The Role of Embedded Capacitors in PCB Design
Mention capacitance, conductors, and dielectric capacitors to a 7th-grade science class, and you may see glazed-over expressions and hear bored sighs. I mean, hey, sometimes I get this with adults and coworkers I’m talking to so I can’t say I’m unsympathetic to the kids, but I have to promise them that circuits and circuit boards are, truly, fascinating.
To help students visualize how capacitors work, construct a simple capacitor from two sheets of aluminum foil separated by a plastic shopping bag layer. Then, connect the lower foil to earth ground and the negative terminal of a DC power supply and the upper foil to the power supply positive terminal. Increasing the voltage caused the two aluminum foil plates to noisily attract one another.
Flux Capacitors Have Layers to Them as Well
The capacitors that we use in our PCB designs are somewhat different than the capacitor built in a classroom. As we design circuit boards, knowing the properties of capacitor types assists with planning and design. When working with through-hole and surface mounted designs, you can use design libraries to select capacitors based on a combination of attributes that include capacitance, dielectric type, nominal value tolerance, rated voltage, and temperature coefficient.
Through-hole mounting (THM) has taken a backseat to surface mount technology. However, the process of drilling holes and securing component leads either axially or radially through the board adds value for specific applications. Military, aerospace and industrial applications require reliability while subjecting components to mechanical and environmental stress. THM capacitor types include non-polarized ceramic discs, silver mica, and plastic film along with polarized electrolytic and tantalum types.
Surface mount capacitors do not require drilled holes through layers and mount directly to the surface of the PCB. Vias replace the through-hole leads and allow a conductive connection between the layers of a circuit board. SMT capacitors can mount to both sides of the board and have much smaller packages than THM capacitors. The different SMT capacitor types include stacked multilayer ceramic, mica, tantalum, film and electrolytic capacitors.
Signals, Circuit Boards, and Capacitors
At one time, capacitors held court in through-hole mount PCBs. Today, popular capacitors from the past have prominent roles in SMT PCBs. A brief look at a few types of capacitors demonstrates how capacitor technology has moved from the past to the present and dived into the future of PCB design.
Surface mount capacitors offer the benefits of small size, automated assembly, and low spurious inductance. The same characteristics seen with older THM capacitor types provide many benefits for PCB designers working with surface mount technologies.
Ceramic Capacitors: Have a dielectric capacitor made from ceramic materials. More often than not, ceramic capacitors have small capacitance values that range from 1F to 1µF and small maximum rated voltages. The capacitors have excellent frequency response and do not become prone to parasitic effects.
Class 1 ceramic capacitors provide high stability, accuracy, and low loss. A class 1 ceramic capacitor can have a nominal value tolerance in the 1% range. Class 2 ceramic capacitors have higher capacitance values but less thermal stability and less sensitive nominal value tolerances. Large power ceramic capacitors have a maximum 100µF capacity and can handle much higher maximum voltages that range up to 100 kV.
Multilayer Ceramic Capacitors (MLCC) make up the bulk of capacitors used on current surface mount PCBs. Each MLCC consists of interleaved silver palladium or nickel-plated silver electrodes covered with plated tin and interweaved within a rectangular block of ceramic dielectric. The type of dielectric used during the manufacture of MLCCs impacts capacitance and thermal stability and, as a result, the frequency and DC voltage properties of the MLCC.
Capacitors with a titanium oxide/calcium zirconate dielectric have lower capacitance and very stable thermal characteristics. When you work with PCB applications that involve time constant high-frequency circuits, select the low capacitance, high-temperature coefficient MLCC. MLCCs with a Barium titanate dielectric offer the high capacitance needed for power supply smoothing and decoupling. Keep in mind that the high capacitance of these MLCCs balances against the properties of the dielectric. The Barium titanate dielectric changes with voltage input and over time.
MLCC capacitors have low equivalent series resistance. As a result, MLCCs have few problems with heat generated by resistance. The capacitors also provide good ripple resistance capabilities.
