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Tips for Beginners in Flexible Circuits Design —Part2

Flex Circuits Design Tips for Materials & Bendability

Introduction

Successful flexible circuit design goes beyond the materials and bending we discussed before. Designers must consider the actual capabilities of manufacturers. By addressing production issues, and cutting down communication time. and avoiding performance losses from pushing production limits, designers can improve their designs.

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They should think about real-world conditions when planning electrical parts, layouts, and lamination. GESFLEX has learned from its past layout and fabrication experiences. It shares these lessons to help others optimize flexible circuit design. Let’s dive deeper into these insights and discuss ways to enhanceflexible circuits design efficiency and performance.

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Electrical Parts in the Design Considerations

Schematic Design Considerations

Flexible circuits mainly connect signals between rigid boards that aren’t aligned. Treat the flexible circuit and its connected rigid boards as one small system during design. It’s crucial to consider how the flexible circuit is routed when arranging signals on connectors. Proper signal order can reduce the need for vias and streamline routing. This reduction in routing complexity helps decrease the FPC board’s size and increase its flexibility.

Therefore, in the design of FPC schematic design. generally is to provide the first draft of the schematic to the engineer. according to the alignment needs to adjust the schematic connector signal sequencing. There is also a general priority to ensure that the FPC signal connection relationship. and then determine the signal connection relationship on the hardboard.

Conductivity

“Flex boards should prioritize the line width, copper thickness. and via size for their current carrying capacity. The current carrying capacity also depends on the application environment. cooling conditions, and allowable temperature rise. Common copper foil thicknesses used in FPCs are 0.50z and 1.00z. with typical line widths ranging from 4mil to 10mil. For the maximum current carrying capacity with a 10°C temperature rise. please refer to the following table (calculated according to IPC-D-275 standards):
Table of current carrying capacity for 0.50z and 1.00z copper thicknesses at a 10°C temperature rise.
Current Carrying Capacity at 10℃ Temperature Rise

Whether using rolled or electrolytic copper. the method for calculating the current carrying capacity for FPCs is the same as that for rigid boards. However, considering flexibility, the copper foils used in FPCs are thinner. in cases requiring high flexibility. FPC surface copper foils use a localized electroplating method. When localized electroplating is used. there is no need to compensate for the thickness of the FPC surface copper foil.

current-carrying capacity

Impedance design

Flexible boards have a range of lamination structures. such as double-sided or three-layer boards. sometimes with added silver paste for microstrip and stripline configurations. You can refer to diagrams for these structures. Use Polar software to calculate trace impedance.

Impedance Control

For microstrips, use a “Coated Microstrip” model. when using a coverlay instead of solder mask to get accurate impedance values. Polyimide, a common dielectric in FPCs. has a dielectric constant of 3.5 at 1 MHz, which allows for controlled impedance.

coated microstrip

Thinner dielectrics in flexible boards place traces closer to the reference plane. resulting in generally lower impedance. For example, a 6 mil stripline can have an impedance of about 33 ohms.

Bandwagon impedance_Sheet1

To meet specific impedance needs with wider traces. you can increase the dielectric thickness or use a mesh reference plane. which also enhances flexibility but complicates impedance calculations.

Mesh designs increase impedance as the mesh holes get larger. Impedance simulation can be done using software like HFSS. or manufacturers can control it based on their experience. Typically, you can manage single-ended impedance from 25-100 ohms. and differential impedance from 75-125 ohms.

Shielding Control

When flexible boards need EMI control, you can add shielding layers to meet the requirements. These layers can be solid copper foil, copper mesh, or silver paste mesh.

Solid Copper Foil:

This is the most common type of shielding. You can place the copper foil on one or both sides of the FPC or just cover the areas that need shielding. Using solid copper foil as a reference layer simplifies impedance control. and allows for direct calculations using the software. However, solid copper foil increases the number of layers in the FPC. which makes the FPC harder to process and increases its rigidity. This type of shielding also makes the FPC thicker. affecting its minimum bending radius.

Copper Mesh:

You can place copper mesh on one or both sides of the FPC or just over the areas that need shielding. Copper mesh reduces the amount of copper used. which increases the flexibility compared to solid copper foil. However, like solid copper, copper mesh also adds layers to the FPC. making it thicker and harder to process, and impacting the minimum bending radius.

Silver Paste or Silver Paste Mesh:

When frequencies exceed 1 MHz. silver paste provides shielding effectiveness comparable to copper foil. You can apply silver paste using screen printing on the surface of single-sided. or multilayer boards (it can overlay on just one side). The surface of the silver paste needs a coverlay for insulation. Using silver paste doesn’t increase the number of circuit layers. making it easy to manufacture and cost-effective. However, silver paste is brittle and not suitable for applications requiring high dynamic bending.

Power and Ground Design Requirements

Designers should ensure the width of the power. and ground wires on the FPC meet the current carrying capacity requirements. Choose the appropriate line width based on the current size and the allowable temperature rise. For high currents, increase the line width, add more traces, and use thicker copper foils. However, thickening the traces can significantly affect the flexibility of the flexible circuit. so it’s important to find a balance.

