I wouldn’t describe myself as athletic, but I do plenty to keep myself in shape between hiking, running, and cycling. Overconfident in my general fitness, I recently accepted an invitation to a friend’s yoga class, where I subsequently suffered in a calming silence for 90 minutes. It’s impressive how flexible the human body is, or conversely, how intractable it can seem when it’s not been properly trained.
PCBs, too, can bend and contort into impossible shapes with the right production methods and materials. Flex PCB materials facilitated the rise of new geometries of circuits without sacrificing network density. Taking advantage of this form requires a few unique considerations than standard rigid boards.
Flex PCB Materials and Circuit Thickness
Flex PCB materials were developed as a specialty subtype of PCB manufacturing. Whereas standard rigid boards are brittle and unyielding, flex circuits can be bent to various angles depending on their thickness. The value of flex circuits is their ability to act as a stand-in for rigid boards and cable assemblies simultaneously.
Therefore, a flex circuit possesses the requisite ductility and support for component soldering. It offers a 2-for-1 space advantage, and the ability to bend the printed circuit in various directions allows designers to maximize space within the enclosure. This feature is especially valuable for devices with small form factors like cell phones and wearable technology.
First, a flex circuit differs from traditional rigid in the layup of the materials:
- Copper foil is etched using an applied photoresist layer.
- Unlike a rigid board, there is no epoxy to flow during lamination to bond the conductor and dielectric layers together. Therefore, an adhesive is used.
- An insulator, typically polyimide, is used as the attachment for the copper foil.
These three materials are layered similarly to the copper-clad laminate cores of rigid boards and can be used as the building blocks for more complex flex circuit constructions. Depending on the density requirements and budget for the circuit, manufacturers can pursue several options:
- Single-sided flex. The simplest flex circuit system. Depending on the need for accessibility to the inner conductor layers, there are three variations of single-sided flex:
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- Uncovered: No covercoat is added to the circuit. This reduces material and processing costs for flex circuits that do not require protection from the environment or additional mechanical support.
- Single access covered: A protective covercoat layer is added to the foil side of the flex circuit. Pre-drilled holes in the covercoat allow access to the conductor.
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- Double access covered: The flex circuit is encapsulated on both sides with covercoat and then selectively removed for conductor accessibility. Removing the covercoat is relatively expensive; designers and manufacturers may first want to see if access is achievable by folding the flex circuit to physically move bottom-side access to the “top” of the circuit.
- Double-sided flex. For a nominal increase in width over single-sided flex, double-sided flex places copper foil on either side of the polyimide for improved routing density.
- Multi-layer flex. As with rigid, higher layer counts can support intricate electrical systems and connections. Layer count increases alongside width, which has the general effect of decreasing flexibility in the circuit. Manufacturers will use lower copper weights and partial bonding between layers to maintain the requisite layer count and facilitate flexibility.
- Rigid-flex. A hybridized form of rigid PCBs and flex circuits, rigid-flex can capitalize on the performance of both.
Differences in Material Criteria for Different Flex Circuits |
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| Conductor | Adhesive | Covercoat | |
| Single-sided | Electrodeposited or foil copper | Standard acrylic adhesives | Can be added to one side, both sides, or excluded entirely |
| Double-sided | Electrodeposited or foil copper | Standard acrylic adhesives | Double-sided covercoat |
| Multi-layer | Lowest copper weight for maximum flexibility | Modified acrylic adhesive to reduce z-axis expansion rate | Top and bottom layer covercoat |
| Rigid-flex | Grain structure of rolled annealed copper improves flexibility over the electrodeposited | Modified acrylic adhesive to reduce z-axis expansion rate | Epoxy prepreg (same as standard rigid) |
The addition of polyimide and acrylic adhesives also introduces some new thermal concerns. They are hygroscopic and absorb enough atmospheric moisture to cause issues like misregistration or outgassing. Therefore, polyimide must be pre-baked before any hot work. Acrylic adhesives, meanwhile, have large coefficients of thermal expansion (CTE) compared to neighboring materials and can cause defects like barrel cracking, pad lifting, and lamination
Qualifying Flex Circuits Mechanical Attributes
As the namesake of these circuits, it should come as no surprise that the most valued mechanical property is its flexibility. There are numerous essential flex characteristics to keep in mind for designers and manufacturers:
- Static vs. dynamic bending. Depending on the assembly application, flex circuits may be intended for a single defined bend angle or a range due to mating cycles or movement in the assembly. Statically engaged flex circuits generally support smaller bend radii than dynamic bending flex circuits. For the stackup, multilayer circuits are generally recommended only for static conditions, while single-layer circuits can take advantage of a reduced thickness for dynamic cases.
- Bend angle. Tighter bend angles, especially bend angles of 90° or more, are more likely to damage the flex circuit materials due to the compounding effects of the stress-strain matrix. Bends this large should only be made once for a printed circuit. Additionally, a reduction in material thickness at the bend will greatly improve the long-term viability of the flex circuit.
- Vibration resistance. A standard PCB in high-vibration applications may experience early failure due to the development and propagation of cracks nearby fasteners. Flex circuits can better withstand vibrations due to the intensive ductility of the material and the reduction in overall mass.
- Conductor staggering and balance. Both staggering and balance refer to similar cases of conductor distribution that improve flexibility. Staggering has designers alternate conductor placement within each layer such that adjacent layers do not have conductors stacked directly on top of each other in the z-axis. A balanced design is one where routes in the plane are more or less split evenly between the center line.
Flexible Circuits and Contract Manufacturers
Flex PCB materials require a different approach to those of standard rigid, and it can be difficult to navigate the nuances for first-time designers. While the mechanical properties of flex materials afford incredible constructions, there is an added element of defects arising from design and manufacturing failing to support these new parameters. Design teams will want experienced manufacturers that can lead production and guide from the design stage to save time and money spent on nonviable circuits.
At VSE, we’re equipped to assist any design team with a team of engineers committed to building electronics for our customers. Alongside our valued manufacturing partners, we provide flex circuit assemblies for life-saving and life-changing devices.
