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Special attention points for flexible circuit wiring
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FPCway: Specialized manufacturer of flexible printed circuit boards and rigid-flexible printed circuits
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About RA Copper and ED Copper
Introduction of Flexible PCB
5 Tips For Designing Flexible PCB
Advantages of FPC (Flexible PCB)
Evolution of the Flex Printed Circuit Board
Benefits of Using Flex Circuit Boards
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Development of Flexible printed circuit board (FPC) market
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The Differences In Rigid PCB, Flex PCB and Rigid-Flex PCB
Flex-Rigid PCB Design Guidelines
Beneficials for Polyimide Flex PCB Boards
About Stiffener on Flex PCB FPC circuit Boards
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Interconnect Solutions for Flexible Printed Circuits and Etched Foil Heaters
Advantages and Disadvantages of Rigid-Flex PCB
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PCB Assembly Blog
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About PCB Assembly
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About SMT Assembly (Surface Mount Technology)
About THT Assembly (Through-Hole Technology)
About Reflow Soldering
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About SMT (Surface Mount Technology)
FPC Research Blog
Preparation of FPC based on ultrasonic spraying method_4_Experimental Results
Preparation of FPC based on ultrasonic spraying method_3_Experimental Procedure
Preparation of FPC based on ultrasonic spraying method_2_Experimental Platform and Principle
Preparation of FPC based on ultrasonic spraying method_1_abstract
Research on Layout Design Method of Ultra-thin FPC_4_Analysis of Layout Design Methods
Research on Layout Design Method of Ultra-thin FPC_3_Analysis of Layout Design Methods
Research on Layout Design Method of Ultra-thin FPC_2_Analysis of Layout Design Methods
Research on Layout Design Method of Ultra-thin FPC_1_introduction
Research progress on polyimide FPC_2_the field of FPC
Research progress on polyimide FPC_1_Introduction
Analysis of Vibration Characteristics of FPCBs _4_Summary
Analysis of Vibration Characteristics of FPCBs _3_Finite Element Analysis
Analysis of Vibration Characteristics of FPCBs _2_Theory of Vibration Analysis
Analysis of Vibration Characteristics of FPCBs Under Random Vibration_1_Introduction
Design Methods for FPCBs_5_Practical Application
Design Methods for FPCBs_4_Electrical Circuit Design and Examples
Design Methods for FPCBs_3_Structure Design Method and Examples
Design Methods for FPCBs_2_Component Selection Methodology and Examples.
Research on Design Methods for FPCBs
Application of MPW technique for FPCBs _4_Summary
Application of MPW technique for FPCBs_3_Experimental results
Application of MPW technique for FPCBs_2_Experimental setup
Application of MPW technique for FPCBs_1_Principle of MPW
Application of FPCB in PC motherboards_4_ Results and discussion
Application of FPCB in PC motherboards_3_ Numerical analysis
Application of FPCB in PC_2_ Experimentation
Application of FPCB in PC motherboards
A Bus Planning Algorithm for FPC Design _4_Experimental result
A Bus Planning Algorithm for FPC Design _3_Proposed Algorithm
A Bus Planning Algorithm for FPC Design _2_Preliminaries
A Bus Planning Algorithm for FPC Design _1_Introduction

Special attention points for flexible circuit wiring

Layer stacking design, device layout, and cutting issues are all obvious, but there are many material weaknesses that can be encountered in flexible circuits.

From relatively high z-axis expansion coefficient adhesives to low viscosity PI substrate copper clad, to copper hardening and fatigue. The following statements of what to do and what not to do will focus on supplementing:


Maintaining the flexibility of the flexible board


It is obvious that the flexibility of the flexible circuit should be determined according to the needs in advance, but it still needs to be emphasized again. If the flexible circuit section is only intended to be folded during assembly and then installed in a fixed position, such as in a handheld ultrasound device, then we have a lot of freedom in choosing the number of layers and the type of copper skin (RA or ED). On the other hand, if the flexible circuit section is to be constantly moved, bent, or rotated, then the number of layers should be reduced and adhesive-free materials should be selected.


