Expanding Options for Rigid

Sep 08, 2023

Ken Ghadia | Mar 10, 2023

Building electronics in unconventional form factors with high packaging density is possible thanks to three-dimensional circuit designs using flex and rigid-flex printed circuit boards (PCBs). Advanced applications in medical devices, automobiles, and aerospace systems largely depend on the flexibility and stability offered by flexible printed circuits (FPC). These benefits have also made them popular in the wearables industry and in miniaturized electronics. The rapid progress in board materials and manufacturing technologies has enabled myriad configurations in the flex and rigid-flex PCB designs.

Based on the required bendability of your product, you can choose either flex or rigid-flex PCBs for your design. The complexity involved in accommodating multiple features will demand higher signal layers in the PCB stack-up. A flex ribbon transfers signal between the boards and features a similar stack as the rigid section. By using the latest design and layout tools, you can easily build the necessary stack-up for your product. It starts from an even-layered, symmetrical structure to an advanced air-gap flex layer construction. We shall discuss some of the configurations that are used in the current electronic designs.

A simple rigid-flex PCB starts with two rigid and one flex layers. This configuration can provide only limited features and is hardly used in today's gadgets. A more-typical structure includes four rigid layers with two flex layers. Table 1 represents an even-layered, symmetrical stack-up that can support impedance-controlled traces.

FR4 is the commonly used rigid insulation material for PCBs. IPC 2221 provides a list of suggested materials based on the product classifications. Ribbons are made from flexible polyimide. They are thinner compared with the boards they connect. But their stack-up will be similar to the inner layers of the rigid PCB. The coverlay in the flex area serves the same functionality as the solder mask of the rigid section. A liquid photo imageable (LPI) solder mask is widely used in rigid-flex PCB fabrication.

Four Rigid Layers with Two Flex Layers

RIGID

FLEX

RIGID

Solder mask

Solder mask

Copper LAYER 1

Copper LAYER 1

FR4 Substrate

FR4 Substrate

Prepreg

Coverlay

Prepreg

Coverlay Adhesive

Copper LAYER 2

Copper LAYER 1

Copper LAYER 2

Polyimide Core - Adhesiveless

Polyimide Core - Adhesiveless

Polyimide Core - Adhesiveless

Copper LAYER 3

Copper LAYER 2

Copper LAYER 3

Prepreg

Coverlay Adhesive

Prepreg

Coverlay

FR4 Substrate

FR4 Substrate

Copper LAYER 4

Copper LAYER 4

Solder mask

Solder mask

Table 1

In Table 1 above, there are four signal layers in the rigid section and two signal layers in the flexible section. There can be up to 20 rigid layers and around six flex layers in any generally used rigid-flex PCB design.

It can be noted that the flex layers are placed exactly at the center of the stack-up to build a symmetrical design. This is preferred to achieve the mechanical stability of the PCB. Though some applications require asymmetrical structures, a balanced stack-up can minimize possible board twists or warping issues.

Zero insertion force (ZIF) connectors are often used to latch delicate ribbon cables like FPC cables. To avoid a separate flexible circuit for ZIF connectors, you can directly extend your design with a ZIF tail construction. This saves a good amount of space in rigid PCB areas and improves signal connectivity.

Considering the same example of an even layered symmetrical stack-up having four rigid layers with two flex layers, you can modify the right section of the stack-up to integrate a ZIF tail construction as shown in Table 2 below.

Four Rigid Layers with Two Flex Layers and ZIF contacts

RIGID

FLEX

ZIF Tail construction

Solder mask

Copper LAYER 1

Polyimide Stiffener

FR4 Substrate

Adhesive

Prepreg

Coverlay

Coverlay

Coverlay Adhesive

Coverlay Adhesive

Copper LAYER 2

Copper LAYER 1

Copper LAYER 1

Polyimide Core - Adhesiveless

Polyimide Core -Adhesiveless

Polyimide Core - Adhesiveless

Copper LAYER 3

Copper LAYER 2

Copper LAYER 2

Prepreg

Coverlay Adhesive

Coverlay

FR4 Substrate

Copper LAYER 4

Solder mask

Table 2

ZIF connector specifications define a strict dimensional tolerance for the precise placement of contact fingers. A maximum of four flex layers can be accommodated in this configuration. But one or two flex layers are commonly used in the design. A polyimide stiffener with a thickness of around 0.05 mm to 0.20 mm is included to meet the ZIF thickness requirements.

