Consider using a loose-leaf approach to the stack-up in the bend regions of flex when there are four or more layers present. Making flex cores on several levels to place two or three flex pieces in between the stiff materials is a solution for a high layer count. In terms of pliability, using three stacked flexes with two or three layers each is preferable to a six-layer flex region. 

Flex thickness, width, and copper content all have an impact on flexibility and bend radius, as many FPCs have a restricted bend radius. There is an option to go below the minimal advised bend radius when using the loose-leaf technique. Calculating the minimum bend radius depending on the thickness of each layer in the stack-up is made possible by separating the layers into a separate flexible printed circuit board. However, each individual flex circuit is constructed to be a little bit longer than the one behind it, giving all of the flex elements enough space to bend without putting undue strain on either the layer next to it or on themselves.

Our HDI loose leaf stack up manufacturing services in Europe provide cost-effective solutions for high-density PCB designs.

Complex Design Rules:

Compared to stiff PCBs, rigid-flex PCBs frequently have more complex design rules. Bend radii, layer stackup, and material limitations are just a few of the mechanical and electrical specifications that designers must be aware of when working with rigid and flexible parts.

Bend Radius Considerations:

Selecting the right bend radius for the flexible sections is a crucial part of the rigid-flex design process. Selecting an excessively small bend radius can result in signal integrity problems and mechanical failure. For example, if the thickest sub-composite is 0.3 mm, then the IPC rules for determining the minimum bend radius yield a result of 3.0 mm. This falls much short of the minimum required by the original design.

Signal Integrity and EMI Control:

It can be difficult to keep signals intact and manage electromagnetic interference (EMI) in the flexible parts. Signal quality can be impacted by the substrate’s flexibility and the signals’ closeness to the bend location.
Layer Transition: It can be difficult to smoothly move signal layers from the rigid to the flexible regions. Impedance mismatches and signal loss can be caused by misalignment or inappropriate layer transitions.

 Flexible circuit boards, also known as rigid-flex circuit boards, are primarily used to enhance product integrity while conserving space or helping to reduce the end product’s size. Although this product can cost up to twice as much as a solid circuit board or much more than a wire assembly, its application and reliability are growing. The success of your product depends on your ability to design rigid-flex or flex PCBs correctly. Working with your supplier and their experts is the best and fastest route to market.

Flexible-rigid PCB stacking

Flex ribbon

You must describe your flex ribbon using a stack up that is comparable to your rigid PCB once you begin designing stiff flex PCBs. Since flex ribbons are thinner than the boards they interface with, they must have the same stack up as the rigid printed circuit board’s inner layers. In addition to carrying signal between boards, flex ribbons usually require a ground return connection.


 Copper design with crosshatching

Use a cross-hatched copper pattern if you want to stretch a power or ground plane all the way across the flex ribbon. Compared to a solid copper film or copper foil, cross-hatched copper is more flexible. In the event that signal layers on the flex ribbon require shielding, cross-hatched copper should also be utilized.

Immediately beneath the cover layer is the signal layer

Unless you want to place components directly on the ribbon, the signal layer on a flex ribbon does not need to be positioned exactly below the cover layer. As long as the signal layer is below the cover lay, SMT components can be positioned directly on the ribbon. This is turning into a more common design option.

The signal layer needs to have solder lands placed in it

In order for SMT components to reach the signal layer, the cover lay must have holes drilled in it and solder lands positioned immediately beneath it. Make sure your fabricator can handle your chosen design by always checking with them. It is always best to put components so that their length is parallel to the bend rather than placing them immediately in the bend area.



Owing to the wide variety of alternatives available when creating flex PCB layer stack-ups, designers should speak with their fabrication company to ensure that the stack-up they have in mind can be produced in a dependable manner. There are many distinctive design options available with flex and rigid-flex PCBs, but designers should make sure their idealized loose leaf stack up can be built as planned.

Bond piles and cover lays

Flexible materials called cover lays are used to shield and insulate the flex circuitry on surfaces and stop them from lifting. They are usually placed outside of a flex sub stack. An adhesive is usually placed on the side that faces the copper in a cover lay structure, with a polyamide dielectric placed on top. bond piles, which have adhesive on both sides and a polyamide dielectric in the centre, are utilized on inner strip line layers and resemble cover lays. They are applied while joining two flex cores.

Quality of PCB Manufacturing

During the process of fabrication and assembly, quality requirements are maintained by:

  • Statistical process control monitoring rates of scrap and defects
  • Thorough inspection verifying the output of the operation
  • Frequent machine calibration and maintenance
  • Adherence to strict industry standards such as ISO and IPC
  • Tight supplier quality control procedures
  • Supervision of the manufacturing floor environment for quality assurance
  • Thorough examination of the completed boards, in-process materials, and raw materials
  • Initiatives for continuous improvement to reduce flaws
  • Quality control guarantees dependable, consistent PCB production. 

