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?
- Difference between stray capacitance and parasitic capacitance
- Calculation of PCB stray capacitance
- Factors affecting the number of PCB stray capacitances
- How to reduce stray capacitance in PCB design?
- Trend of stray capacitance
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.
Difference between stray capacitance and parasitic capacitance
Stray capacitance and parasitic capacitance both refer to unintended capacitance in electronic circuits, but they have distinct characteristics and implications:
1.Stray Capacitance: This term describes the unintentional capacitance that occurs between circuit elements or between a circuit and its surroundings due to the physical layout of the PCB or wiring. Stray capacitance usually arises from the proximity of conductive traces or components, and it can affect signal integrity and introduce noise. For example, two closely spaced traces on a PCB might develop stray capacitance, which could affect high-frequency signal performance by causing unwanted coupling between signals.
2.Parasitic Capacitance: Parasitic capacitance is a more general term that includes all types of unintended capacitance within a circuit, including stray capacitance, but also extends to capacitance inherent in semiconductor devices, such as between a transistor’s gate and its source or drain terminals. Parasitic capacitance affects the behavior of electronic components and circuits by influencing switching speeds, signal propagation delays, and overall circuit performance. It is an important consideration in the design of high-speed and analog circuits where even small amounts of capacitance can have significant effects.
Key Differences:
Source: Stray capacitance typically originates from the physical layout of circuit elements and their proximity, while parasitic capacitance can come from both the physical layout and the inherent properties of electronic components.
Impact: Stray capacitance often affects signal integrity and can introduce noise, whereas parasitic capacitance can impact the overall performance and behavior of the circuit, particularly in high-speed or sensitive applications.
Both types of capacitance need to be managed in circuit design to ensure optimal performance and minimize undesirable effects on circuit functionality. Proper PCB layout, component placement, and careful design considerations are essential to mitigate the impact of both stray and parasitic capacitance.
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
Trend of stray capacitance
The trend of stray capacitance in modern electronics has become increasingly significant due to advancements in technology and miniaturization. Here’s a breakdown of how stray capacitance trends are evolving:
1.Miniaturization: As electronic devices shrink and components become smaller, the physical distance between conductive traces and components decreases, which can increase stray capacitance. This is particularly notable in high-density PCBs where closely packed traces and smaller components result in higher levels of unintended capacitance. Designers must carefully manage these effects to maintain circuit performance.
2.High-Frequency Design: With the rise of high-speed digital and RF (radio frequency) applications, stray capacitance becomes more critical. High-frequency signals are more susceptible to the effects of stray capacitance, which can lead to signal degradation, crosstalk, and interference. Engineers need to use advanced design techniques, such as differential signaling and shielding, to mitigate these issues.
3.Increased Power Density: Modern electronics often feature higher power densities, meaning more power is packed into smaller areas. This can exacerbate the effects of stray capacitance, leading to potential issues with heat dissipation and signal integrity. Effective thermal management and layout strategies are crucial to address these challenges.
4.Advanced Materials and Technologies: The use of advanced PCB materials, such as high-frequency laminates and low-loss substrates, can help manage stray capacitance by providing better control over dielectric properties. Additionally, new technologies, such as embedded capacitors and advanced shielding techniques, are being developed to mitigate stray capacitance in complex circuits.
5.Design Tools and Simulation: Modern PCB design tools offer sophisticated simulation capabilities that allow engineers to model and analyze the effects of stray capacitance more accurately. This helps in identifying potential issues early in the design process and implementing solutions to minimize its impact.
The trend towards smaller, faster, and more powerful electronic devices has heightened the importance of managing stray capacitance. Engineers are increasingly focused on optimizing PCB layout, using advanced materials, and leveraging simulation tools to address these challenges and ensure reliable circuit performance.
