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Layered Rigid-Flex PCB

  • layered-rigid-flex-pcb

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OVERVIEW

Item Rigid Flex PCB
Max Layer 36L
Inner Layer Min Trace/Space 3/3mil
Out Layer Min Trace/Space 3.5/4mil
Inner Layer Max Copper 6oz
Out Layer Max Copper 3oz
Min Mechanical Drilling 0.15mm
Min Laser Drilling 0.1mm
Aspect Ratio(Mechanical Drilling) 12:1
Aspect Ratio(Laser Drilling) 1:1
Press Fit Hole Ttolerance ±0.05mm
PTH Tolerance ±0.075mm
NPTH Tolerance ±0.15mm
Countersink Tolerance ±0.15mm
Board Thickness 0.4-3mm
Board Thickness Tolerance(<1.0mm) ±0.1mm
Board Thickness Tolerance(≥1.0mm) ±10%
Impedance Tolerance Single-Ended:±5Ω(≤50Ω),±10%(>50Ω)
Differential:±5Ω(≤50Ω),±10%(>50Ω)
Min Board Size 10*10mm
Max Board Size 22.5*30inch
Contour Tolerance ±0.1mm
Min BGA 7mil
Min SMT 7*10mil
Surface Treatment ENIG,Gold Finger,Immersion Silver,Immersion Tin,HASL(LF),OSP,ENEPIG,Flash Gold;Hard gold plating
Solder Mask Green,Black,Blue,Red,Matt Green
Min Solder Mask Clearance 1.5mil
Min Solder Mask Dam 3mil
Legend White,Black,Red,Yellow
Min Legend Width/Height 4/23mil
Strain Fillet Width 1.5±0.5mm
Bow & Twist 0.05%
Table of Contents
Primary Item (H2)

A single-layer PCB is fast becoming a thing of the past. Today, you will witness it in only the simplest of the PCBs. Layered PCBs are more widely used, particularly the rigid-flex ones. These not only offer the ability to bend and flex how you like them to be but also have enhanced functionality. 

Through the course of this article, we will learn about its features, design considerations, the manufacturing as well as its application across industries. 

But, before we proceed to the layered rigid-flex PCB, it is highly important to understand what a rigid-flex PCB is.

What is a Rigid-Flex PCB?

In simple words, a rigid-flex PCB is a printed circuit board that incorporates and exhibits the qualities of both sturdy circuit boards and flexible circuit boards. A successful combination of the two ensures that the resultant PCB can be flexed comfortably and can hanlde the harshest of opearting environments.

Now, let's define the layered form.

What is a Layered Rigid-Flex PCB?

A layered rigid-flex PCB is a kind of rigid-flex board that has many layers or rigid and flex circuits amalgamated to reap the benefits of a a flex board and a rigid PCB.

Each layer is carefully selected to form a PCB that works as intended for the application that it is made for.

The number of layers in a rigid-flex PCB can range from 1 to 100. Thanks to several manufacturers, it is now easy to create PCB prototypes and assemblies at a much faster rate. Although the process is slightly complicated, there is no stopping them from constructing these for different industries. 

Green printed circuit board with yellow flexible connectors and surface-mount technology

Key Features of a Layered Rigid-Flex PCB

  • The layered rigid-flex PCB, as we know, has parts that can bend over and stiffen. The latter is characterized by layers that are arranged on top of one another. While some of them act as a ground, the others are combined to carry the signals. 
  • The rigid part is not as thin as the flexible part. However, it is constructed in the same fashion as the inner layers of the stiff PCB sections.
  • It is the flex ribbons that create signals in between the rigid sections. So, to make it possible, a minimum of one layer is needed in addition to a ground plane is required for the currents that return.
  • A cross-hatched copper pattern is used to make power or ground connections stretch over the flexible ribbon. This makes the PCB highly flexible. 
  • You can place the components directly on the flex ribbon. But only if the overlay has a signal layer right above it. Make sure the overlay is always on the signal layer. Button plating methods can also be used. 
  • Embedding of components is not required. Components can be placed directly on the flex ribbon if there is a signal layer below the overlay. Solder lands need to be in the signal layer below the overlay. An alternative for mounting components is to use plated through-hole vias under the components, sometimes called button plating.
  • Components or critical features are placed directly in the flex ribbon bend areas.
  • The rigid and flex sections are streamlined by using the correct layer alignment. All the components are evenly stacked on the circuit. 