Rigid-flex PCBs can be designed with different shielding methods to guard products like cell phones, MRI machines, and radar systems against external EMI radiations. One such configuration with a shielding film layer is shown in Table 3 below. Again the configuration with four rigid layers with two flex layers is considered here. The shielding film layer is laminated to the external surface of the coverlay under heat and pressure. This method is cheaper and more popular compared with copper layer or silver ink shielding techniques.

Four Rigid Layers with Shielded Flex Layers

RIGID

FLEX

RIGID

Solder mask

Solder mask

Copper LAYER 1

Copper LAYER 1

FR4 Substrate

Shield Layer

FR4 Substrate

Prepreg

Coverlay

Prepreg

Coverlay Adhesive

Copper LAYER 2

Copper LAYER 1

Copper LAYER 2

Polyimide Core - Adhesiveless

Polyimide Core - Adhesiveless

Polyimide Core - Adhesiveless

Copper LAYER 3

Copper LAYER 2

Copper LAYER 3

Prepreg

Coverlay Adhesive

Prepreg

Coverlay

FR4 Substrate

Shield Layer

FR4 Substrate

Copper LAYER 4

Copper LAYER 4

Solder mask

Solder mask

Table 3

The increasing number of signal layers impacts the bending ability of rigid-flex PCBs. But to retain the benefits of flexibility and reduced cost, IPC recommends configuring two or more flex layers together as one set. Such a configuration is known as an air gap construction, and it eliminates flex adhesives in the rigid areas of the PCB. The flex sets bend separately with this configuration and minimize via-hole inconsistency issues.

Table 4 shows a rigid-flex configuration with the air-gap construction. It includes a stack-up with six rigid layers and four flex layers. Each pair of flex layers is grouped with coverlay on both sides. An air gap is constructed between the two pairs of flex layers as per the IPC 2223 guidelines. IPC's interconnect stress test has successfully established greater reliability of a rigid-flex PCB with the air gap construction by subjecting a sample board to rapid temperature cycling.

Though this process adds extra coverlay on both sides of flex pairs, the benefits of a higher yield and reliable performance make it worth implementing.

Six Rigid Layers with Four Flex Layers Having Air Gap Construction

RIGID

FLEX

RIGID

Solder mask

Solder mask

Copper LAYER 1

Copper LAYER 1

FR4 Substrate

FR4 Substrate

Prepreg

Coverlay

Prepreg

Coverlay adhesive

Copper LAYER 2

Copper LAYER 1

Copper LAYER 2

Polyimide Core - Adhesiveless

Polyimide Core- Adhesiveless

Polyimide Core - Adhesiveless

Copper LAYER 3

Copper LAYER 2

Copper LAYER 3

Prepreg

Coverlay adhesive

Prepreg

Coverlay

AIR GAP

Coverlay

Coverlay adhesive

Copper LAYER 4

Copper LAYER 3

Copper LAYER 4

Polyimide Core - Adhesiveless

Polyimide Core - Adhesiveless

Polyimide Core - Adhesiveless

Copper LAYER 5

Copper LAYER 4

Copper LAYER 5

Prepreg

Coverlay Adhesive

Prepreg

Coverlay

FR4 Substrate

FR4 Substrate

Copper LAYER 6

Copper LAYER 6

Solder mask

Solder mask

Table 4

All the configurations discussed till now are mainly even layered and symmetric constructions. But it is possible to build an odd-layered stack also. Applications requiring GND-SIGNAL-GND configurations for double-side shielding in flex areas can preferably adopt the odd layers strategy. This reduces the design cost and improves bending ability.

Instead of placing flex layers at the center of the stack-up, they can be shifted above or below for an asymmetric design. Such a configuration may be employed if a design has varying dielectric thickness requirements. But the unbalanced configuration will impact the board's mechanical strength, so the board has to be suitably supported during the assembly process.

Designs with different rigid area thicknesses or varying flex layers are also viable. Many PCB manufacturers offer diverse rigid-flex PCB configurations like breakaway panels, edge plating, bookbinding formats, etc.

There are several alternate design configurations possible in rigid-flex PCBs. You can implement multiple functionalities with improved circuit density and excellent packaging. An experienced rigid flex PCB manufacturer can suggest a feasible and cost-effective configuration for your design requirements.

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