Rigid Flex PCB Design

If you’re new to the world of rigid flex PCB design, or a professional who needs more insights, this the guide you’re looking for. Today’s guide covers basic to advanced rigid flex printed circuit board design technologies. From basic definition, classification, benefits, types of material, working principle, design software to manufacturing process – by the end of it all, you’ll be an expert in rigid flex PCB design. Well, let’s jump right into the main subject of this guide.

• What is a Rigid Flex PCB?
• Types of Rigid-Flexible Printed Board Circuits
• Benefits of Rigid-Flex Printed Board Circuit
• Rigid Flex Circuit Board Material
• How Rigid Flex PCB Work
• Rigid Flex PCB Design Considerations
• Popular Rigid Flex PCB Design Software
• Tips and Tricks for Rigid-Flex Design
• Rigid Flex Circuit Board Gerber Layout Guidelines
• Rigid Flex PCB Manufacturing Process
• Main Applications of Rigid-Flex PCB
• FAQs on Rigid-Flex PCBs
• Conclusion

What is a Rigid Flex PCB?

A rigid-flex printed circuit board is a board that uses a combination of rigid and flexible technologies in the application.

Rigid Flex PCB – Photo courtesy: All About Circuits

Most of the rigid-flex boards have “multiple layers of flexible circuit substrates” that attach to rigid boards. The attachment is on the outside or the inside of the board depending on the application design. You can also describe it as being a pattern of conductors that have circuit prints on a “flexible insulating film.” The name rigid flex comes as a combination of the rigid circuits and flexible circuits. The mixture is of the rigid and flexible circuits is a result of exploiting the benefits of rigid and flexible circuits. The rigid section will carry the bulk or part of all the components that you will need on the circuit. The flexible section will act as the interconnections between one rigid section and another rigid section. It was the initial idea for space programs as they were looking to save on space and weight. You are most likely to find them in many connections because of space and weight that they save. It is common in devices such as tablets and mobile phones that you can pack quickly. It also eliminates the need of wiring on such devices making them as simple as possible. You will find the Rigid-Flexible PCB on devices such as radios, pagers, computers and computing devices among others.

Rigid Flex on TVs – Photo courtesy: CNet

The circuits are a result of a slight layer of conductors on the outside of the insulating board known as substrates. There are different components on the surface which connect to the substrate by soldiers.

Types of Rigid-Flexible Printed Board Circuits

Rigid Flex PCB design is available in different categories that you can choose from depending on the application. You should be careful to make the right choice of the type of Rigid-Flex PCB design per the application. Continue reading and find out about the different kinds of Rigid-Flex PCB designs that are available for your choice.

· Flexible PCB

The substrate in use when making the board is flexible plastic. It allows the board to that the rigid forms are incapable of doing. It gives it the flexibility that enables it to move without breaking the (PCB) printed circuit board. They are costly when it comes to creation in comparison to the rigid PCB’s but are more advantageous.

                    Flexible PCB in a Camera – Photo courtesy: Wikipedia

They can assist in the restoration of bulky and heavy wiring in superior technologies and gears such as satellites. These types of techniques do not require heavy material and wide spaces. You can find the flexible printed circuit board in the following forms:

1. Single sided flexible printed circuit board.
2. Double sided flexible printed circuit board
3. Multilayer flexible printed circuit board

§  Single Sided Flex

This type of Rigid-Flex PCB has the conductive layer with the copper material on one side of the board. They are essential for dynamic applications or any equipment that will need to be very flexible. Their advantage includes the low cost of acquisition and the easy assembly.

Single sided flex PCB

When assembling this type of Rigid-Flex PCB, you will need only one type of tooling. It makes it easy to reproduce many copies of the board. It is the perfect solution for replacements of the wire harnesses.

§ Double Sided Flex

It is an extension of the single-sided PCB where the copper conductors are on both sides of the printed Circuit board.

Double sided flex PCB

It has the copper insulators in connection with one another through vias or plated through holes. The vias are responsible for creating active circuitry between the two layers. It is among the most common designs because it is easy to manufacture. They are also very light, and you can quickly reproduce them.

§ Multilayer Flex

Just from the name, you will realize that the board has more than a double copper conductor. You can find up to 10 conductive layers on a typical connection of the circuits.

Multilayer Flex

The relationships between the layers is also as a result of the vias or the plate through holes. They are essential for applications that need high-density connector PCBs and conductors’ route through tiny areas. The PTH is making it easier and creates more space for soldering joints.