Why use Layered Rigid-Flex PCB?

There are several benefits of using these layered rigid-flex PCBs, including:

Enhances functionality

The many layers allow you to achieve higher circuit density in comparison to when constructing single-layer rigid-flex PCB. This also makes it possible to add more complicated functionality into a smaller space.

Saves space

PCB assembly in the case of layered rigid-flex PCB includes only a few assembly steps. In it, the flex PCB uses polyimide films. These sheets reduce the surface area and, in turn, lower the packaging size of the final PCB. It also makes the final circuit much lighter in weight.

Multiple designs can be incorporated into one. This is yet another way space is saved. 

Freedom of designing

With rigid-flex circuits, you can bend and flex things in three dimensions. This means you can create more innovative designs. This flexibility allows a manufacturer to create appealing products that are more durable and functional. 

Assembly cost is cheaper

With many layers involved, you don't have to worry about the assembly cost shooting up. Automation is possible such as in the form of robots and machines. This speeds up the fabrication process, thus saving time. Efficiency too increases with this.

More reliable

Although many layers are involved in the construction, layered rigid-flex PCBs handle heat dissipation really well. With more layers, more components are involved that allow precise routing of signals, thus reducing signal distortion. This also ensures consistent performance all over the board, making it reliable. 

Higher Flexibility

The ability to flex and bend implies that they can be molded into various desirable shapes and sizes. For example, in aerospace systems, the space is limited and the circuits must be fitted into complicated shapes. Layered rigid-flex circuits allow inclusion into irregular surfaces. 

This way, you can optimize the space to its full potential. 

Fewer connectors

Connections are either constructed in the substrate directly, or they are embedded. What this means is that you don't need separate connector parts or wires to attach them. This kind of PCB assembly simplifies the process by minimizing the individual parts. There is also a lesser risk of failure. 

Blue multilayer PCB with intricate circuits and flexible connectors visible

The Fabrication Process of Layered Rigid-Flex PCB

Depending on the number of layers you decide to build, here are a few considerations:

The Substrate materials

Overall performance, durability, and flexibility depend on the materials used in making both flex and rigid PCBs. Generally, for flex PCBs, polyimide films such as UBE Upisel and Dupont Kapton are used as these have high electrical and mechanical properties. High Tg materials like Arlon, Isola, and Nelco make rigid circuits.

Even the bonding film can be customized to match the PCB design. Epoxy, acrylic, and nitrile phenolic work well. For coating traces that are durable, liquid photo-imageable coverlays are best.

Once these are selected, the next step is to prepare them.

Preparing the base material

The initial step in the fabrication of rigid-flex is cleaning the laminate. The surface comprises a copper layer, which may or may not have an adhesive layer over it. Before proceeding, this is prepared with intense cleaning. 

Bonding of layers

The substrate materials are placed in layers. These may comprise prepgs, copper foils, cores, and adhesives. After this, the books and other components are organized to move into the lamination press. At this point, high pressure and temperature are applied to bond them together. 

Generating a pattern

Start by putting a dry film on the copper layers on the exterior. Even liquid photoresists can be used. With the help of lithographic imaging, circuit patterns are established. Last, copper that is not protected by the resist is removed. This creates the conductor traces for the circuit.

Etching

As you might know, etching is a process used to eliminate unwanted copper from the circuit. When building a layered rigid-flex circuit, each laminated side is etched with an etchant spray or by giving the laminate a nice etch bath. 

Mechanical drilling

All drilling work, whether for pads, circuit holes, or via patterns, is done with the help of precise techniques. Then, these holes are covered with copper using the electroplating method. This forms connections between layers.Here's an example - the laser drilling method. This is known for making exact results without taking much time. 

Plating copper

With this procedure, the required amount of copper metal is released into the PTAs (plated through holes). This helps in making electrical connections in the PCB. 

