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Multilayer Rigid-Flex PCB

  • Multilayer-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 multilayer rigid-flex printed circuit board(PCB) combines rigid and flexible materials. Three or more copper-conducting layers are connected through holes in these circuits. 

This type of PCB has the strength and support of rigid materials and is easy to bend due to flexible sections. Rigid-flex PCBs are lightweight and compact and fit even in the smallest electronics with high-tech features. 

Some of the common applications of multilayer rigid-flex circuits are medical instruments, aerospace, and automotive sections. In this article, we’ll discuss all about multilayer rigid-flex circuit boards and their design considerations. 

How Does Rigid Flex PCB Work?

Multilayer PCBs have a 3-dimensional interconnection between conducting layers. It eliminates the use of multiple individual PCBs while doing an even better job. Flexible PCB materials such as polyimide are combined with rigid sections made from FR-4. 

The two layers are connected using strong adhesives. Vias and component holes are present in each layer of the circuit to make vertical connections. The rigid portion of PCBs has components and flexible regions are used to form bendable connections for dynamic functionality. 

Multilayer rigid-flex PCBs allow compact bending, folding, and wrapping to fit various electronics. They are a one-stop integrated circuit choice for advanced electronic designs with high performance. 

Each flexible layer has an external polyimide layer for insulation. Plated through-holes are added for circuit connections. Additionally, a surface stripline is added for high-speed controlled impedance. 

Overall, rigid-flex circuits can conform where electronics may have design challenges such as crosstalk, overheating, and component density. 

These types of PCBs are able to dissipate heat even in complex component areas as they have a layered structure. Moreover, the flexible feature allows the PCBs to be installed in uniquely shaped geometries. 

Types of Rigid Flex Circuit

There are two types of rigid-flex printed circuit boards.

Rigid-Flexible Composite PCB

It is the combination of rigid and flexible boards with a common blind and buried via. Blind vias connect the two layers without penetrating the boards. On the other hand, buried vias are used for augment circuit and trace routing. Composite PCBs have high-density designs. 

Rigid-Flexible PCB

Both rigid and flexible boards are made separately and then laminated together. These result in advanced boards with high performance that can fit into small design spaces. 

The main applications of rigid-flexible PCBs are medical devices, TVs, and lighting. Due to their high-temperature resistance and ability to bend, rigid-flex circuits are ideal for high-end electronic devices with long running hours. 

Design Considerations for Rigid-Flex PCBs

The design requirements of rigid-flex PCBs are a bit complex. Here are some critical ones to help you design reliable circuit components: 

  • Layer Stackup

There should be ample layer stackup with both rigid and flex layers to ensure reliable bending and functioning. If there are more flex layers, the circuit won’t have a strong structure. 

If the rigid layers increase, the PCB will be thick. The presence of the right rigid and flex layers gives a sturdy structure to the circuit when it’s bent in unique design shapes. 

Rigid layers can be formed using reinforced laminates or FR-4. Flex regions of the circuit use dielectric materials that can be bent in different shapes. 

An advanced adhesive should combine these two layers into one structure. Note that the balance of these layers is essential to add impedance control and stability.

  • Bend Radius

Careful attention should be given to not going over the minimum bend radius. If the bend radius is less, it will lead to copper fracturing and component damage. In addition, it can delaminate the layers, making the PCB dysfunctional. 

To make flex-rigid PCBs, a bend radius of 3mm to 10mm is generally used. The bend radius can be adjusted by using different copper thicknesses, adding more flexible materials, and changing the stackup design. 

  • Rigid-to-Flex Transitions

Many factors, such as gradual transition geometry and annular rings, can impact the rigid-to-flex connections. If the connections are not carefully done, the PCB will crack or fail to function. 

To avoid this, reduce copper use near transitions to avoid material piling up. Skip using acute angles and gradually taper the corners for easy transitioning. 

  • Placement of Component

No components can be added to the flexible layers as they only support conformity and movement. Therefore, circuit components should be placed in rigid sections. Besides, overlapping should be avoided to ensure that flexible sections are freely bendable. 

It will also remove stress from the connectors. Another thing to keep in mind is avoiding component placement around the rigid and flex section transitions. Pay attention to the thermally adjusting components and provide ample copper area. 

  • Routing

Routing should be done along the neutral axis to avoid breaking and peeling of PCB. Avoid acute angles during transitions. Pay attention to the copper density, as it may make the circuit hot due to the lack of heat dissipation in some sections. Use wider traces and space them appropriately to support flexibility. 

