An Overview of Flex Circuit Materials and Layer Stacks

There are so many applications of flexible PCBs, it is sometimes odd we do not see these more often. Many devices that require some type of mechanical motion or that have a complicated enclosure shape can benefit from a flex PCB design. These systems began their life in mil-aero, but since that time they have begun to appear in consumer and commercial products.

When designing a flexible PCB, the designer must pair up the appropriate materials for the design and understand how these work into the flexible layer stackup. The material ordering in the flex region of a PCB follows a specific order to provide the required copper connections and the insulating materials needed to build the layer stack. However, as we will see in this article, adding in a rigid section and extending this to multiple flex layers creates additional challenges in flex PCB design.

Flex PCB Materials Overview

Before looking at the typical stackup for a flexible PCB, it’s important to know the standard materials used in these systems.

• Base flex material: Polyimide/polyamide is most often used as the base material in flex PCBs. Rigid-flex PCBs have an additional rigid region that will contain the polyimide as an inner layer (see below).
• Copper conductor: Manufacturers prefer rolled-annealed copper foil instead of using a traditional electrodeposited copper foil.
• Coverlay: The exposed copper circuitry of the flexible PCB is protected by a coverlay material (sometimes called an “overlay”). This material plays the same role as a solder mask in a rigid PCB. The coverlay film is also polyimide.
• Adhesives: These epoxy-based materials are used to bond flex base material and coverlay onto the copper conductor. Note that processes involving plating directly on polyimide are available which do not require adhesives.
• Stiffeners: These non-flexing regions of rigid-flex PCBs use FR4 or polyimide to provide rigidity. There are two methods for attaching stiffeners to the flex PCB: lamination or adhesion using a pressure-sensitive adhesive (PSA).

The polyimide base material gives a flex PCB its characteristic orange color, as shown below. Note that there are also solder mask materials available for flex PCBs that can be used for extra environmental protection. These flex solder mask materials are specifically formulated for use on polyimide boards and they can provide a very unique look.

Rigid-Flex PCB Layer Stackup

There are many different layer stacks that can be used in a flexible PCB or in a rigid-flex PCB. The most basic uses a single flex region with two copper layers built on a flexible substrate material (polyimide) as shown below. This flex region can be directly incorporated into a rigid-flex PCB stackup or it can be used on its own. In a rigid-flex PCB, the coverlay film is bonded to the prepreg during lamination so that the FR4 stiffener regions can be assembled on the flex ribbon.

Example rigid-flex board with 6 layers in the rigid region and 2 layers in the flex region.

The rigid-flex stack-up typically consists of at least one flexible layer pair between rigid sections. The most commonly designed rigid-flex boards usually have double-sided flexible PCB laminated with rigid sections with layers of pre-preg as shown in the above figure.

Note that the stiffener sections use a prepreg to bond to the flex section in each of these arrangements. However, the layer stack shown above does not need to be limited to a single internal copper layer and two rigid FR4 layers. Multiple rigid layers can be stacked on each side of the flex region. Also the FR4 layers on each end can have different layer counts as these will be laid up and press laminated individually.

Multilayer Flex PCB Stackup

Two flexible layer pairs are usually not stacked on top of each other when the flex ribbon is just being used as an integrated connector. However, if we are talking about a multilayer flexible design, then multiple coverlay-substrate layers can be laminated in succession to give a multilayer flex PCB.

Example flex stackup with 4 layers.

The above image shows four flex layers, but realistically we can have an odd number of flex layers by bonding substrates directly and etching away one layer. Flex fabrication houses can help with planning a flex PCB stackup with an odd layer count.

Bookbinder Rigid-Flex PCB Stackup

We can also have multiple rigid layers between stacked flex regions in a bookbinder-type flex stackup. In this stackup, the flexible layer pairs are stacked on top of each other with some prepreg creating space in between the flex ribbons. By making one of the flex regions slightly longer, the entire stack can be folded without creating excess stress on the outer flex ribbon layer. This is outlined below.

Bookbinder rigid-flex stackup.

Due to the diverse range of options in building flex PCB layer stackups, designers should contact their fabrication house to make sure the stackup they want can be reliably fabricated. Flex and rigid-flex PCBs offer many unique design approaches, but designers should check to ensure their idealized stackup can be produced as intended.

When you’re ready to specify your flexible PCB stackup and specify materials in your flex PCB, make sure you use the complete set of design features in OrCAD from Cadence to specify your design requirements and create your PCB layout. OrCAD includes the industry’s best PCB design and analysis software with flex PCB design tools. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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