Applying coverlay

Mostly, polyimide is used as a coverlay substance. Once done, an adhesive is added to the surface of the printed circuit board with the help of the screen printing method. 

Lamination

Using the right amounts of pressure, heat and vacuum, the coverlay is attached properly. 

Applying stiffener

Before making any more lamination, apply local stiffeners. However, make sure you are doing this addition only if the design requires it. 

Soldering

Holes are created wherever soldering is required. After that, a copper film that is left exposed is covered with immersion silver or ENIG. Last, marks and print labels are printed. 

Cutting the flex board

After the above additions, it is now time to separate the flex board from its production panel. So, the two easiest and most widely used methods are specialized blanking knives and hydraulic pinching methods. 

Verification

After you have carried out the above steps as described, test the layered rigid-flex board for its efficacy. You can test it electrically to evaluate its quality and standards. This helps establish that the product has met the set design specifications. Some common methods employed for testing include grid testing systems and flying probes. 

illustrating the layers of a flex-rigid PCB with labeled components and transition zones

Types of Layered Rigid-Flex PCB

Here are a few examples of layered rigid-flex PCB.

  • 2-Layer Rigid with 1 Flex Layer
  • 3-Layer Rigid with 1 Flex Layer
  • 4-Layer Rigid with 2 Flex Layers
  • 4-Layer Rigid with 2 Flex Layers and ZIF Contacts
  • 5-Layer Rigid with 3 Flex Layers
  • 6-Layer Rigid with 4 Flex Layers:

A Few Things to Remember

  • The circuit should enable you to organize the wires and the ground planes in a way that stops signals from preventing electromagnetic interference. 
  • PCB boards that have lots of pins may need more signal layers. However, if there are too many layers and you feel they are unnecessary, you can take some away. Remember, arrange the layers in a way that does not stop signals from mixing up or causing problems.
  • In a layered rigid-flex PCB board, the different layers switch between signals and power/ground areas. These are then separated by materials like dielectric core or prepreg. While this setup helps reduce electromagnetic interference and signal mixing, it is also important to deal with the heat.
  • If the device to be constructed has both analog and digital parts, then keep them in separate layers. Connect the ground planes at just one point, as this will help check the noise. You can also design the ground plane in a way that both analog and digital parts have separate sections.
  • If you decide to keep digital and analog parts separate, then place the ground plane between the two, and it will act as a barrier. Put the analog ground plane next to the analog signal layer and the digital ground plane next to the digital signal layer. This ensures that each signal layer will only affect its own ground plane.
Green PCB with gold tracks and multiple outlined cutouts for electronic components

Applications of Layered Rigid-Flex PCBs

Consumer electronics

The ability to bend and create compact designs makes layered rigid-flex highly suitable for electronic devices such as tablets, laptops, smartphones, and wearables. 

Medical devices

Apart from bending and flexing, medical equipment requires body movement. The rigid-flex technology, in combination with layering, makes this possible. Examples of such devices include pacemakers, hearing aids, and imaging equipment.

Automobile industry

Several electronic systems in cars, such as engine control tools, navigation tools,, and sensors, must be sturdy enough to handle vibrations and work optimally. The rigid-flex circuit can do both, making cars more reliable.

Defense and aerospace

This sector is tough terrain. Machines such as radars, avionics, and missile guidance systems are required to operate in harsh environmental conditions, such as extremely high temperatures and vibrations. The circuits in rigid-flex are created to work in such challenging conditions without compromising the results. 

Industrial equipment

Places like factories and other such places require machines that can fit into smaller areas yet move and function freely. By using rigid-flex tech, equipment like control systems, robots, and instruments are created. This makes sure the interconnections are flexible and can flex independently. 

Parting Thoughts

By combining the sturdiness of rigid circuits with the flexibility of flex circuits, layered rigid-flex PCBs offer several advantages over other circuits. These are significant advancements that are delivering innovations across industries.

Not only this, the technological development is expected to push new limits and expand its reach. For more detailed information about Layered Rigid-Flex PCB, feel free to get in touch with our experts.

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