Mathematics and science follow a simple rule of precision and accuracy. They mainly give accurate measurements of events, actions, and items. It still leaves a person thinking; therefore, it needs key consideration while designing, manufacturing, and analyzing the physical structure. The uncertainty principle of Heisenberg is found in most mathematical, scientific, and engineering projects. However, this principle states that the speed and position of an item cannot be measured at the same time. Therefore, scientists and engineers are still looking for solutions to measure them altogether.
Determining an ideal value of impedance plays a vital role in designing a PCB, especially a high-speed PCB design. This gives the definition of trace parameter control and the choice of board material. Exclusively calculating and analyzing the impedance of a PCB brings various challenges. However, the impedance control significance of a board makes it necessary to use a reliable and durable method to give efficient results. Thus, let’s dive into some available options and find a solution for calculating the impedance control of a PCB, which gives practical and accurate results.
Methods of PCB Trace Impedance Calculator
Propagation of signals in the layout of PCB occurs in the transmission line. But this task seems quite challenging. The circuits of today comprise compact, multifunctional, and complex designs. This literally implies that a PCB has to encounter various types of signals like DC, RF, and digital, distinct trace types like stripline and microstrip with variant orientation and size. However, there are some considerations to follow, which include clearance and creeping, edges of the board, design of footprint pad, and spacing and width of the trace. Moreover, there may add up more concerns like matching the impedance of output and input connections and the type of material that may affect the impedance.
Thus, managing these considerations may end up in unmanageable situations. Besides these parameters, it may also include the goals of signal integrity maximization, removing reflection, and reducing the EMI or noise. Other than managing these parameters, the impedance control of PCB still comes first and needs serious interest and a determined approach. To make calculations simple, it’s important to deduct pre-hypothesis. This is associated with each design’s uniqueness which implies that it gives various possibilities for measuring the impedance control of the board. Eventually, the result you will get in the end may vary to a certain aspect because of different techniques. Anyhow, the methods described below are fully practical and applicable.
Methods for Calculation
· Smith Chart
A Smith chart is one of the oldest methods. It came into being ages before PCB layout software and computers were developed. Its main purpose is to determine the relation of the reflection coefficient between load and source impedance. It then uses a mathematical manipulation series to get the value of impedance for a trace or line.
· Computer Simulation
The advanced design software of PCB packages contains a feature of impedance control calculation. It gives benefits: you can access the available design parameter and can modify it automatically to perform different simulations. This way, you can choose an ideal design for PCB manufacturing. However, the only drawback of such type tools makes the software much more expensive.
· Online Calculator
Online calculators prove an ideal than software with design packages of PCB for control impedance. As online calculators offer features to calculate impedance control or parameters of trace with desired impedance. Although it does not act as completely as simulation software, these may place you under the ballpark, which needs a little but necessary tweaking from your manufacturer for the PCB manufacturability. Most online calculators follow the principle of the Design Guide of standard IPC-2141A for Controlled Impedance of PCB with High-Speed.
How Does PCB Trace Impedance Calculator Works?
Each trace possesses a small, almost unnoticeable series of inductance spread alongside the trace, which has a proportional relationship with a trace cross-sectional area. The impedance becomes noticeable as the time of rise increases. In the same manner, each trace also has capacitance alongside the return signal path and the trace. This serves as a function of trace width and material of dielectric between the trace and the return path of the signal. Here again, if the time of rise increases, the generated impedance becomes significant as the electrical current flies across this capacitance.
The drivers interpret traces in the form of transferred LC circuits and the AC impedance of the trace generated from this dispersed LC circuit. This refers to uncontrolled impedance. However, manufacturers do not try to design the environment for the trace for this very impedance. This allows the capacitance and the inductance to change along with the trace, as this impedance does not usually affect the operations, so you don’t have to spend any money or time designing methods to manage it.
· Controlled Impedance
However, a question arises: what if the impedance affects the operations? In this situation, design a circuit where traces resemble the transmission lines. This allows us to terminate the trace capability of impedance to bypass the reflections.
While controlling impedance, ensure that the trace has constant impedance at each end rather than changing from time to time, just like in a standard uncontrolled impedance case. Impedance control requires three parts of the geometry of the circuit to control. This includes trace width, the coefficient of dielectric of the substance around the trace, and the value of spacing between the trace signal and the return path signal. For instance, the coaxial cable serves as a transmission line with impedance control.
You can also alter these geometric aspects and get controlled impedance only as far as you keep altering other aspects as necessary. So that the relation between these features does not modify and the value of impedance stays constant.
How To Confine Trace Impedance with PCB Trace Impedance Calculator?
The process of manufacturing circuit boards with controlled impedance must have the ability to calculate the impedance. The impedance trace calculator of PCBs helps in calculating the trace impedance. You can access these calculators in the CAD software or online. There are various factors to consider while calculating the impedances. This includes trace thickness, trace width, laminate thickness, copper weight, and dielectric thickness.
