In today’s competitive electronics industry, there is a growing need for innovative, efficient printed circuit boards (PCBs). As the industry grows, so does the need for PCBs that can withstand various environmental conditions and meet the requirements of complex electronic devices. This is where the concept of flex rigid-flex PCB comes into play.
Rigid-flex boards offer a unique combination of rigid and flexible materials, making them ideal for applications that require durability and flexibility. These boards are commonly found in medical equipment, aerospace systems, and other high-reliability applications.
Impedance control is a key aspect that greatly affects the performance of rigid-flex boards. Impedance is the resistance a circuit provides to the flow of alternating current (AC). Proper impedance control is critical as it ensures reliable signal transmission and minimizes power loss.
In this blog , Capel will explore five factors that can significantly affect the impedance control of rigid-flex boards. Understanding these factors is critical for PCB designers and manufacturers to deliver high-quality products that meet the demands of today’s technology-driven world.
1. Different substrates will affect the impedance value:
For Flex Rigid-Flex PCB, the difference in the base material does have an impact on the impedance value. In rigid-flex boards, the flexible substrate and the rigid substrate usually have different dielectric constants and conductivity, which will cause impedance mismatch problems at the interface between the two substrates.
Specifically, flexible substrates have a higher dielectric constant and lower electrical conductivity, while hard substrates have a lower dielectric constant and higher electrical conductivity. When the signal propagates in the rigid-flex circuit board, there will be reflection and transmission at the interface of the rigid-flexible pcb substrate. These reflection and transmission phenomena cause the impedance of the signal to change, that is, impedance mismatch.
In order to better control the impedance of the flex-rigid pcb, the following methods can be adopted:
Substrate selection: choose a combination of rigid flex circuit substrates so that their dielectric constant and conductivity are as close as possible to reduce the problem of impedance mismatch;
Interface treatment: special treatment for the interface between pcb rigid flex substrates, such as using a special interface layer or laminated film, to improve impedance matching to a certain extent;
Pressing control: In the manufacturing process of rigid flexible pcb, parameters such as temperature, pressure and time are strictly controlled to ensure good bonding of rigid flex circuit board substrates and reduce impedance changes;
Simulation and debugging: Through simulation and analysis of the signal propagation in the rigid flexible pcb, find out the problem of impedance mismatch, and make corresponding adjustments and optimizations.
2. Line width spacing is an important factor affecting impedance control:
In the rigid-flex board, the line width spacing is one of the important factors affecting impedance control. The line width (i.e. the width of the wire) and the line spacing (i.e. the distance between adjacent wires) determine the geometry of the current path, which in turn affects the transmission characteristics and impedance value of the signal.
The following is the influence of the line width spacing on the impedance control of the rigid-flex board:
Fundamental Impedance: Line spacing is critical for controlling the fundamental impedance (i.e., the characteristic impedance of microstrip lines, coaxial cables, etc.). According to transmission line theory, factors such as line width, line spacing, and substrate thickness jointly determine the characteristic impedance of a transmission line. When the line width spacing changes, it will lead to a change in the characteristic impedance, thereby affecting the transmission effect of the signal.
Impedance matching: Impedance matching is often required in rigid-flex boards to ensure the best transmission of signals throughout the circuit. Impedance matching usually needs to adjust the line width spacing to achieve. For example, in a microstrip line, the characteristic impedance of the transmission line can be matched to the impedance required by the system by adjusting the width of the conductors and the spacing between adjacent conductors.
Crosstalk and Loss: Line spacing also has an important impact on the control of crosstalk and loss. When the line width spacing is small, the electric field coupling effect between adjacent wires is enhanced, which may lead to an increase in crosstalk. In addition, smaller wire widths and larger wire spacings result in more concentrated current distribution, increasing wire resistance and loss.
3. The thickness of the material is also an important factor affecting the impedance control of the rigid-flex board:
Variations in material thickness directly affect the characteristic impedance of the transmission line.