Rigid-flex PCB

This type has the rigid and flexible printed circuit board in one position. The flexible part is in integration with the rigid part, and the PCB assembly is in line with the plate through holes.

Rigid Flex PCB Design

It creates smaller interconnect areas reducing any chance of the PCB failing during the application process. The flexible connection also prevents the firm or rigid part from twisting and breaking. You can find them in use on the motherboard of computers.

Benefits of Rigid-Flex Printed Board Circuit

The advantages of using the rigid flex printed board circuit are not only in saving the space and weight of devices. The other benefits of the technology include:

• It minimizes the space requirements by the application of 3D
• It removes the need of having cables and connectors between the parts that are rigid. It will help in reducing the size of the board thus reducing the weight of the whole system
• Working on maximizing space will reduce the number of parts that you will be using in any application
• Reducing the number of solder joints ensures that you can rely on the connectivity
• It is simple, and the work of handling it during the assembly process is straightforward in comparison to flexible boards
• It also simplifies the process of Printed Board circuit assembly
• “Integrated ZIF contacts” will provide a simple modular interface to that particular environment
• The condition of testing is simple, and you can do prior tests before connecting the circuits
• Assembly and logistical costs will also reduce when you use the rigid flex printed board circuit
• You can increase the complex nature of mechanical design and improve degree and freedom for housing solutions.

Rigid Flex Circuit Board Material

There are many materials that manufacturers use in making the Rigid Flex Circuit Board. The flex circuit is a result of stack up “flexible substrate material” with copper. The manufacturers laminate the material together under heat and pressure using adhesives. Selecting a material for use in making the Rigid Flex PCB is critical as it determines durability and strength. You should choose the substrate that you will use after analyzing on:

• Dimensional Stability
• Thermal and heat resistance
• Electrical properties such as conductivity
• The flexibility of the material
• Chemical resistance of the material

Rigid Flex PCB Stack up

Building a multilayer PCB is a process in comparison to making using Legos. The parts should be fitting together very easily, but you should still follow directions for the pieces to stick together. Multilayer PCB is the modern norm and manufacturers can stack up more than 30 layers. The layer stack-ups are very important in many applications all over the world. You should be ready to experience shifts in circuit board strategies. Continue reading and find out the kind of material that you will find in a Rigid Flex PCB design.

Flexible Material

It is a technology where you will assemble electronic circuits by putting electronic devices on plastic substrates that are flexible. The flexible plastic substrates that you will come across include PEEK, polyimide or transparent polyester that can conduct electricity. Additionally, you can also screen print the silver flex circuit on polyester.

Flexible material

Polyester (PET) is readily available in 25 – 125 microns and is a material of choice due to flexibility and electrical conductivity. It is also resistant to moisture and chemical properties hence an excellent material for industrial environments. The advantages of using the flexible material are:

• It gives the potential for replacements of rigid connectors and boards.
• The circuits with single sides are suitable for high or dynamic flex applications.
• It also allows for the stack up in different configurations.
• The disadvantages of using the flexible material are:
• It will involve an increase in cost if you replace them with the rigid material.
• The risk of damage during use and handling is very high, and you have to be extra careful.
• The process of assembly becomes complex
• Reworking or repairing the material is complicated and, in many cases, impossible
• Utilizing the panel poorly will increase the cost

Rigid Material

There are many materials that manufacturers use when making the rigid part of the rigid flex PCB. The materials vary depending on the application. You will make a selection on the material after carefully analyzing the application.

Rigid Material

Apart from the application, there are other factors that you will consider when choosing the rigid material. The factors are:

• Reliability requirements
• Shelf life or duration of application
• Methods of manufacturing
• Budget and cost that you intend to use

The most common rigid materials that manufacturers use is: Substrate material – It involves the use of woven fiberglass which uses a lot of epoxy resin to make it better. It is, however, not reliable because it does not resist frequent shocks, constant movements, and vibrations. It is the reason why manufacturers shift to use polyimide. Polyimide – manufacturers use it due to its toughness, versatility, and resistance to constant vibrations and movements. It is also resistant to heat making it the best material in very hot environments.

Polyimide

The substrate that manufacturers usually prefer to use is the polyimide which is a flexible and robust thermoset. There are many examples of the polyimide that you might use in the manufacturing process of the device. They include Kapton, Apical, UPILEX, Norton TH, VTEC PI, and Kaptrex. Conductor Material The conductor material that manufacturers prefer to use in the assembly of the Rigid Flex PCB is copper.