Once you get the values of all the parameters, adjust them to a certain level to calculate the impedance. Once you reach a desirable level of impedance, test the effectiveness of the board through test coupons. These coupons are built altogether in the same plane. This way, you get an appropriate value of impedance without facing any trouble while accessing actual board traces. It gives an accurate value if the traces of the test coupon and the traces of the boards are similar.
Generally, the fabricator creates the test coupons over the edges of the manufacturing panel so that you can get the authentic value of impedance through these test coupons without defecting the circuit.
The trace impedance can also be calculated through a network analyzer, TDR, or test system of controlled impedance with TDR techniques. A specialized and skilled engineer uses test systems of impedance to measure the impedance, which ensures high-end results.
How Does Design and Layout Affect Trace Impedance?
In an actual PCB design, signal impedance can be quite distinct from the ideal desired value from the design schematic. This occurs because of the substrate’s presence and trace arrangement on the circuit. This increases some significant effects, such as crosstalk. This alters the value of impedance from the desired value. PCB may experience problems of power integrity, like ringing when a PCB shifts high frequencies. The power-delivered net impedance also turns from the desired capacitance behaviors of PCBs at higher frequencies. This takes part in the integrity of power and potential signal problems.
· Transmission Line Impedance
The transmission line impedance is categorized through the values of impedances. Characteristics impedance plays one of the essential. This simply refers to the transmission line impedance of PCBs in complete isolation from other transmission lines. The value typically equals 50 ohms. Though, it takes distinct types of values based on the standard of signaling in a device. For instance, LVDS mainly features that the impedance differential of distinct parts must equal 85 Ohms.
Some other metrics that define the impedance of transmission lines mainly depend on the alignment of two distinct transmission lines. Because of the capacitance of parasitic effect which arises due to circuit substrate material and the shared inductance between 2 close transmission lines. Transmission lines are also categorized with odd and even values of impedance. Differential and common impedance is linked with other values, which brings the complete value of impedance that defines the transmission line up to five.
· Power Delivery Network Impedance
The network of power delivery may show the impedance capacitive at a very low frequency, which decreases power bus resistance in the series along with the return path of ground and load components at DC. The physical spacing between the traces, internal planes, and power planes in the circuit dominates this impedance. However, as the frequency of driving increases, the mutual inductance inside the circuit enhances the power delivery system’s impedance. Ultimately, the power delivery net impedance may exhibit various peaks at a high frequency.
Ideally, the power delivery net impedance must be flat inside the band that you will work with. The bandwidth depicts all the frequencies between a knee frequency and a clock rate for digital. Moreover, if the harmonics that contain digital signals read a similar frequency, then a function of transfer for the return signals in the ground layer must be flat. Therefore, the same approach applies to the analog signals which travel throughout the ground planes and the board.
Whereas the spectrum of impedance becomes essential for determining the bandwidth that possesses minimum impedance of power delivery. The spatial impedance distribution in the ground planes plays a more important role, especially in devices with mixed signals. Signals pass through the path which has the least reactance on the return ground when passing through ground planes. Furthermore, the least reactance path in multipoint topology, point-to-point, and a star must lie under the conductors of the circuit. This ensures that the circuit must have a minimum inductance of the loop and have less EMI susceptibility.
· PCB Substrate Material Selection
Because of the parasitic interference, you have to select the PCB substrate material carefully and design the stack-up. The substrate material has a dielectric constant which affects the design geometry to create transmission lines alongside specific impedance. This directly affects the net power delivery impedance. The conductors inside the planes also define the impedance of the loop in a PCB, which directly affects the EMI susceptibility of the circuit.
· Impedance Matching Networks
The PCB stack-up affects various features of the design circuit. This includes routing strategy and thermal resistance. PCB design with appropriate substrate material can help in reducing the risk of loss of signals while managing the constant impedance across the circuit. The maintenance of PCB impedance to a certain value during the process of routing is essential. It ensures the matching of impedance throughout the net. Once the signals transfer to a transmission line mode, make sure that the drive, receiver, and transition line must have a constant impedance to avoid the reflections of signals.
Conclusion
Impedance control refers to a method of regulating PCB trace strength by calculating its impedance. The control of impedance has become the standard practice of PCB because of the frequently altering frequencies of high-tech devices and applications.
Impedance control can be measured by utilizing the test coupons after the manufacturing of the PCB. Test coupons serve as a PCB which gauges the efficiency of the manufacturing process of PCBs. However, test coupons also make on the same line as a PCB, typically on the edges. After the production process, test coupons go through an evaluation process that checks the alignment of layers, internal structure, and electrical connectivity. You can choose coupons from the collection of suppliers or specially created for the PCBs. PCB manufacturers can also design test coupons or can place them on functional panels.
