Of all FCBs, the 2 layer flexible circuit board is perhaps the one that’s used the most, and for good reasons.
With these boards, you can increase design flexibility, routing space, durability, and reliability, thereby increasing power handling capability.
Plus, they cost less to manufacture.
For these reasons, they’re widely employed in modern-day electronic applications, so there’s an ever-increasing demand for these boards.
This article explains more about double-layer flex FPCs and how they can contribute to your design.
Table of Contents
- What is 2 Layer Flexible Circuit Board?
- Types of Double Layer FPCs
- Materials Used to Make 2-Layer FPCs
- Double Sided Flexible PCB Stackup
- Double Sided Rigid Flexible PCB
- Advantages of Doubled-Sided Flex PCBs
- Double-Sided Flexible PCB Applications
What is 2 Layer Flexible Circuit Board?
A 2 layer FPC is a flexible circuit with two conductive copper layers or membranes on each side of the base frame, typically a polyamide.
That means you can produce trace designs on each side of the substrate that suit your needs.
Usually, engineers connect these trace designs using through-holes that are copper-plated. However, it’s possible to build this board without through holes.
Additionally, these boards allow greater flexibility, and it’s easier to significantly reduce the size of using them.
For this reason, they’re commonly used in applications with limited space or requiring a lot of movement.
Types of Double Layer FPCs
Depending on the design requirements, the cover layer on double-layer FPCs can be on one side, both sides or non-existent.
Now depending on the presence or lack of the cover layer as well as plated through holes (PTH), the 2 layer FPCs fall under the following four classifications:
- Those with both cover layer and PTH
- Those with a cover layer but no PTH
- Those with PTH but no cover layer
- Those with neither the cover layer nor PTH
Materials Used to Make 2-Layer FPCs
Ultimately, the materials you choose to build a 2 layer flexible PCB depend primarily on how you intend to assemble and use the board.
But manufacturers mostly use the following materials to make these boards.
First, copper foil
and alloys of aluminum, gold, nickel, and silver. The copper foil forms the conductor material laminated onto the substrate.
And it’s often rolled and annealed (RA) and electrodeposited (ED).
The RA foil is a smooth and evenly structured type of foil, while the electrodeposited one forms a sturdy foil type that, strong as it is, cannot withstand the etching process.
Then there’s the photosensitive polyimide coating.
And the reason polyamide is popular is it allows remarkable stability in terms of dimension.
Also, with this substrate, you can attain a high component density.
Further, base materials like films, foils, and adhesives.
What makes LCP more popular is the fact that it absorbs less moisture which makes it ideal for the process of manufacturing polyimide.
Double Sided Flexible PCB Stackup
Below are the different copper and insulating layers that make up a 2 layer flexible circuit board:
- Inner layer: Both the inner and outer layers are copper. And it’s on the inner layer that vias are, forming electrical connections through the layers. That way, electrical signals can flow from layer to layer.
- Ground plane: The ground plane consists of a metal layer of copper foil connected to the circuit ground. But besides being ground, it helps with heat dissipation and minimizes noise in electrical signals. Many engineers also use the inner layer as the power layer when necessary.
- Signal layer: Double-layer flexible PCBs have two signal layers. One on the top layer for routing signals on the external part of the board, and another on the bottom layer for signals that you’ll route on the internal part of the board.
- Prepreg layer: This layer has a dielectric membrane that provides support and insulation between the copper layers.
- Solder mask layer: Now, this is a thin lacquer-like polymer layer that protects the board from corrosion and oxidation. Thus, it prevents performance deterioration, enhancing the board’s longevity.
- Silkscreen layer: The silkscreen consists of a thin layer of ink traces on which you print details of the board name and part numbers to help identify components. Also, you can print any warning symbols, logos, or test points.
Double Sided Rigid Flexible PCB
Double-sided rigid, flexible PCBs combine the features of a double-sided rigid circuit board and double-sided flex boards.
See, rigid boards comprise materials and substrates that make them solid and unbending.
On the other hand, flexible boards comprise materials that enable the base of the flex circuits to bend into different shapes and dissipate heat.
So with double-sided rigid-flex PCBs, engineers connect the rigid parts to the flexible parts so that the board can bend on the flexible parts.
Also, electrical signals can transfer from the rigid sections of the board to the flexible ones.
Advantages of Doubled-Sided Flex PCBs
- Flexibility: First, these boards offer users more layers without the negative impact on flexibility as they’re not too thick.
- Lower cost: Secondly, the boards cost relatively less to make and are also easier to design.
- Small footprint: Also, because the dielectric substrate used in 2 layer designs is very thin, it allows more streamlined designs.
- Wide application: In addition, because the design can accommodate complex layouts, you can use it on a wide range of applications.
(Caption: Flexible circuit board with components)
- More routing space: Typically, double-layer FPCs have two routing layers and, therefore, more routing space. That means there’s ample room for traces and pads.
- Signal integrity: For some applications, you must use a 2 layer FCB. A good example of such an application is one that’s meant for use in a high-speed environment. In such cases, signal integrity is one common issue you’re likely to face, and a 2 layer helps counteract that.
- Lightweight: Because of their thin and light nature, 2 layers are flexible, and it’s possible to make them smaller in size. Thus, you’ll use a smaller amount of materials for these boards and therefore cut down on costs, especially for copper, which is very expensive. More importantly, this lightweight feature comes in very handy in the making of small devices like hearing aids, implantable medical devices like pacemakers, and mobile devices.
Double-Sided Flexible PCB Applications
Below are some common applications of the double-sided flexible PCB:
- Aerospace – ground and power plane apps.
- Medical – hearing aids, pacemakers, etc.
- Military equipment
- Signal transmission
- Digital cameras
- Gaming consoles
- GPS systems
- RFID systems
- Devices like calculators, printers, and phones
What Are 2 Layers in a PCB?
2 layers in a PCB means the circuit board has two conductive layers, one on each of the two sides of the base frame.
How Many Layers Can a Flex PCB Have?
While most designs have one to three layers, and some up to six, a flex PCB can have as many as 40 layers.
Still, as much as it’s possible to have very many layers, just remember that when a design exceeds three layers, it’s likely to have errors.
Often, it will have limited bendability because of its thickness.
How Thick is 2-Layer PCB?
The 2-layer PCB’s standard thickness is 0.6mm.
Now you understand better about the 2 layer flexible circuit board.
For example, you now know the different types of double-sided flexible PCBs, the materials used to make them, and the benefits and advantages of using them.
We hope you find this article helpful and that you’re better placed to make an informed choice on whether or not to use this board type.