Rigid-Flex PCB Assembly

The following steps are followed for rigid-flex PCB assembly:

  • Material Prep

The first step is to clean the copper-clad laminates before the fabrication. After this, the prepreg, polyimide, coverlay and stiffeners are cut as per the requirement. 

  • Inner Flex Core

The flex board will have a flexible inner core that can be done using polyimide(PI) and wrapping in thin copper foil. In addition, copper foils are laminated when adding more than two flex layers.

  • Making Flex Core Circuits

After lamination, the flex core circuit is made with copper foil coated with a photoresist to prevent UV irradiation. A non-transparent film is used to make PCB connections on the foil. This film is cured under UV light to safeguard circuit patterns. 

The last step is to wash away the uncured photoresist and soak the exposed copper in NaOH solution. It will dissolve extra copper, and the copper circuits in the inner core will become more prominent.

  • Circuit Lamination

When there are more than two rigid-flex layers, alternate copper foil and PI layers are laminated. The lamination is carried out before adding any circuits. Besides, copper is used to electrolate the circuit with tin. 

Any excessive material is dissolved in a NaOh bath, leaving only electroplated copper traces. The next step is drilling holes in the plates using a laser. The coverlay is laminated to complete the flex layers of the PCB.

Rigid Section Circuits, Drilling and Lamination

The rigid section of the PCB is laminated with alternating prepreg layers and copper foil. Holes are added to the board along with circuits on rigid sections. You may use a laser to make an HDI PCB. 

  • Cut Excess Material

Use a laser to cut off the extra prepreg so that the section is exposed.

  • Test PCB Functioning

Some common PCB testing includes impedance checks, bending, and thermal performance. These tests are done after adding silkscreen and surface finish over the PCB. also check for the quality of holes, connections, and mechanical resistance.

Benefits of Multilayer Rigid-Flex Circuits

Multilayer rigid-flex PCBs have several benefits, including: 

  • Better Functionality

Multilayer circuits have a higher circuit assembly density, offering improved functionality. It enhances the capacity and signal speed due to layers, even in a small-size PCB. 

  • Reduced Errors in Assembly

Since the PCB layer production is automated with machines, there’s less room for human error. Many hand-built wires have these errors and lead to failure in functioning. Automated assembly ensures perfect routing and vias for connections.

  • Affordable

The manufacturing does not rely on manual labor, reducing separate soldering, wrapping, or routing costs. Moreover, the increase in production volume due to automation makes bulk orders cheaper.

  • Flexible Designing and Installation

The multilayer three-dimensional design gives freedom to design. With an advanced design, multilayer rigid-flex PCBs are able to function better under heavy loads and chemical changes and withstand harsh weather.

Another plus point is the flexible installation, as the bendable circuits can be adjusted anywhere to achieve better electronic movements and function. Also, the high density of these circuits allows more space to add multiple features.

  • Improved Heat Dissipation

Rigid-flex PCBs with multilayer designs have better flow. As a result, they are able to dissipate heat better. Moreover, better airflow ensures a lower overall operational temperature. The increased surface/volume ratio also increases the lifecycle of the circuit.

  • Lightweight

Multilayer rigid-flex circuits are made using thin dielectric substrates. Therefore, they are more flexible with sturdy structures. Their small size makes the assembly of smaller electronics easier. 

  • Durable 

The fewer interconnections in rigid-flex circuits improve durability. These circuits flex up to 500 million times before they stop working. Even if the thermal conditions are extreme, they will still function as normal. 

Applications of Rigid-Flex PCBs

Rigid-flex circuits have reliable performance at a low manufacturing cost. They are used in various high-tech devices, such as aerospace technology. Rigid-flex circuits are also used in simple lighting to make them long-lasting. 

Their industrial applications include radiofrequency devices or military communication devices that require high-quality signals. They are ideal for high-shock impact devices without getting damaged. 

Due to its flexible nature, they are used in medical devices such as health monitoring wearable equipment. Some of the high-tech equipment like X-ray machines, heart monitors, and CAT scan devices use rigid-flex PCBs. 

Another significant application is shipment tracking and scanning for eCommerce departments. It makes the job easier and faster. 

Conclusion 

Multilayer rigid-flex PCBs are versatile digital circuits that are used in radiofrequency, high-speed communication, and microwave systems. Rigid-flex PCBs offer increased signal quality while being lightweight and compact. When designing these circuits, consider the materials, bend radius, and internal component placement. Partner with a professional PCB manufacturer to ensure premium quality with ultimate design flexibility. 

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