Capacitor Selection in High-Speed ​​and High-Density PCB Design

Capacitors are one of the basic components in electronic circuits
and have important and extensive uses. According to application classification, most capacitors are often divided into four types: AC coupling, including bypass (passing AC and blocking DC); decoupling (filtering AC signals or filtering
high-frequency signals superimposed on DC signals or filtering low-frequency components in power supplies , reference power supplies and signal circuits); active or passive
RC filtering or frequency selection networks; analog integrators or sample-and-hold circuits (capturing and storing charges). There are many types of capacitors and
many classification methods. According to different manufacturing materials and processes, the following types are commonly used: NPO ceramic capacitors, polystyrene
ceramic capacitors, polypropylene capacitors, polytetrafluoroethylene capacitors, MOS capacitors, polycarbonate capacitors, polyester capacitors,
monolithic ceramic capacitors, mica capacitors, aluminum electrolytic capacitors, tantalum electrolytic capacitors, etc. These capacitors have
their own characteristics to meet different application needs.
Now high speed and high density have become one of the important development trends of electronic products. Compared with traditional PCB design, high-speed and high-
density PCB design faces many new challenges and puts forward many new requirements for the capacitors used. Many traditional capacitors
can no longer be used in high-speed and high-density PCBs. This article combines the basic characteristics of high-speed and high-density PCBs to analyze the main parasitic parameters and their influences of capacitors in high-frequency applications
, points out some traditional understandings or practices that need to be corrected or abandoned, summarizes
the basic characteristics of capacitors suitable for high-speed and high-density PCBs, and introduces several new developments in capacitors suitable for high-speed and high-density PCBs.
1 The influence of parasitic parameters when capacitors are used at high frequencies
A large number of theoretical studies and practices have shown that high-speed circuits must be designed as high-frequency circuits.
The basic requirements for capacitors used in high-speed and high-density PCBs are good high-frequency performance and small space occupation. Actual capacitors have parasitic parameters. For
capacitors used in high-speed and high-density PCBs, the influence of parasitic parameters is particularly important, and many considerations are based on reducing the influence of parasitic parameters
.
Actual capacitors have many parasitic parameters, and the main parasitic parameters are equivalent series resistance RS and equivalent series inductance LS.
When analyzing a circuit, for simplicity, the simplified capacitor equivalent model shown in Figure 1 is usually used. It is generally believed that RS is
composed of the capacitor pin resistance and the equivalent resistance of the two plates of the capacitor in series, and LS is composed of the capacitor pin inductance and
the equivalent inductance of the two plates of the capacitor in series.
The equivalent impedance of the capacitor is

So that the capacitor has the smallest possible impedance and the highest possible self-resonant frequency.

It can be seen that in the design of high-speed and high-density PCB, when selecting and applying capacitors, some traditional cognitions
and practices need to be corrected or abandoned. The actual application effect of the capacitor depends not only on its own performance, but also on the application design and
the specific situation on the PCB. Only by considering these factors comprehensively can the conclusions drawn accurately reflect
the role of the capacitor in the circuit.
It may be difficult to make theoretical analysis for the more complex application design of the capacitor. UltralCAD ​​Design provides specialized
computer-aided analysis software that can solve this problem well.
2 Basic characteristics of capacitors suitable for high-speed and high-density PCB
In the design of high-speed and high-density PCB, although different specific applications have different specific requirements for capacitors, most of them
require capacitors to have the following basic characteristics.
2.1 Chip-type
The parasitic inductance of chip capacitors is almost zero, and the total inductance can be reduced to the inductance of the component itself, which is usually only
1/3~1/5 of the parasitic inductance of traditional capacitors. The self-resonant frequency can reach 2 times that of leaded capacitors with the same capacity (some data say
it can reach 10 times). Therefore, the capacitors used in high-speed and high-density PCBs are almost all chip capacitors.
2.2 Miniaturization