Conductor material -copper foil

It is the conductor material of choice for many manufacturers because of the following benefits:

• it is readily available
• high workability
• it can conduct electricity

There are two forms of copper foils that you can use in making the circuitry applications. These include the electro-deposited copper and the rolled copper foil. The forms are available for you to choose from in various thickness, sizes, and weights. You will subject the copper to surface treatment before using it on board assembly. You will apply a thin layer of zinc to increase the longevity or shelf life of the copper foil. The foils undergo chemical treatment to:

• Reduce the degradation of the bond
• Increase adhesion of the bond
• Augment the strength of the bond
• Protect the board from oxidation

Adhesive Types Manufacturers use adhesives to increase and extend the service life for the Rigid-Flex PCBs. They are the main connectors between the conductor materials and the substrates. Deciding on the type of adhesive that you will use in bonding the conductor and substrate is crucial. There are many adhesives available in the market for manufacturers to choose from. Remember that you will choose an adhesive depending on the application. The most common adhesives that you can use include:

• Polyimide Adhesives – is good due to its temperature resistance up to 500 degrees centigrade.
• Polyester Adhesives – is cheap and has low bond strength. Since it has low thermal conductivity, manufacturers opt for modified polyester adhesives. The modified type has excellent heat resistance, excellent versatility for use in the assembly of complex boards.
• Acrylic Adhesives – it has excellent thermal stability, resistance to chemical corrosion, easy to apply, cheap and readily available.
• Epoxies – common in the assembly of rigid-flex designs due to heat, chemical and solvent resistance. They are very flexible and have excellent bond stability and is very stable with the addition of polyester.
• Protective coatings – you will use it protect the surface of the Rigid Flex PCB’s. It will help in resistance of oils, chemicals, dust, hydrocarbon solutions, and different hydrocarbon solutions. You will select the protective material after knowing the kind of material in use for the assembly of Rigid-flex PCB.

You are likely to find protective coatings such as cover lays and cover coats. Insulators The material that manufacturers use in making the substrates also act as the material for the insulators. They provide excellent electrical insulation between the parts that conduct electrical currents. The materials that manufacturers use in making the insulators are ceramics, Teflon and particular polymers. The most common substrate that doubles up as an insulator is the FR-4.  The FR-4 is a laminate of a fiber-glass epoxy that is readily available and affordable. It is an excellent insulator to electrical conductivity and can resist flames than the fiberglass.

How Rigid Flex PCB Work

The Rigid Flex PCB works under the same principals but in different applications. In many applications, they act as connectors allowing current to flow from one point to another. It makes it very suitable for use in many applications as you will learn in a short while. You will find it in use on computer motherboards and keyboards to help in the transmission of information.

Rigid flex PCB

In their case they work as connectors that transmit information from one part of the computer to the next. In the keyboards, they work by enabling the switch matrix so that when you type everything translates and transforms correctly.

Laptop keyboard with rigid flex PCB

In the LCD applications or fabrications, the glass will act as the substrate that has the flexible connections on it. They work in conjunction with other transmitters to produce the liquid crystal display. In this case, they enhance reliability and maximize the adaptability of the device. They also pass as interconnecting circuits, capacitors, and resistors either directly or by using other connectors to assist. This way they complete the circuit and make it relevant for the transmission of data and current. They also make the transmission of signals possible as the level of conductivity is very high. The principals of operation are in use in various applications such as:

• Cell or mobile phones
• Medical equipment for the transmission of signals to the computer
• Computer especially under the keyboard and the motherboard
• Solar Panels as they help in the transfer of heat for conversion into electric power
• Electronics such as television sets, radios, and many other applications.

Rigid Flex PCB Design Considerations

When you want to design a Rigid-Flex PCB, there are certain things that you need to consider. These considerations will enable the smooth operation of the Rigid Flex PCB and avoid hitches.

Rigid flex PCB design

Many designers have gone through a lot of demands expecting them to reduce cost and increase efficiency. It is the reason why many of them often turn to rigid flex circuits that are 3D to meet the rising demands. It requires very close collaboration and supervision between the fabricator and the designer so that they can work on eliminating mistakes. Continue reading and find out more about the things that you will consider.

Popular Rigid Flex PCB Design Software

Designing a Rigid Flex PCB will require you to have specific software that will guide you in making the right thing. It will also make the job of designing very simple and save you time and money in the long run. There is almost 46 software that you can employ in designing the Rigid Flex PCB.

