Capacitors are important components on printed circuit boards. They impact the functionality and quality of a circuit. Hence, its importance can’t be overlooked. Also, capacitors are commonly used in electronic circuits as they can block direct currents. This electronic component is popular among applications like power circuits and electronic circuits.
Furthermore, the capacitor is a passive component that forms the basics of electronic circuits. In PCB manufacturing, capacitors have been a game-changer for a long time.
What is a PCB Capacitor?
A PCB capacitor is a core electronic component on a printed circuit board. This electronic component saves energy and discharges it into a circuit. A capacitor is indispensable in PCB. Therefore, it passes an alternating current but doesn’t pass a direct current.
This electronic component is one of the most common passive components in a circuit board. PCB Capacitance measures the amount of energy or charge a capacitor can carry. Typically, a capacitor features two conducting plates. An insulator separates these two conducting plates. Capacitors are available in different types. Each of which comprises a different dielectric material.
Capacitors play significant roles in several applications. They shield microchips from noise. Circuits featuring capacitors have a frequency-dependent behavior. Two metal plates separated by an insulator forms a capacitor. The two metal plates are on both sides of the insulator. This insulator is known as dielectrics.
A capacitor helps in storing energy in an electric field. This component doesn’t dissipate energy. It rather stores energy electrostatically between its metal plates. In addition, a capacitor is also referred to as a condenser. You can liken a capacitor to a battery. This is because they both store electrical energy. The capacitance is the amount of charge moved in the capacitor.
Types of Capacitors
Capacitors are available in different sizes and shapes. The dielectric used between the metal plates differentiates these capacitors.
Ceramic capacitor
This type of capacitor features coating on the two sides of the ceramic disc. Ceramic capacitors store a smaller charge. However, it leaks less current. These capacitors feature ceramic as their dielectric. The capacity of this component changes as there is voltage. Also, one of the properties of a ceramic capacitor is its high dielectric constant. Ceramic capacitors always feature a 3-digit code on their body. This helps to identify the capacitance value in pico-farads.
Film capacitor
This is the most common capacitor used in circuits. It comprises polypropylene, Teflon, polyester, and more. Film capacitors are also available in different case styles and shapes. These capacitors feature Teflon, polycarbonate, or polystyrene as their dielectrics. These capacitors feature no dielectric loss and have high insulation resistance. Also, they feature good temperature characteristics.
Electrolytic capacitor
This type of capacitor is available on circuit boards. Electrolytic capacitors look like small tin cans. They can hold much electric charge in their tiny bodies. This is the only polarized capacitor. This means that they can only function when wired in a specific way.
Electrolytic capacitors feature a positive pin, anode, and a negative pin, cathode. You connect the anode to a greater voltage. If the anode is wired up the other way, there will be an explosion. Electrolytic capacitors are available on a circuit when large capacitance values are necessary. The dielectric of this capacitor is a thin layer of oxide.
Supercapacitors
These capacitors can hold a high amount of energy. However, these capacitors can’t handle too many voltages very well. Supercapacitors release their energy at once.
Dielectric capacitor
This capacitor requires a continuous capacitance variation for tuning transistor radios.
How do PCB Capacitors Work?
Generally, PCB capacitors have two major functions. They store electric charge and allow alternating current to flow while withholding direct current. Also, PCB capacitors charge and discharge electricity in an electric field. So, how do they work?
Capacitors start working with a charge. Electric current flows into a capacitor and stays in the first plate. Why does it remain on the first plate? It remains there because the insulator won’t allow negative charge in. As more electrons remain in the first metal plate, the plate becomes negatively charged. This plate pushes away the excess electrons to the other plate. The second plate will now be positively charged
The two metal plates continue to charge. However, the positive and negative electrons would try to come together. Then, the insulator in the middle of the two plates prevents this from happening. The capacitor will continue storing electric charge. This is because there is continuing tension between the negative and positive sides of the metal plates.
At a point, the metal plates will no longer be able to store a charge. So, what happens at this point? If your PCB has a path for flow of electric charge, all the electrons in the capacitor will discharge.
Importance of Capacitors on PCB
Capacitors play a significant role in PCBs. These electronic components function beyond storing electrical energy between metal plates. PCB capacitors offer the following benefits.
Keep voltage at the same level
Capacitors on PCBs help to keep the voltage at a particular level. These electronic components minimize voltage pulsation. The capacitor charges when the voltage is on the parallel circuit. Furthermore, the electricity that flows out of a capacitor is an alternating current. Since most electronic circuits work with direct current, capacitors convert AC to DC. Capacitors offer a voltage boost in a circuit.
Remove noise
When it comes to noise reduction, the capacitor is one component that helps to achieve that. Capacitors interrupt DC and enable the flow of AC. The DC in a capacitor helps to eliminate noise.
Charges and discharges energy
Due to their structures, these components can charge and discharge energy. This is the greatest benefit of capacitors on PCB. Also, capacitors store electric charge onto the two metal plates. This makes the charging and discharging process quicker.