The package size of chip capacitors has developed from 1206 and 0805 to 0603, 0402, 0201, and the mainstream has transitioned from 0603 to
0402. Murata Manufacturing has already produced 01005 miniature capacitors [8]. Miniaturization not only meets
the need for high density, but also reduces the influence of parasitic parameters and distributed parameters.
2.3 High frequency
The speed of many modern electronic products is getting higher and higher. The clock frequency of computers has increased to hundreds of megahertz or even gigahertz,
the frequency of cordless phones has increased from 45MHz to 2400MHZ, and the frequency of digital wireless transmission has reached more than 2GHZ. Therefore, the noise caused by the signal and its higher
harmonics also appears in a higher frequency range, which puts higher and higher
requirements on the high-frequency performance of capacitors. The self-resonant frequency of Vishay Intertechnology's silicon-based surface-mount RF capacitors has reached
13GHZ [9]. The self-resonance frequency of miniaturized chip microwave single-layer ceramic capacitors (SLC) has reached 50 GHZ [10].
2.4 Multifunctionality
Combining capacitors with other components in a package (many have been realized in chip form) not only realizes multifunctionality, but also saves
PCB area and is easy to use. Murata Manufacturing has developed three-terminal chip capacitors containing resistors NFR series
, three-terminal chip capacitors containing inductors
NFW series, and three-terminal chip capacitors containing two magnetic beads NFL series based on three-terminal chip capacitors (stacked chip feedthrough filter capacitors)
[8]. Syfer Technology integrates two Y capacitors and one X capacitor to form a
stacked chip X2Y capacitor component, which can suppress common-mode and differential-mode noise at the same time. Its package size is 2012 (0805) and 3216
(1206), which is used for DC power supply filters [11]. AVX has carefully designed the internal circuit of the multilayer chip-type through-hole filter capacitor
, transferring 70% of the parasitic branch inductance into the series inductance on the input/output line, which plays
the role of a T-shaped low-pass filter, thereby significantly increasing the self-resonant frequency, widening the bandwidth of noise suppression, and improving the strength of noise suppression. The
company has also developed a new material that uses multilayer technology to solve the RC combination problem, avoiding
the complex process of ceramic film-silver electrode-ruthenium series resistor film co-firing, and developing a series of components called Z products, such as RC components, RCR low-pass filters
and their arrays [12].
3 Several new developments in capacitors suitable for high-speed and high-density PCBs
3.1 Taking into account several aspects of performance
The development of some capacitors pursues several aspects of performance while taking into account the application needs of high-speed and high-density PCBs.
Vishay Intertechnology has launched the industry's first surface-mount RF
capacitor with a package size of 0603 and based on silicon wafers - HPC0603A [9]. Developed based on Vishay's proprietary semiconductor
process, this capacitor has a construction that reduces parasitic inductance. Compared with traditional RF capacitors, the self-resonant frequency of this capacitor is 2 to 3 times higher. The high-performance, high-precision
HPC0603A has a capacitance range of 3.3 to 560pF and a self-resonant frequency of up to 13GHz.
The HPC0603A, which provides E12 values ​​in this range, can operate stably in the frequency range of 1MHZ to several GHZ, with a parasitic inductance of only 0.046nH. The capacitor
has a Q factor of 4157, a tolerance of ±1% or 0.05pF, and a low equivalent series resistance. The HPC0603A has an area of
​​1.60×0.80mm2 and a height of 0.56mm, and is available in 6V, 10V, 16V and 25V voltages.
The high capacitance range and relatively small package of the HPC0603A improve the circuit Q, transmission range and reliability. The unique structure of the HPC device
reduces parasitics caused by shortened interconnect lines on the PCB and improves circuit performance by shortening the distance between components
. This innovative design significantly increases the self-resonant frequency of the capacitor.
3.2 Highlighting individual performance aspects
Some capacitors are developed to pursue outstanding performance in individual aspects to meet the special application needs of certain high-speed and high-density PCBs.

Since current integrated component technology cannot produce capacitors with larger capacitance, it is very difficult to obtain larger capacitance through integrated circuits
using existing technology , so passive component suppliers continue to develop smaller packages for discrete components. Murata
Manufacturing has begun to produce micro capacitors with a package size of only 01005 [8].
This capacitor is so small that it is almost invisible to the naked eye, and the area and volume occupied by the PCB are 50% and 70% smaller than those of 0201 capacitors, respectively.
The company's 01005 capacitor code is GRM102, the capacitance range of the COG series is 2 to 15pF, and the capacitance range of the X5R series
is 1000 to 10000pF. It is also reported that
the capacitance range of Samsung Electro-Mechanics' COG series 01005 ceramic capacitors is 1 to 10pF, and the capacitance range of the XR5 series is 1000 to 4700pF.
3.3 Improvement of traditional capacitors
Use relevant new materials and new processes to improve some traditional capacitors, fundamentally overcome their main shortcomings, give full play to
their advantages, and meet the application needs of high-speed and high-density PCBs. The most representative is the aluminum electrolytic capacitor. Solid chip aluminum electrolytic capacitors are developed
using organic semiconductor materials such as TCNQ (1S/cm) and conductive polymers such as polypyrrole (120S/cm) as cathode materials. Since the new cathode material has a much higher conductivity than the traditional electrolyte (below 10-2S/cm), the new aluminum electrolytic capacitor not only achieves chip-type, but also overcomes the shortcomings of poor temperature and frequency characteristics of traditional aluminum electrolytic capacitors, achieving impedance-frequency characteristics close to ideal capacitors , making the electrical performance and reliability of aluminum electrolytic capacitors qualitatively improved, and greatly broadening the application range of aluminum electrolytic capacitors [13]. 3.4 Capacitors that can be packaged in chips The development of capacitors that can be packaged inside large-scale integrated circuits (LSI) is also one of the important development directions of capacitor technology. ALPS Electric is working with North to develop a technology for packaging high-capacitance thin film capacitors in the internal base of LSI packages . Experts believe that this technology is essential for high-speed logic LSIs operating at frequencies above several GHz to 10 GHz [14]. The technology developed this time is to package the decoupling capacitors that were previously packaged outside the LSI package inside. This will minimize the distance between the capacitor and the flip chip. Since the parasitic inductance of the enclosed internal wiring is reduced, the flip chip can be quickly supplied with charge when switching , resulting in a more stable power supply voltage. It is expected that this technology will soon become practical. This type of technology also brings new concepts and conditions to high-speed and high-density PCB design, which deserves full attention. 4 Conclusion High-speed and high-density PCB design technology continues to develop, and the performance requirements for the capacitors used are getting higher and higher; with the continuous advancement of capacitor technology, new capacitors continue to emerge; and research on capacitor application technology for high-speed and high-density PCBs continues to deepen. All these make it difficult to properly select capacitors in high-speed and high-density PCB design. Although there are many types of capacitors, there are usually only one or two most suitable for a specific application. It is necessary to fully understand the characteristics of high-speed and high-density PCB and the high-frequency characteristics of capacitors, keep abreast of relevant new devices and technologies, and comprehensively consider factors such as specific application needs, technical difficulty, and economic cost to properly select capacitors. The discussion in this article has a certain guiding role in the selection of capacitors in high-speed and high-density PCB design.