PCB design is a crucial part of the entire PCBA manufacturing process. This article mainly discusses PCB stray capacitance, calculation of PCB stray capacitance, factors affecting PCB stray capacitance and how to reduce stray capacitance in PCB design.
Table of Contents
What is PCB stray capacitance?
From the name, we can know that the meaning of stray is: “without rules, aimless, not where it should be.”
So we can say that PCB stray capacitance is the unavoidable, unintentional and unwanted capacitance present between various parts of a circuit.
Capacitance doesn’t just exist inside a capacitor. In fact, any two surfaces that are at different potentials and are close enough to create an electric field have capacitance, just like a real capacitance.
This effect is often found in circuits, for example between conductive traces and component leads. This unexpected capacitance is called stray capacitance.
PCB stray capacitance can cause interruption of normal current flow within a circuit.
Calculation of PCB stray capacitance
In common calculations, the stray capacitance formula: C = Q/V, is a measurement of the charge accumulated at the differential potential.
In PCB design, the formula for stray capacitance becomes: C= ϵA/D, which means the relationship between the capacitance value and the dielectric constant of the insulator, the area, and the distance between conductors.
Factors affecting the number of PCB stray capacitances
There are three factors in capacitor construction that determine the amount of PCB stray capacitance produced. These factors all affect PCB stray capacitance by affecting the amount of electric field flux produced by a specific amount of electric field force (the voltage between any two boards).
1. Board spacing
Keeping all other factors constant, the greater the spacing between the boards, the smaller the stray capacitance.
On the other hand, smaller board spacing creates more stray capacitance.
Tighter board spacing results in higher field forces, and for any particular voltage applied to the two boards, this results in relatively high field flux (charge accumulation on both scales).
2. Board area
All other variables are constant, a larger board area gives more stray capacitance, while a smaller board area gives less stray capacitance.
3. Dielectric materials
Holding all other variables constant, a higher dielectric constant of a dielectric material will produce a larger stray capacitance, while a lower dielectric constant of a dielectric material will produce a smaller stray capacitance.
Relative permittivity represents the dielectric constant of a material, which is close to that of a vacuum (pure).
For example, the standard dielectric constant of glass with a relative dielectric constant of 7 is 7 times that of vacuum. Therefore, when all other variables are equal, it will result in an electric field flux that is 7 times stronger than a pure vacuum.
How to reduce stray capacitance in PCB design?
In many applications, stray capacitance between multiple signals can deplete or affect the entire design.
At lower frequencies, stray capacitance is often negligible. At high frequencies, stray capacitance may be a major problem in the circuit. We can minimize stray capacitance during layout.
Stray capacitance is usually caused by electrical coupling between a signal line and another signal line or between a substrate and a signal line.
Here are some ways to reduce PCB stray capacitance:
1. Control the leads of electronic components and keep them relatively short
Keeping electronic components leads very short and grouping components in a way that eliminates capacitive coupling can reduce the generation of PCB stray capacitance.
Stray capacitance can be used to block or block low-frequency signals. This is because capacitors or devices that act as capacitors have high impedance to low-frequency signals. Therefore, it is difficult for low-frequency signals to pass through a circuit with capacitive characteristics. When you add unnecessary capacitance to a circuit, the circuit can prevent low-frequency signals from passing through. If it is a wireless circuit or an audio circuit, the entire circuit frequency range may be blocked.
Therefore, the inductor leads must be kept short (the ideal length is less than 1.5mm) to effectively prevent stray capacitance and produce capacitive effects, which will limit the inductor’s ability to pass low-frequency signals.
2. Increase the spacing between components
It is important to increase the spacing between components, traces, or cables to reduce stray capacitance.
Because the stray capacitance is inversely proportional to the distance, the larger the distance, the smaller the stray capacitance, and the smaller the distance, the larger the stray capacitance.
3. Add shielding conductor
Place another reference signal, a shielded conductor, between the various networks with low requirements. For example: add a grounded copper strip between adjacent traces.
The copper tape acts as a shield to prevent charge accumulation, thereby reducing the generation of stray capacitance.
4. Reduce trace width
As the cross-sectional area of a conductor increases, stray capacitance increases, thus reducing trace width, especially for traces that conduct high-frequency signals.
5. Remove the inner ground plane
A large inner ground plane may be great for thermal management and EMI control, but it does little good for stray capacitance.
If stray capacitance is to be mitigated, it is recommended to remove the inner ground plane.
6. Avoid excessive parallel wiring of metals
Stray capacitance is the unintentional manifestation of electric charge in a circuit or non-capacitive components. Much like a stray dog or cat, stray capacitance just happens to be where it is due to the circumstances. It is quite easy for stray capacitance to materialize on a circuit.
1. Board spacing
2. Board area
3. Dielectric materials
1. Control the leads of electronic components and keep them relatively short
2. Increase the spacing between components
3. Add shielding conductor
4. Reduce trace width
5. Remove the inner ground plane
6. Avoid excessive parallel wiring of metals
