Flexible Rigid-flex Circuits Technical Engineering Guide

24 June 2022
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Simply put, flex circuits are printed circuit boards that can bend. They are a hybrid of conventional printed circuit boards and round wire. Flex circuits can be shaped into design configurations that are not possible with other circuits. Most designers use flexible and rigid-flex circuits for their limitless geometrical possibilities without sacrificing repeatability of printed circuits or precision density.

Hemeixin PCB specializes in printed circuit boards with expertise in engineering and manufacturing. Keepr eading to get more insights on technical engineering basics fooor flerxible and rigid-flex PCBs.

Basics of Flex Circuits: The Main Types You Should Know About (IPC 6013)

Single Layer Config

Type 1
A single laminated structure of conductive layer sandwiched between two insulating layers or uncovered on one side;
It is optional to use connectors, pins, stiffeners, and other electronic components.

Double Sided

Type 2
This circuit has two conductive layers, one on each side of the polyimide within the circuit, mostly connected with a plated thru-hole.
It is optional to use connectors, pins, stiffeners, and other electronic components.

Multi-Layer

Type 3
Combination of three or more single-sided or double-sided layers with complex interconnections, electronic components, pins, and stiffeners in a multilayer design.
The layers are connected with the use of thru-holes.
It is possible to control the impedance.

Rigid Flex

Type 4

Rigid outer layers of the circuit are connected to inner flexible layers with the help of copper-plated vias.
Outer layers can have exposed pads or covers
Rigid-flex circuits have conductors on the rigid layers, which differentiate them from multi-layer circuits containing stiffeners.

Main Benefits of Flexible and Rigid-Flex Circuits

The basic premise of flex and rigid-flex circuits is the ability to design circuits that fit into a device’s specific enclosure. This ensures that manufacturers don’t have to sacrifice aesthetics and architectural features of their project.

Rigid flex assemblies are deployed in applications that require reliability and longevity, such as medical, engineering, and aerospace industries. Flex circuits can be folded and configured into smaller spaces that allow devices to achieve a small form factor, while also reducing the overall weight of a device.

Overall Reduction of Cost

The low weight and thin structure of a flexible circuit allows it to meet the needs and requirements of any system.
Another advantage is that flex circuits minimize the overall number of interconnects required, which means fewer connectors and solder joints. This also means that there will be fewer potential sources of failure and greater longevity across the circuit.

A Better Substitute for Wire Harness and Cable Assembly

Flexible circuits are faster and subject to less error because they snap into place in a single high density connector instead of individual wires. This allows flex circuits to be repeatable, with every rigid flexible circuit board behaving exactly as predicted.

Higher Tolerance for Heat

Flex circuits are so thin that the heat buildup can dissipate from both sides to a heat sink. Additionally, the compact design of the circuit shortens the heart path and increases cooling effectiveness.
The lower buildup of heat also minimizes the risk of expansion and contraction because of the chemical and heat resistance properties of polyimide material.

The Importance of Stiffeners

Stiffeners are used to provide mechanical support to every rigid flex PCB board. They are often used to increase the thickness of a rigid flex PCB in certain areas that require them or alter bend areas to predefined locations. Manufacturers may also use stiffeners to meet ZIF connector specifications. In short, stiffeners allow manufacturers to strengthen specific areas of the rigid flex circuit board.

Various Stiffener Materials for PCBs

Stiffener thickness depends on the designer’s needs and are not written in stone. The most common range for stuffers is in the range of 008” to .059”.

FR4 (also known as G10)
Polyimide
Stainless Steel
Aluminum

As a rule, manufacturers strive to make the stiffener as thick as possible to provide more support to the PCB. This is usually achieved by adding a thin layer of polyamide at the contact fingers. Certain applications may require other materials such as stainless steel or aluminum. These materials are more expensive but have better heat sinking properties and rigidity.

Flex Circuit Material Properties

Below are typical properties of dielectric material for flexible circuits.

Property	Polymide	Polyester
Thickness range	0.5-5 mil	2.5 mil
Break elongation	25,000 @25°C	20,000-35,000 @25°C
Tear Initiation Strength (lb/in)	1000	1000 - 1500
Resistance to Acidity	Good	Good
Resistance to Grease and Oil	Good	Good
Resistance to Alkalis	Poor	Poor
Resistance to Organic Solvents	Good	Good
Resistance to Water	Good	Good
Resistance to UV Light	Good	Satisfactory
Service Temperature (°C)	-125 to +200	-60 to +105
Change in Linear Dimension (100°C, 30min)	<0.3	<0.5
Electrical Dissipation Factor (ASTM D150) 1 MHz	0.01	0.018

Importance of Impedance

Impedance is used to determine the performance of a high speed circuit. The primary purpose of PCBs are to transfer the signal power from the driver device to the receiving device. The use of impedance matching allows PCBs to achieve maximum signal power. It can be used in different configurations including coplanar, differential, and characteristic modes.

Design Recommendations for PCBs

Corners should be radiused within the flex band areas to eliminate stress contractors and improve reliability.
The use of staggered layer to layer trace positioning to eliminate the “I-Beam” effect and improve both reliability and flexibility.
The use of teardrops and fillets to eliminate stress concentrations and maximize reliability.
The use of vias within bend areas are not recommended because they can contribute to stress and lead to breakage.
The use of overlay and stiffener terminations to prevent the buildup of stress concentrators.

Pre-Bake Specifications Prior to Circuit Assembly

Most flexible and rigid-flex circuits still contain moisture and require extensive pre-baking to remove all the water content before the assembly practice can commence.

Over the years, this pre-baking of all flex circuits before assembly has become an established industrial practice without exception, even if the product recently arrived from the supplier in a vacuum-sealed packaged. Baking the product before packaging does not eliminate the need for pre-baking before assembly.

For more detailed guides on flexible and rigid-flex circuits, visit Hemeixin. As a leading rigid flex circuit manufacturer, we can provide ultra-quick turnaround within 10 working days. Our PCBs are specially designed for use in aerospace, military, and medical applications.