For shielding and impedance control, you can evenly coat one or both sides of the FPC with silver paste. Keep in mind that due to its properties, such as higher resistance. you cannot use silver paste for high-current circuits.

In high-speed signal design, arrange as many ground connection pins as possible to minimize the signal loop.

If there is no ground reference plane, like in a single panel. run a ground wire between the two signal traces to serve as a return path.

To improve ground noise without compromising flexibility. consider punching ground vias in the upper and lower ground traces or planes.

In multilayer FPCs, route the signal lines as close to the ground plane as possible.

Crosstalk Control

To minimize interference between critical networks. simulate crosstalk to find out the minimum required spacing between signal lines. You can use design rules for spacing or set limits on how long lines. That can run parallel to each other to control crosstalk effectively. Techniques to reduce crosstalk include increasing line spacing. and using ground planes for isolation.

Timing Control in Flexible Circuit Board Design

Timing Control Meeting setup and hold times is essential for timing circuits. Here are the basic formulas for timing calculations:

Tpropmax=Tcycle -Tmin setup -Tmax out valid +/- Tskew – Tiitter – Tcrosstalk

Tpropmin=Tmin in hold _ Tout hold +/- Tskew + Tiitter + Tcrosstalk Where:

  • Tpropmax is the maximum propagation delay allowed on the transmission line.
  • Tpropmin is the minimum propagation delay allowed.
  • Tcycle is the clock cycle.
  • Tmin setup is the minimum setup time required by the input devices.
  • Tmax out valid (often defined as Tco) is the maximum valid output time from the output devices. meaning the time from the clock edge to when valid data is output.
  • Tskew is the relative delay at the clock input PIN of input and output devices, reflecting the difference in clock timing.
  • Tjitter represents the delay caused by clock jitter, which can change the clock cycle.
  • Tcrosstalk is the delay introduced by crosstalk on the bus.
  • Tmin in hold is the minimum hold time for input devices
  • Tout hold is the hold time for the output of devices.

You can find these parameters in the device datasheets. with Tjitter and Tcrosstalk often approximated as 0.5 ns. Calculating these allows us to determine the maximum. and minimum propagation delays the transmission line can handle. Static timing analysis helps guide chip device selection and the layout and routing of circuits. Generally, the setup time requirements determine the maximum length of the transmission line in synchronous circuits. while hold time requirements determine the minimum length. These setup and hold times are specific to the devices receiving the input signals.

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Layout Design

Before flexible circuit design starts, get to know the function and interconnections of the flexible board. Check the size and pin sequence of key components like connectors. Usually, Import the DXF format structural element diagram into the design software. and use the “change” command to outline the FPC board. Position connectors and locate holes according to the diagram. Make sure to note the orientation of the first pin and the center position of key components. Arrange other components based on how the circuit signals flow and connect. Layout must meet the client’s Design for Manufacturability (DFM) and Design for Testing (DFT) standards.

Coverlay Design

The coverlay on fpc acts as insulation and protects conductors from contamination, moisture. and scratches, similar to a solder mask. However, unlike solder mask which is applied by printing. the coverlay laminated onto the conductor surface like copper foil. You need to drill or punch holes for pads through the coverlay. Minimize the number of coverlay openings. When small openings are necessary, opt for round ones over square. The smallest possible hole size is 0.25 mm, depending on the capability of the manufacturer. and the edge distance of the holes should be more than 0.25 mm. If pad spacing is tight, consider overall coverlay openings around the pads or use solder mask methods.

Coverlay Design Requirements

Try to reduce the number of overlay openings in flexible circuit design. For some vias and metalized holes, you can cover them completely. similar to a solder mask without openings.

Coverlay Machining Capabilities

The minimum drill or punch size for a coverlay is 0.25 mm, which varies by manufacturer. The edge distance of coverlay holes should be more than 0.25 mm. If pad spacing is too close and the standard coverlay opening cannot meet the 0.25 mm requirement. consider overall coverlay openings or solder mask techniques.

Coverlay Over Pad Design

For high-performance needs, consider designs that use coverlay over pads to increase adhesion. Make sure pads are at least 0.36 mm larger than the coverlay opening on each side. If not feasible, design the pad toe to extend into the coverlay by more than 0.18 mm. For small or isolated pads, use one of these methods to enhance reliability. Manufacturers typically apply treatments to meet these standards. like pressing the coverlay onto connected teardrop pads and traces.

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Conclusion

As we conclude our guide on designing flexible circuit boards. it’s crucial to focus on the core aspects that ensure success. Effective flexible circuit design involves selecting the right materials like Polyimide. and carefully planning layout and lamination to meet both electrical and mechanical requirements. Remember, maintaining flexibility while achieving desired impedance levels. That can require innovative approaches like using mesh reference planes. Always use simulation tools such as HFSS to verify your designs. and consult with manufacturers for expert advice. By balancing flexibility, thickness, and performance. your designs will be well-equipped to handle the demands of modern electronic applications.

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