The following is the effect of material thickness on the impedance control of rigid-flex boards:
Transmission line characteristic impedance: The characteristic impedance of a transmission line refers to the proportional relationship between the current and voltage on the transmission line at a specific frequency. In the rigid-flex board, the thickness of the material will affect the value of the characteristic impedance of the transmission line. Generally speaking, when the material thickness becomes thinner, the characteristic impedance will increase; and when the material thickness becomes thicker, the characteristic impedance will decrease. Therefore, when designing a rigid-flex board, it is necessary to select an appropriate material thickness to achieve the required characteristic impedance according to system requirements and signal transmission characteristics.
Line-to-Space Ratio: Variations in material thickness will also affect the line-to-spacing ratio. According to the transmission line theory, the characteristic impedance is proportional to the ratio of line width to space. When the material thickness changes, in order to maintain the stability of the characteristic impedance, it is necessary to adjust the ratio of line width and line spacing accordingly. For example, when the material thickness is reduced, in order to keep the characteristic impedance constant, the line width needs to be reduced accordingly, and the line spacing should be correspondingly reduced to keep the line width to space ratio unchanged.
4. The tolerance of electroplated copper is also a factor affecting the impedance control of the flexible rigid board:
Electroplated copper is a commonly used conductive layer in rigid-flex boards, and changes in its thickness and tolerance will directly affect the characteristic impedance of the board.
The following is the influence of electroplating copper tolerance on the impedance control of flexible rigid boards:
Electroplated copper thickness tolerance: The thickness of electroplated copper is one of the key factors affecting the impedance of the rigid-flex board. If the thickness tolerance of electroplated copper is too large, the thickness of the conductive layer on the plate will change, thereby affecting the characteristic impedance of the plate. Therefore, when manufacturing flex rigid boards, it is necessary to strictly control the thickness tolerance of electroplated copper to ensure the stability of characteristic impedance.
Uniformity of electroplating copper: In addition to thickness tolerance, the uniformity of electroplating copper also affects the impedance control of rigid-flex boards. If there is an uneven distribution of the electroplated copper layer on the board, resulting in different thicknesses of the electroplated copper on different areas of the board, the characteristic impedance will also change. Therefore, it is necessary to ensure the uniformity of electroplated copper to ensure the consistency of characteristic impedance when manufacturing soft and rigid boards.
5. Etching tolerance is also an important factor affecting the impedance control of rigid-flex boards:
Etching tolerance refers to the deviation of the thickness of the plate that can be controlled when etching is carried out in the process of manufacturing flexible rigid boards.
The following are the effects of etching tolerances on the impedance control of rigid-flex boards:
Impedance matching of rigid-flex board: In the manufacturing process of rigid-flex board, etching is usually used to control the characteristic impedance value. Through etching, the width of the conductive layer can be adjusted to achieve the impedance value required by the design. However, during the etching process, since the etching speed of the etching solution on the plate may have a certain tolerance, there may be deviations in the width of the conductive layer after etching, which affects the precise control of the characteristic impedance.
Consistency in characteristic impedance: Etching tolerances can also lead to differences in the thickness of the conductive layer in different regions, resulting in inconsistent characteristic impedance. The inconsistency of the characteristic impedance may affect the transmission performance of the signal, which is especially important in high-speed communication or high-frequency applications.
Impedance control is an important aspect of Flex Rigid-Flex PCB design and fabrication. Achieving accurate and consistent impedance values is critical to reliable signal transmission and the overall performance of electronic devices. So by paying close attention to substrate selection, trace geometry, controlled dielectric thickness, copper plating tolerances, and etch tolerances, PCB designers and manufacturers can successfully deliver robust, high-quality rigid-flex boards that meet the industry’s stringent requirements. 15 years of industry experience sharing, I hope Capel can bring you useful help. For more circuit board questions, please consult us directly, Capel’s professional circuit board expert team will answer you online.
Post time: Aug-22-2023
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