Flexible PCB Materials: Polyimide, PET and LCP Compared
A flexible circuit is only as good as the film it is built on, and the choice of that film sets the temperature limit, the bend life, the electrical loss and the cost of the finished part. The materials used in flexible PCB construction fall into a small number of families, and the differences between them matter far more than the differences between rigid laminates.
What Makes a Flexible Circuit Different
A flexible circuit bends, which means the material has to survive strain rather than merely support copper. It is also thin, which changes how it is handled, how it is laminated and how it behaves thermally. The substrate is not a mechanical base in the way FR-4 is; it is a film whose properties determine what the circuit can do.
Substrate Films: Polyimide
polyimide is the default substrate for flexible circuits and has been for decades. It has high tensile strength, retains its properties from very low temperatures up to around 300 degrees, resists most chemicals and keeps its elasticity after curing. It tolerates soldering, which polyester does not, and it can be processed through standard PCB equipment.
Its weaknesses are equally well known. Polyimide tears relatively easily at a sharp edge, and it absorbs moisture, which must be driven out before lamination or reflow. Both properties have to be designed around rather than ignored.
Polyester, PEN and Other Films
Polyester is cheaper and has good mechanical and electrical properties, but its temperature capability is limited, and it cannot survive a soldering process. That restricts it to circuits that are connected by a different means, such as a pressure connector or a conductive adhesive.
Polyethylene naphthalate sits between polyester and polyimide. It offers better thermal performance than polyester, costs less than polyimide, and is a reasonable choice where the circuit will not be soldered and the temperature range is moderate. PTFE and aramid films appear in specialised applications, the first for its low loss at high frequency and the second for its dimensional stability.
Liquid Crystal Polymer
LCP has become the material of choice where flexible circuits have to carry high-speed or radio-frequency signals. It absorbs far less moisture than polyimide, and its dielectric constant is low and stable, which means a transmission line built on it holds its impedance across temperature and humidity. Its loss is also lower than polyimide’s at high frequency.
It can be melt-processed and laminated, which allows constructions that would be difficult with other films, and the material is recyclable. The cost is higher than polyimide, and the process window is narrower, so it is used where the electrical performance justifies it rather than as a general replacement. High frequency traces and data bus routing covers the transmission-line design that makes those properties useful.
Adhesives and Adhesiveless Construction
The adhesive is what bonds the copper to the film, and its properties are often the limiting factor in the finished circuit. Acrylic and modified epoxy adhesives have the best bonding performance, but they also have higher dielectric loss, lower thermal conductivity and worse dimensional stability than the films they join. A thick adhesive layer also reduces the bend life, because it is usually the part that cracks first.
Adhesiveless laminates deposit copper directly onto the film or use a thin cast layer, which removes that weak link. They cost more, they are the standard choice for circuits that will flex repeatedly, and they have better electrical performance. For a static bend, an adhesive laminate is often adequate and cheaper.
Flexible Copper Clad Laminate
The combination of film, adhesive and copper is called FCCL, and it is supplied as a finished laminate rather than assembled by the circuit manufacturer. The copper is usually electrodeposited for static applications and rolled annealed for dynamic ones, because the rolled material has a grain structure that withstands repeated bending without work-hardening to the point of cracking.
FCCL is made by pressing the stack under controlled temperature and pressure, and the properties that matter for the finished circuit, peel strength, dimensional stability and dielectric constant, are decided at that step as much as by the choice of film. PCB dimensional stability explains why the laminate’s movement has to be characterised before tight registration can be promised.
Coverlay and Surface Protection
A flexible circuit needs protection over its conductors, and a rigid solder mask is a poor choice because it cracks when the circuit bends. The usual answer is a coverlay: a layer of polyimide film with an adhesive, drilled or punched with openings over the pads.
Coverlay is thicker than solder mask, so it has to be included in the bend geometry and in the keepout around a pad. Where the circuit must flex repeatedly, a photoimageable coverlay that is thinner and more compliant may be preferable, at higher cost. Both options have to be defined on the drawing, because the pad openings are part of the mechanical design.
Stiffeners and Their Materials
Stiffeners are bonded to the flexible circuit to support connectors and components, and they are chosen for stiffness, thickness and thermal expansion rather than for flexibility. Polyimide, FR-4 and thin aluminium and stainless steel are all used. The stiffener has to be defined with its thickness and its exact position, because it changes the local stiffness and therefore the bend behaviour of the circuit around it.
The thermal expansion of the stiffener is a real consideration where a component is soldered to a stiffened area, since a mismatch between the stiffener and the film will stress the solder joints during thermal cycling.
Matching Material to Application
Thermal Expansion and Its Consequences
Flexible films expand and contract far more than the copper bonded to them, and the difference is a design parameter rather than a footnote. Over a soldering cycle the mismatch puts the copper in compression or tension depending on the direction of the temperature change, and repeated cycling can crack a pad or a via even when the circuit never bends.
This is why a stiffener under a component is chosen with its expansion in mind, and why the copper in a plated hole through a flexible film needs more annular ring than the equivalent hole in a rigid board. The material will move; the design has to accommodate the movement rather than resist it.
Storage, Handling and Moisture
Polyimide absorbs moisture, and absorbed water has to be driven out before lamination and before reflow. A flexible circuit that has been stored in a humid environment without a bake will blister or delaminate at soldering temperature, and the failure looks like a process defect rather than a storage problem. The material supplier’s bake profile should be followed, and circuits should be kept in dry packaging between operations.
Handling matters for a different reason. A crease in a flexible circuit is permanent damage to the copper in that area, and it will not be visible on the finished part. Keep the circuits flat, support them during any operation that involves pressure, and treat the bend area as the most fragile feature of the part rather than as a convenient place to grip.
The selection follows from the requirements. A circuit that will be folded once during assembly and never moved again can use an adhesive laminate with electrodeposited copper on polyimide, or polyester if it will not be soldered. A circuit that flexes thousands of times needs adhesiveless construction and rolled annealed copper. A circuit that carries a high-speed link needs LCP or another low-loss film with a controlled dielectric. Multilayer PCB prototype requirements covers what the fabricator needs to build the first article from that decision.
FAQ
Is polyimide always the best choice? It is the most versatile, but LCP is better for high-frequency work and polyester is cheaper where no soldering is involved. The right answer follows the application rather than the material’s reputation.
Why does the adhesive matter so much? It is usually the weakest layer mechanically and electrically. Thick or lossy adhesive reduces bend life and increases loss, which is why adhesiveless laminates are used for demanding circuits.
What copper should be used for a flexing circuit? Rolled annealed copper for anything that flexes repeatedly, and electrodeposited copper only where the bend is static or the number of cycles is very small.