Other benefits of capacitors
- They discharge current almost immediately
- Capacitors are capable of filtering unwanted frequencies
- They are a better option for AC applications
- These components can handle high voltage applications
- Capacitors function well in varying temperatures
- They are capable of handling power loss
Factors to Consider when Choosing a PCB Capacitor
Capacitors are available in different types. It is very important to know the type that suits your application. There are certain variables to consider when choosing a capacitor for a PCB.
Leakage current
Capacitors leak out their stored energy. This leakage drains slowly. Therefore, you will have to consider how much current a capacitor leaks. This is very important, especially when storing energy is the primary function.
Tolerance
These electronic components feature a variable tolerance. Normally, the tolerance for these components ranges between ±1% to ±20%. Therefore, it is important to consider the tolerance value of a capacitor before you opt for it. The tolerance value describes how the capacitance can vary from the nominal value.
Capacitance
This refers to the electrical characteristic of a capacitor. It is the first variable to consider when choosing a PCB capacitor. How much capacitance does your circuit need? However, this depends on the requirements of your application. Therefore, if an application requires filtering a rectified voltage, a big capacitance is necessary.
Voltage rating
Voltage stress will cause damage to a capacitor. Therefore, it is important you take voltage rating into consideration. It is essential to know the voltage level of the area you are installing the capacitor. Most times, capacitors are installed parallel to a circuit.
Operating temperature
This factor has to deal with environmental factors. When incorporating a capacitor on PCB, you need to consider the operating temperature. That is, the environment temperature, the PCB will work under. If a circuit will function in an environment temperature of 100°C, avoid using a capacitor rated at 85°C.
Life expectancy
This is the length of time a capacitor will remain healthy. It is important to consider this when choosing a capacitor.
Measuring PCB Capacitance
PCB capacitance measures how much energy is in a capacitor. Every capacitor can hold a particular amount of capacitance. Farads represent capacitance unit. A farad holds a ton of energy. Typically capacitance can be in microfarads or picofarads.
So, how do you determine the amount of electric charge a capacitor is currently holding? Here Q=CV. The Q represents the total charge. C represents the capacitor’s capacitance and V represents the voltage. Therefore, Q=CV. That is, Capacitor’s capacitance multiplied by voltage equals total charge. It is important to note that a capacitor’s capacitance has a direct correlation to its voltage. Therefore, a decrease or increase in the voltage source will reflect in the capacitor.
Capacitance is an expression of the ratio of electric energy. The capacitance value is between 1 pF (10−12 F) to about 1 mF (10−3 F). The capacitor’s capacitance is higher when the surface area of conductors is larger.
What is PCB Parasitic Capacitance?
Parasitic capacitance is the effect of a virtual capacitor between two traces. A dielectric material separates these traces. Parasitic capacitance happens as a result of the possible difference produced when traces are close.
You can calculate parasitic capacitance as C= q/v. q represents the charge in coulombs. C is the capacitance while v represents the voltage. Therefore, the effect of parasitic capacitance is a major problem in high frequency PCBs. Each pad in a PCB has its parasitic capacitance. Each trace features parasitic inductance.
Parasitic capacitance can occur between conductors on PCBs and within surface mount devices. Furthermore, parasitic capacitance is different from stray capacitance, although people use them interchangeably. Parasitic capacitance deals with the fact that it will hinder the operation of a circuit. Stray capacitance is primarily concerned with how a circuit introduces unwanted capacitance.
How to reduce parasitic capacitance in circuit board layout
Capacitors prevent low-frequency and allow high-frequency signals in circuits. The ability of capacitors to allow high-frequency signals is one of the reasons they are widely used. This capability causes stray capacitance issues in high-frequency circuits. The stray capacitance can result in noise or EMI. It is not possible to get rid of stray capacitance. However, there are ways to reduce it during PCB layout.
- Don’t use excessive vias
Vias help to connect different layers of a circuit board. However, excessive use of vias can maximize the capacitance. You can minimize PTH coupling by reducing annular rings surrounding vias.
- Avoid parallel routing
Parallel routing enables maximum area between the two metal plates. This results in maximum capacitance between these metals.
- Separate component carefully
Separating components, power planes, and wires carefully helps to reduce parasitic capacitance. Also, the appropriate removal of transmission lines can be helpful.
- Moating
Power planes function in a similar way as a ground plane. Therefore, removing the power planes from the surrounding of a conductor is important.
- Impedance matching
There is an impedance mismatch in high-speed digital applications. Any mismatch as a result of parasitic will generate reflections on the line. Ensure you match impedance throughout the signal lines transmitting high-speed data.
- Incorporate low permittivity dielectric material
The higher permittivity of a dielectric material results in higher stray capacitance. Low permittivity results in low stray capacitance. Therefore it is ideal to use dielectric material with low permittivity.
Frequently Asked Questions
What is a non PCB capacitor?
Non PCB capacitor is a type of component that has no polychlorinated biphenyls. This type of capacitor meets the requirements and standards of the EPA.
Polarized capacitor vs non-polarized capacitor: What is the difference?
A polarized capacitor features both negative and positive sides. Therefore, you have to place it in an area with the most positive voltage. On the other hand, a non-polarized capacitor can be placed anywhere you desire.