Rigid flex PCB design

Some of the software that is available for you to choose from include:

§  PCB Artist

This is a reliable tool that you can use when designing your PCB and it has some of the best features. The key features include a components library of more than 500,000 parts, free Gerber files, and integrated schematics. Apart from that they also have a parts list plus their reports in CSV and a design rule that checks errors between select items.

§  Ultiboard

It provides fast PCB prototypes and several applications on it. It also has seamless integrations with the multisim and complete schematics of the circuits. More software includes:

• Altium designer 17
• Solidworks PCB
• Diptrace
• PCBWeb
• BSch3V
• XCircuit
• GerbV
• KiCad Eda
• Eagle PCB
• Circuit maker
• Pad2Pad
• OrCAD
• ZenitPCB
• CircuitStudio
• PCB123
• Cuspice
• FreePCB
• Mentor Graphics Xpedition
• ExpressPCB
• Tina
• Fritzing
• EasyEDA
• Proteus
• Upverter
• Circuit wizard.

Tips and Tricks for Rigid-Flex Design

Originally, designing the Rigid Flex was done in two separate parts where the hard part and flexible part were separate.

Rigid flex PCB design

It was very costly and time consuming, and they came up with better solutions to this problem. You can use the 3D animation or create paper models or mylar models. In design, there are tips that you need to watch out for, and they include:

• Cost of the design
• Working together with your fabricator will ensure that you get a perfect device at a low cost given the benefits.
• Board Shape
• Consider a shape that will be able to contain more circuits on it as you do the designing. It will help in conserving space and also the cost of designing. Think of the bending radius when looking at the designs and come up with flexible parts.
• Tracerouting – avoid the use of the sharp corners and curves to reduce the stress on the device.
• The path through holes and vias – try avoiding the use of vias in the bend regions, especially when designing for dynamic applications.

Other tips that you have to look at include a cover layer, planes, staggered lengths, service loop, and conservation of copper. You should also look at panelization, tear resistance, routing and the static bend ratio.

Rigid Flex Circuit Board Gerber Layout Guidelines

The guidelines based on IPC 2223C for the construction of rigid-flex PCB’s. Ensure that the degree of reliability and performance is very high. The key elements to look at are the adhesive flex cores. Selective flex areas and lamination of the rigid region. Look at the hard flex transmission line where the flexible layers and the rigid layers meet. After that consider the vias or the path through holes with a minimum requirement of 0.05 inches. Do not drill the PTH through specific cover lay adhesives. Look at the spacing on the copper features with a minimum clearance space of 0.025 inches on the external layers. Then check on the flex layer thickness and balance the construction of the elements. Make sure that they have the same finish at the end of the development considering the count on flex layers. You can use the flex layers in pairs and eliminate flex adhesives from the areas that are rigid. Control any form of impedance when doing the shielded designs. Offset the traces from one layer to the next in the flex area if you can. When doing the trace layout consider parallel or straight layouts to get rid of the potential of mechanical stress.

Rigid Flex PCB Manufacturing Process

What is the difference when assembling Rigid Flex PCB and Flex PCB? The production of the flex PCB is straightforward in comparison to producing the rigid flex PCB.

Rigid Flex printed board circuit

Rigid-flex PCB requires repeated laminations, and the factory requires the rigid PCB production machines. It will also need the mechanisms for producing the flex PCB’s making the quality control difficult.

Manufacturing Process of Rigid Flex PCB

Prepare all the material that you will need by cleaning the production panels using chemicals. Place the “circuit forming photoresist film” and ensure that there is proper adhesion of the film. To prevent any damage on the thin cores, use the handling equipment carefully. The second step is the exposure of circuit patterns by photo resisting the coated panels. The coated panels usually have circuit artwork patterns overlaying them. In the process, UV lights on the machine will transfer the circuit pattern on to the film. Etch process comes through as the circuit patterns etch on the thin core using specialized equipment. You can etch both sides of the film depending on the type of design you are making. You will then take it to the drilling machine to make the circuit holes. You can use laser systems in making the conductor holes that are very small. You will take the film for copper plating to fill the holes with the right copper elements. It will be the start point of making the device conductive. The next step is passing it through cover lay applications to align and tack them into the right place before laminating. After that, the next thing is to laminate using the cover lay technology under enough pressure and heat. It will ensure that the lamination and adhesion are proper and will not cause hitches in the future. The next step is the application of stiffeners before putting it back for lamination under the same conditions. You will subject the device through electrical tests making sure it conducts electric current properly. It will also help in isolation and continuity of the process.

