How to Place a PCB Bypass Capacitor in Process
The placement of bypass capacitors is one of the most critical phases of the design process. Failure to place them correctly can completely negate their performance. Also critical is a situation in which there are too few capacitors for particular components. This information should be communicated back to the engineer, whenever it occurs, so that the schematic can be updated.
1. A major factor in determining where to place the bypass capacitors is whether components can be placed on the bottom side of the board.
It is better to place components on the bottom because capacitors can usually be placed under the pads of top-side SMT components. Placing them on the bottom side usually frees up more space for fanout traces and vias. If capacitors must be placed on the top side, they should be located as close as possible to the power pins of the components.
2. When multiple capacitors of different values are assigned to the same supply pin on an IC, you should place the lowest value capacitor closest to the device pin.
The lowest value capacitor provides switching current for the highest frequency supply current requirement. When the output of a digital device switches from an ‘off’ state to an ‘on’ state and vice versa, the current required to do this becomes quite high for a very brief period of time. If only large value capacitors are available to provide this near-instantaneous current, then the output will be unable to switch at the required speed due to the longer time constant of these larger capacitors. This can cause serious timing problems in the design. Placing low value capacitors close to the pin helps to supply a small current very quickly to the switching device. This is because of the shorter time constant of these smaller capacitors. Once the output reaches a steady state again, the current requirement is reduced.
3. Larger non-polarized capacitors and tantalum capacitors should be placed near the pin or device in ascending order of value. Tantalums are typically used as an ‘area storage tank,’ which provides the required current faster than the system power supply. These tantalums recharge the high frequency capacitors more quickly than the system supply can respond.
5. Devices with multiple power pins usually need to have at least one bypass capacitor per power pin. Looking at the figures below, if the design only allows for two bypass capacitors on U3, then place one on either side of the device,
6. Always refer to the schematic when placing bypass capacitors because there are often logic input pins on digital devices that are ‘tied high’. Make sure when placing the bypass capacitors that you are placing them at the device power pins and not at tied high logic pins.