layers as possible. This is especially important in high-speed digital designs, where the inductance of the traces can be significant.
As for the local decoupling capacitors, they should be positioned directly next to the ICs, in order to reduce the total current loop length. It’s also a good idea to use two local decoupling capacitors of the same nominal value, rather than one larger capacitor. This reduces the overall connection inductance by a factor of two, and it also provides better high-frequency filtering to the rest of the power bus.
Whether it’s a high-speed digital or an analog design, the proper placement and selection of decoupling capacitors is essential for ensuring that your board will run smoothly. If you skip the necessary steps, then you’ll find yourself dealing with power surges and other issues that can negatively impact your final product. By taking the time to properly select, place, and size these capacitors, you’ll be able to ensure that your boards are running at their optimal performance. And that’s something we all want.