CPU

The final step will be final fabrication where the individual parts are die cut using precision female/male dye sets. You can use other methods such as laser cutting, steel rule dyes, mechanical routing and use of chemically milled dies.

Main Applications of Rigid-Flex PCB

The current situation for Rigid-Flex PCB manufactures in the world Manufacturers are in a position where they have to update their software from time to time to keep up. It will also involve the update of machines from time to time which is usually very expensive. There is an emerging trend which requires them to introduce or integrate IoT to trigger the demand for flexible PCB. You will use the rigid-flex PCB in forms when you are looking at the reliability and adaptability of your device. It comes in handy when warping and flexing are unavoidable. The technology has come a long way, and they are in use in more compact limiting situations. The demand has also been on the increase in the recent past for applications on different devices. The expectations in it are huge, and it will grow further soon.  They are present in almost all the market segments including medicine, military, information, aviation, instrumentation, automotive, industrial sensors and controls.

FAQs on Rigid-Flex PCBs

Rigid Flex PCB

1)Do you allow the client to contribute on the design that he may require for his Rigid Flex PCB?

As a customer, you are at liberty of calling in and requesting for a custom-made Rigid Flex PCB. The input of the customer is very important, and we accept them because they also improve the mode of development. A team of Engineers is always available to guide you through the design you are requesting.

2)Will you allow me to choose from the options available of different equipment to use for assembly of my PCB?

There are quite many options especially on the equipment that you can choose. When you are requesting a custom-made Rigid-Flex PCB, you can also select the kit for assembly.

3)I have seen the PLCC plug in many connections, may I know its function?

There are wide varieties of the PLCC plug that you will come across in different Rigid Flex PCB designs. They serve as connecting links between the PLCC production sockets and the flexible PCB.

4)Can you bend the FR-4 Semiflex towards both directions?

The design rules state that the FR4 Semiflex is possible with the milling area inside.

5)What is the difference between the Polyimide and the LCP?

LCP or Liquid crystal polymer has low moisture intake and better dimensional stability in comparison to polyimide. It is not as reliable as polyimide, and the cost is high.

6)Do you have the UL listing for the rigid-flex?

There are four UL listing available which include flammability, full recognition, DSR, and CTI.

7)What is the cost of the FR4 Semiflex?

The cost of the FR4 Semiflex will vary accordingly depending on the value it provides you. The cost of using it will bring a comparative advantage to you, so it is fair.

8)Which software can I use when I want to design a flex-rigid?

You can use the EDA-software for the designs from the easy eagle to the sophisticated Mentor.

9)What is the maximum thickness of copper that I can use?

The nominal thickness of the copper you intend to use should not exceed 70 millimicrons.

10)Is it okay to use the Gigabit signal in the rigid-flex?

It is okay to use the Gigabit signal as many of the USB3 use it. You have to agree on a design that is well tuned with impedances well defined.

11)What is your recommendation on flexing inside or outside?

Flexing outside is better because it is cheaper than flexing on inside. Putting more layers inside will be more expensive.

12)Can I apply heat to help me bend the Semiflex?

It is possible but not under the recommendations as it may lead to embrittlement and aging. You can increase the number of stack-ups and use a bending aid instead.

13)What is the thermal resistance of the FR4 Semiflex when over molding it with plastic?

The thermal resistance does not change and is the same as that of the initial material.

14) What is the size of the smallest plate through holes that you can get?

“0.006 for a 0.062 finish thickness part or 10:1 ratio.”

15)Do you have the smallest dielectric that exists between layers?

The smallest that is available is as follows:

1. 1 oz copper cladding = 0.004-0.005 millimicrons
2. 2oz copper cladding = 0.004 millimicrons
3. Do we have anything by the name hybrid PCBs?

Yes, hybrid PCB’s are available mostly for the RF and the broadband industry.

16)What is FPC?

FPC stands for flexible printed circuit and is an interconnection for the transmission of signals for electronics.

17)  Which file guideline do you prefer to use in making the rigid-flex PCB?

We use the Gerber files which are in use in all manufacturing processes.

Conclusion

As you can see, rigid flex PCB design can be a simple process if you have the right tools and accessories. Besides, rigid flex PCB play a fundamental role in modern electrical and electronic systems. The best part – I have made everything about rigid flex PCB simple for you to understand. More importantly, you need to choose a reliable rigid flex PCB manufacturer to help you implement your project.