We can use IPC-2223B formulas (Formula 1 represents single-sided, Formula 2 represents double-sided, etc.) to determine the minimum allowable bending radius based on the allowable deformation of copper and other material properties.




This example formula is for a single-sided flexible board.


We choose EB based on actual usage conditions, with 16% for applications with little bending, 10% for flexible mounting applications, and 0.3% for dynamic flexible designs.


Do not bend at corners


Generally, we recommend keeping the copper trace of the flexible circuit bent along the vertical direction. But sometimes it's not possible, so try to minimize the bending amplitude and frequency, or use tapered bending according to mechanical design requirements.






                    Figure 1: Preferred bending location


Use arc routing


As shown in Figure 1 above, it is best to avoid using abrupt right angles or rigidly straight 45° angle traces, but rather to use an arc angle routing pattern. This can reduce the stress on the copper during bending.


Do not suddenly change the width of the trace


When the trace is connected to the pad, especially when arranging and arranging the flexible circuit terminals (as shown in the figure below), a weak force point will be formed, and the copper skin will easily age over time. Unless a reinforcing plate is used or the application process will not bend, it is recommended to use a gradually narrowing connection method such as the one shown below.





Figure 2: Sudden changes in trace width or connections to pads can create weak focal points



Use polygons


 it is necessary to place a power supply or ground plane on a flexible board. If you don't mind significantly reducing flexibility and potentially causing the copper skin to wrinkle, you can choose to use solid copper. Generally speaking, it is best to use shadow polygon copper plating to maintain a high degree of flexibility. In mentioning this, I also think that traditional shadow polygons will have excess copper reinforcement in the directions of 0°, 90°, and 45°. A more optimized pattern is the hexagonal approach. This problem can be solved by using negative layer and arrayed hexagonal pads, and the shadow polygon can be established more quickly using copy and paste methods.



Figure 3: Using hexagonal copper plating can evenly balance the stress in three angles.



Provide reinforcement for pads


Due to the use of low-viscosity adhesives (relative to FR-4), copper on flexible circuits is more likely to detach from the polyimide substrate. Therefore, providing reinforcement for exposed copper is particularly important. Coated through holes provide appropriate anchoring for two flexible layers, so using vias is a very good reinforcement method. Because of this (providing z-axis expansion), many processing plants recommend adding coated through holes with depths up to 1.5 mils to rigid-flex boards and flexible circuits. Surface mount pads and non-coated through hole pads do not have reinforcement measures themselves, so additional reinforcement is needed to prevent detachment.




Figure 4: Reinforcement methods for flexible circuit pads, plating, increasing anchors, and reducing cover film openings


Referring to Figure 4, the second option is suitable for adhesive-type cover layers, and the third option is suitable for non-adhesive-type cover layers. Protective films using adhesives may exhibit "overflow" phenomena, so the gap between the pad and the opening must be large enough to ensure excellent soldering formation.


SMT component pads are the most fragile, especially when flexible circuits bend under the rigid pins and pads of the component. Figures 5 and 6 show how to reinforce the pads on both ends of the pad using a cover layer. To achieve this, the pads on the flexible board must be larger than the pads on a typical rigid board.


Looking at the comparison in Figure 6, SMD pads for installing components on a flexible board. This significantly reduces the installation density of flexible circuit components, but compared to rigid circuits, the density of flexible circuits cannot be too high in the first place.


Figure 5: SOW package cover film opening, showing its reinforcement on both ends of each pad. Figure 6: Adjusting pad size and cover layer openings



Figure 6: Adjusting pad size and cover layer openings.


Maintain double-sided flexibility


 For dynamic double-sided flexible circuits, try to avoid placing traces in the same direction, but instead need to stagger them (Figure 7) to evenly distribute the copper trace (Figure 8).



Figure 7: Not recommended adjacent layer copper trace routing.



Figure 8: Preferred staggered adjacent layer copper trace routing.

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