Read More: Guide to PCB Component Placement for Efficient Assembly

Principal Uses for Rigid-Flex PCB and Loose Leaf Layer Stackups

 Because of their special advantages and design, rigid-flex PCBs (printed circuit boards) have found wide-ranging applications across many industries. These boards are ideal for a wide range of applications because they blend the structural stability of rigid boards with the flexibility of flexible circuits. Some of the primary uses for rigid-flex PCBs are listed below:

Aerospace and military:

A lot of applications in aerospace and military use rigid-flex PCBs. They are used in systems in aviation and spacecraft where it is essential to reduce weight, optimize space, and have great dependability. Rigid-flex PCBs are perfect for military and aerospace electronics because they can tolerate severe weather, vibrations, and high temperatures.

Medical Devices:

Rigid-flex PCBs loose stack ups are used in the medical industry for a number of applications, such as implanted devices, patient monitoring systems, and medical diagnostic equipment. These PCBs’ flexibility makes it possible for them to fit into small, asymmetrical medical devices, guaranteeing sturdy and dependable connections.

Consumer electronics:

Wearables, tablets, and smartphones are just a few examples of devices that frequently use rigid-flex PCBs. Their flexible parts allow electronic devices to be bent and folded, which makes them more portable and easier to use.

The automotive industry:

Uses rigid-flex PCBs in engine control units, dashboard displays, GPS navigation, and infotainment systems for vehicles. They are appropriate for the rigorous automotive environment because of their sturdy design and tolerance to temperature variations.

Industrial Automation:

Robotics, control systems, and sensors are just a few of the areas where rigid-flex PCBs are essential. When there is a shortage of space and dependability is crucial, they can be included in machinery and equipment.


Base stations, networking equipment, and antennas all depend on rigid-flex PCBs loose leaf stack ups for their infrastructure. Their small size aids in making the best use of available space in telecom infrastructure.

Wearable Technology:

Rigid-flex PCBs loose leaf stack ups are advantageous to the wearables market, which includes fitness trackers and smartwatches. These PCBs make it possible to design wearables with small, light weights and complex electronic components.

Quality and Compliance

In the electronics manufacturing sector, PCB Runner, a seasoned provider of expert PCB creation services, has over ten years of experience. The majority of the businesses we manufacture PCBs for are global in scope and have been with us since the beginning. We work hard to keep our clients’ satisfaction levels at or above 99%, and we take pride in offering the best service available. We are able to adapt to our clients’ changing needs because of our flexibility. Because we are adaptable, we can work with the most precise, efficient, and economical design and manufacturing methods.

Engineers, designers, R&D innovators, product managers, supply chain/manufacturing specialists, sales/customer service representatives, and quality assurance workers make up our multinational team of professional experts.

 Our production facilities are of the highest calibre, and we have synchronized our quality processes and technology roadmaps to meet the requirements of IPC Class 2, 3, and 3A. We are also accredited for ISO9001.

 Why Choose PCB Runner for special PCB design

The Netherlands-based PCB maker is aware that every client and their needs are unique and distinct. Customers need to be able to trust their source to fulfil their needs, in our opinion. Our speciality is assisting clients in launching their goods into the market as quickly as feasible.

One of the rare businesses that actually manufactures boards in-house is PCB Runner. We manufacture every kind of board, including:

  • Aluminum-based metal PCBs
  • PCBs using Rigid-Flex (FR-4 and Polyimide)
  • Board flex (polyimide)
  • Through-Holes, SMT, BGA, and other HDI boards
  • Lead-Free Boards that Comply with WEEE and RoHS
  • Intel PCB Mil spec (IPC Class 1, 2, 3).
  • 40+ layer multi-layered circuit boards
  • Materials for Boards: FR-4, Polyimide, Rogers, and Others
  • Spacing and Trace to 3 mils
  • Blind and Buried vias
  • Filled vias Boards with Heavy Copper Via-in-Pad
  • Skilled Assembly and Fabrication Services

In addition to the aforementioned, we provide professional manufacturing and assembly services for:

  • Prototype circuits
  • Board Runs
  • Small and Medium
  • Quick Turn PCBs
  • Burn-In for Boards


In a rigid-flex circuit, a loose leaf stack ups bookbinder construction is challenging since it necessitates creating a board that is not flat because each sub-composite has a varied length to accommodate the shape that the bent flex portion will take. Because additional tooling is needed, manufacturing might only be able to produce a few boards per panel. This could result in a much lower manufacturing yield and higher unit costs. Due to the need for separate forms for every flex board, the initial setup costs are very considerable. Several important components make up a rigid-flex circuit design, and each one requires careful thought during creation. Close cooperation between the designers, material supplier, and implementers is necessary for the successful implementation of a practical and elegant solution, even when their combined presence may provide a substantial challenge.

PCB Runner offers custom loose leaf stack ups Rigid Flex manufacturing for all industries and critical design requirements. Please do submit your schematics & BOM files to our email at or