Impedance Design Considerations for Flex-Rigid PCB

As electronics products witness their fast development, market calls for increasingly higher demands for flex-rigid PCB (printed circuit board) and impedance control PCB simultaneously together with increasingly rigorous requirement on them. The leading issue flex-rigid PCB with requirement on impedance is faced up with lies in a large difference between measured value and design value as high as over 20Ω, leading to failure of design compensation and difficulty to get manufacturing controlled. This article mainly discusses how to meet rigorous impedance control accuracy in the perspective of PCB design and is expected to be beneficial to staff serving for PCB manufacturing industry.

Impedance Control Analysis

Leading elements affecting impedance include dielectric constant, medium thickness, trace width and copper thickness.


Based on cross-section analysis, when practical cross-section data is applied in modules, the difference between calculated value and practical measured value obtained by impedance instrument lies in the range from 14Ω to 33Ω which is summarized in the following table.


Theoretical Value (Ω) Measured Value (Ω) Difference (Ω)
113 143 30
109 134 25
95 112 17
93 107 14
120 153 33
110 139 29
96 119 23
95 116 21
125 153 28
110 141 31
100 123 23
90 110 20
124 151 27
112 137 25
104 123 19
95 113 18

Based on the difference demonstrated above, difference between theoretical value and measured value is too large possibly due to the following reasons:


During engineering design, the access is incorrect substituted to software parameter.


In accordance with factors affecting impedance and cross-section data, perhaps only dielectric constant leads to inaccurate access. Based on combined concept of dielectric constant, it can be known that dielectric constant of PCB substrate material is a comprehensive result of dielectric constant of dielectric material in substrate material, which can be approximately indicated by weighted sum of dielectric constant of resin in dielectric material and dielectric constant of reinforcing material. When it comes to flexible material, however, it is composed by adhesive and PI (polyimide). Thus, dielectric constant of flexible material is the comprehensive dielectric constant of both adhesive and PI.


Measuring module design is incorrect concerning PCB


During impedance design process, impedance line measurement usually involves transmission line design and reference plane and it should be guaranteed that certain distance can be maintained between copper edge of reference plane and impedance line. As far as this situation is concerned, the distance is only 0.5mm that may be too short, leading to complete ignorance of this reference plane.


• Experimental Scheme


Step 1: engineering data is designed to respectively verify:
i. Influence of transmission copper foil on impedance when it is either added to or not added to measurement module.
ii. What is influence of distance between copper foil edge and impedance line on impedance in measurement module. Horizontal distance between design edge and impedance line is respectively 0.5mm and 4.5mm.
iii. Measurement module design decides the influence of grid reference plane and copper foil reference plane on impedance.


Step 2: Flexible board is fabricated and impedance of flexible board is measured.


Step 3: Cross-section access is substituted to theoretical impedance of module computation and figured out according to comprehensive dielectric constant of dielectric material so that errors can be eliminated aroused by access.


Step 4: Conclusions can be made through data comparison: parameter access method and design regulations of measurement module.


• Experimental Result


1) In accordance with the experimental scheme with and without transmission copper foil added to measurement module, original measurement data indicates that impedance leads to such a small difference between transmission copper foil adding and not adding. Therefore, it can be concluded that no influence is laid to impedance no matter transmission copper foil is added or not added.


2) In accordance with experimental scheme designed based on the distance between reference plane copper foil edge and impedance line, impedance difference is so small that it can be concluded that distance between reference plane copper foil edge and impedance line features no influence on impedance.


3) In accordance with experimental scheme designed based on grid and copper foil module designed for measurement module reference plane, it can be concluded that impedance will be dramatically influenced when measurement module reference plane is designed to be copper foil and grid.


4) In accordance with experimental scheme concerning different trace widths, grids and copper foil modules with different sizes, it can be concluded that when grid is designed to be a reference plane, it is associated with residual rate of copper. The higher residual rate of copper is, the smaller difference it will lead to with copper foil. The lower residual rate of copper is, the higher difference it will lead to with copper foil. Therefore, as grid is used as reference plane, copper should be coated at reference place compatible with impedance line position.


5) In accordance with practical design measurement module, cross-section access is substituted to module to figure out theoretical impedance that is then compared with practical measurement impedance. Since flexible material is composed by adhesive and PI, dielectric constant of flexible material should be a comprehensive dielectric constant of both constituents or single dielectric constant is acquired through the application of software for impedance calculation. Based on previous experimental results, it can be concluded that the dielectric constant of PI is 2.8 while that of adhesive is 3.5. As a result, as data is substituted to software for calculation, dielectric constant accuracy will be verified.

Flex-Rigid PCB Designer's Considerations on Impedance Design

Consideration#1: Reference plane should be grid reference plane and copper foil reference plane.


Based on the experimental results listed above, it can be concluded that engineering design based on copper foil reference plane is capable of meeting the impedance requirement of flex-rigid PCB. When grid reference plane is designed, the larger grid is, the larger difference it will generate between grid impedance for the minimum residual rate of copper and copper foil impedance while the smaller grid is, the smaller difference it will generate between grid impedance for the maximum residual rate of copper and copper foil impedance.


In conclusion, grid design as reference plane is closely related with grid size, that is, residual rate of copper. The higher residual rate of copper is, the smaller difference will be between it and copper foil impedance and theoretical design data. The lower residual rate of copper is, the larger difference will be between it and copper foil impedance and theoretical design data. As a result, when grid is selected as reference plane, copper should be coated on reference plane compatible with corresponding impedance line position.


Consideration#2: Flex-rigid PCB impedance should be designed depending on adding functionality of impedance calculation software.


Compared with ordinary impedance calculation software, impedance calculation software with adding functionality contains access acquisition function for each medium layer and performs more accurately in terms of access acquisition. Moreover, it's easier to simulate practical situations and more convenient to be applied for engineering design.


Consideration#3: Dielectric constant of each single layer is acquired on flex-rigid board.


It can be verified based on full-scale experiments that dielectric constant of PI is 2.8 while that of adhesive is 3.5, which can be used as a reference for flex-rigid board designer. Theoretical data calculation based on the application of impedance calculation software with adding functionality is capable of meeting the demand of flex-rigid PCB customers.

PCBCart Can Manufacture Flex-rigid PCBs with Strict Impedance Control.

Established in 2005, PCBCart has been providing quality guaranteed Flex-rigid PCB manufacturing services since then. We can provide strict impedance control on each piece of flex-rigid circuit boards. If you need any flexible rigid PCB fabrication, please reach us here for practical and cost-effective solution.


Helpful Resources
How to Analyze and Prohibit Impedance of High-Speed PCB Power
Elements Affecting Characteristic Impedance of PCB and Solutions
PCBCart Offers Advanced Flex-rigid PCB Fabrication Services Starting from 1pc

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