Introduction to PCB Impedance Control
Science is the experimental knowledge obtained from the observations. The scientific knowledge is based upon the precision and accuracy. The values in the scientific and engineering works have the precision and accuracy according to the given conditions.
However, there are many uncertainties occurring in the numerical values and experimental data. We have to consider these uncertainties while checking and analysing our values and experiments. In case of the design formation, experimentation and construction of the physical systems, we give attention to certainty and precision.
A common example is the Heisenberg uncertainty principle. This law states that we cannot determine the position as well as speed of a particle simultaneously. In this way, we are unable to find a confirmed value of π.
While designing a printed circuit board, the impedance value of the PCB design is very important. The impedance value of the PCB design gives the control of the parameters that are in trace in the PCB design. Moreover, it also helps in the selection of the specific material for the board.
However, it is quite a challenge to find out and calculate PCB impedance control. It was quite a task when the designers used the Smith charts for the design of the PCBs. Even in today’s age, it is even a difficult task to calculate the PCB impedance.
The PCB impedance is important because it can give us the information about the reliability of the process. In this way, we can obtain the accurate and reliable results. Therefore, in this article, we will dive into the methods to calculate PCB impedance control.
Methods Calculate PCB Impedance Control
The type of the PCB design that relates to the signal propagation is transmission line design. This signal transmission is quite a difficult task. The reason for this difficulty is present secretly within that question. The point is that the circuit boards today are very complex and multifunctional.
These small sized PCB circuits have several types of the signals. The types of the signals confronting in the designs are digital, RF and DC. Moreover, there are many types of the traces in the PCB designs such as micro strip and strip line. In addition, the PCB boards have the different lengths and varying orientation.
There are different factors to consider during the design of the PCB boards. They include the board edge, the spacing of the traces, the clearance of the board and spacing. On the other hand, the input and output connection should match in the impedance. There are several methods for the calculation of the PCB impedance control. Here we will discuss some of them.
Ø Smith Chart
The smith chart is a classical method to calculate PCB impedance control. This method is so old that it was present when computers and PCB design software were not available. This method is best for the calculation of the relationship between the load impedance and source. There are a series of the mathematical calculations that take place and give the impedance value.
This method gives the reflection coefficient relationship in the graphical form. Managing all the factors and components of the PCB is not an easy task. The purpose is to increase the signal strength by decreasing the reflection and amount of noise. Therefore, to calculate PCB impedance control is an important task and involves the certain types of the methods.
In order to simply the calculations, the assumptions play a significant role. Moreover, every PCB board has the unique design and structure. Therefore, we can determine the PCB impedance control with the help of the different types of methods. Smith chart is one of those plenty of methods used in this context.
Ø Computer simulation
In this advanced age, the software for the PCB designs are present in the upgraded and advanced form. These types of software have the function of to calculate PCB impedance control installed in them. These types of the software have the advantageous features that make the process easy for you.
The big advantage is that the parameters essential for the design of PCB are already present in the software. The software can manage and modify these parameters according to the given condition and need. The software automatically performs the different simulations and shows you the various designs. You can select one of them for the manufacturers.
There is a limitation of this method for the calculation of the PCB impedance control. To cover all the features and functions in the software, it can be too much expensive.
Ø Online calculator
This method to calculate PCB impedance control is more common than the above method. The reason is that this method is more flexible than software simulations for the PCB design. The advantage of this method is that you can find the controlled impedance in the online calculators.
These calculators for the calculation of the impedance control also provide the trace parameters that have the impedance that we desire for our PCB design. However, these calculators are not as comprehensive as are the simulation software. But these calculators are useful to put you on the point where you can find the value in the manufacture of your PCB.
All these methods listed above are helpful for the calculation of the impedance control of your PCB design. You can use of them for your PCB design. However, it is advisable to you that you must choose a reliable and relatively simple method for the design of your PCB as well.
A Quick Method to Check PCB Impedance Control
This quick method to calculate PCB impedance control is very helpful and beneficial for your PCB design. This method has the best use if you apply it in the start of your PCB design layout. If you apply this method at the end, then it does not give the better results.
This method is very useful if you want to incorporate the effects among the spaces of the stack up parameters. Moreover, this method is also useful if you want to engrave the effects between the spacing of the surface elements. The management of the dielectric height is the contribution of this method before finalizing the routing of PCB.
The procedure described below will help you to calculate PCB impedance control value with easy steps.
- In the first step, you have to estimate and determine the rise time for the trace for your PCB design.
- In the next step, you should calculate the maximum length of the trace. In this step, keep in mind that the transmission effects should not disturb the process.
- In the final step, determine the impedance value for the trace. The impedance value will come variable according to the type of the trace. Moreover, the impedance value also depends upon the geometry of the trace.
Impedance Control Issues in PCB Manufacturing
Impedance control requirements often accompany the fabrication of complex multilayer PCBs. Understanding the specifics of meeting these requirements helps not only to shorten the start-up time for boards in production, but also contributes to obtaining a better result.
Impedance control at a PCB manufacturing plant can be roughly divided into several stages:
- Selection of the structure, if it is not specified by the customer, and the calculation of the specified thicknesses of dielectrics in accordance with the characteristics of the conductive pattern of a particular project;
- Calculation of the optimal parameters of conductors in accordance with the structure, the real parameters of the materials used and the impedance requirements specified by the customer;
- Coordination of design data with the board developer;
- Making boards and performing testing.
If the board structure is specified in the initial data, then the manufacturer selects the board structure from the materials available in the warehouse that is closest to the specified one. If the board structure is not initially specified, then, based on preliminary calculations, the board structure is selected that provides the smallest deviation of the impedance values actually obtained on the finished board from those specified in the customer’s project.
For the selected board structure, an updated calculation of the width of the conductors and the gaps is performed, at which the impedance values will be provided most accurately. The board structure formed by the plant and the optimal parameters of the width of the conductors and gaps can be agreed with the developer by e-mail.
The Standard Accuracy for PCB Impedance Control
The standard accuracy with which factories control impedance is ± 10%. In some cases, in agreement with the factory, the accuracy of the controlled impedance can be increased to ± 7% or even to ± 5%. When designing and manufacturing boards with impedance control, some designers sometimes want to “strictly” meet their specified impedance requirements.
How to choose the best manufacturing option, and will the expectations correspond to reality?
The impedance is influenced by several factors: the parameters of the conductors, the properties of materials, manufacturing accuracy, and the internal features of the project. Let’s try to consider these factors in more detail.
Factors Affecting Impedance
When calculating the refined prepreg thicknesses in the finished board, the filling of the space between the conductive layer conductors must be taken into account, which is determined by the percentage of copper on the adjacent conductive layers and the foil thicknesses. These values used in the calculation should be close to the real values in the finished board.
The percentage of copper on the conductive layer in relation to the board area can be calculated using a Gerber inspection and processing program such as CAM350 or other programs.
The final thickness of the foil in the finished board depends on its starting (base) thickness, as well as on the location – the outer or inner layer in the board structure.
Key points to remember
Along with the usual PCB specifications, the PCB designer should also specify:
- Which layers contain controlled impedance traces?
- The impedances of the traces since there can be more than one value of impedance trace per layer.
- Separate aperture codes for controlled impedance traces, e.g., 4mil non-controlled impedance trace and 4mil-controlled impedance trace.
Remember, trace Impedance is a critical factor in the effort to control reflections. Impedance must match the driver and load.
If you are passionate and ready for the design and want to manufacture your design, you can try our tools. Our tools will help you in this context and you can upload your CAD files accordingly.