Choose the right fit of capacitor for your design
Mica Capacitors: Work as decoupling capacitors and ripple filters in resonance, time constant, coupling, and power conversion circuits. Manufacturers build mica capacitors from sheets of mica with both sides coated with deposited metal. THM and SMT mica capacitors provide excellent reliability, stability, and precision with nominal value tolerances of +-1%, +-2%, and +-5%.
Plastic Film Capacitors: Use different types of dielectric materials that segment the components for specific applications that include filtering, general coupling, and decoupling. Metallized film capacitors, such as mylar and several different types of polyester and polystyrene capacitors consist of a thin layer of metal deposited into a plastic film that connects to each lead. Film foil capacitors such as polytetrafluoroethylene (PTFE) capacitors use plastic film to separate two metal foil electrodes.
Type FCN Capacitors: Have a non-inductive metallized polyethylene naphthalate (PEN) film construction that provides the same stable temperature and frequency characteristics seen with traditional polyester film capacitors. The low ESR seen with Type FCN capacitors yields superior high-frequency performance. As a result, FCN capacitors work for EMI filtering, power supply input and output filtering, signal coupling, and IC power bus bypassing or decoupling.
Type FCP Stacked Metallized Polyphenylene Sulfide (PPS) Film Capacitors: Pack high capacitance values and excellent high-frequency response over a wide temperature range within small SMT packages. As with FCP capacitors, Type FCA acrylic film capacitors offer high capacitance values as well as improved high-frequency filtering and excellent DC power bus noise attenuation. FCA capacitors consist of non-inductive stacked layers of metallized resin film and plated copper alloy terminals.
Aluminum electrolytic Capacitors: Provide higher capacitance than other capacitor types but have very wide nominal tolerance values. Higher capacitance values allow electrolytic capacitors to smooth ripples when used in power supplies and work as coupling capacitors. Due to wide tolerance values and because the equivalent series resistance increases with frequency, electrolytic capacitors do not handle high frequencies. SMT electrolytic capacitors offer high capacitance, low impedance, and high temperature stability. In addition, the surface mount electrolytic capacitors resist vibration.
Tantalum Capacitors: Use a very thin film of oxide on tantalum as an electrolytic. While the layer of oxide covers a small tantalum anode and works as the dielectric, the conductive cathode envelops the dielectric and anode. Although tantalum capacitors do not have the current capacity seen with aluminum electrolytic capacitors, tantalum capacitors offer a combination of high capacitance that ranges from 1µF to 100µF, endurance, and stability. Surface-mount tantalum capacitors provide the same properties along with a wide operating temperature range in small package sizes.
Embedded Components and Circuit Boards Now
Embedded capacitance materials consist of a very thin dielectric material sandwiched between two layers of copper. During the manufacturing process, a coating of epoxy material laminates the copper foils together. Capacitors embedded into the rigid or flex PCB substrate directly below the pin of an IC have extremely short electrical paths that reduce parasitic capacitance and inductance, lower power bus noise and minimize EMI.
Because of the increased capacitive density, embedded capacitors decouple power supplies and facilitate the removal of discrete capacitors. Combining those capabilities with the reduction in size has made embedded capacitors valuable assets for telecommunications, computing, medical, and handheld electronics equipment.
What technologies will feel old in the future?
The need for even smaller product footprints has started a revolution in capacitor technologies. One next step involves the development of high-density, ultra-thin solid aluminum capacitors that offer stability in high-voltage, high-temperature applications and the circuits which power them.
Another type of ultrathin electrolytic supercapacitor works for low power microprocessor and RF applications. Designed as embedded components, the supercapacitors provide extremely long power life for next-generation IoT devices while resisting vibration and shock.
Altium Designer® offers a comprehensive set of tools to optimize your PCB, regardless of the type of capacitor technology you’re dealing with. To learn more about the best practices for incorporating embedded capacitors into your design, talk to an Altium Designer expert today.