Polyimide PCB Material: Heat, Flex and Reliability

Polyimide is the material that made the flexible circuit practical. It withstands soldering temperatures, it can be made in very thin films, it is mechanically tough and it is dimensionally stable when it is processed correctly. Those properties are why it is still the base film for the majority of flexible and rigid-flex circuits.

It is not a single product. The film is available in several chemistries and thicknesses, and the way the copper is attached to it changes the electrical and mechanical behaviour significantly.

What Polyimide Is

Polyimide is a polymer with an imide group in its backbone, which is what gives it its thermal stability. Aromatic polyimides, the type used in circuit materials, are usually supplied as a film or as a varnish that can be coated onto a carrier and cured.

The film is typically 12.5 to 50 micrometres thick for a flex circuit, and it is supplied with the copper already bonded or as a bare film for a build up process. Its colour is amber, which is why a flex circuit without a coverlay has a characteristic appearance.

Thermal Behaviour

The glass transition temperature of a polyimide film is high enough that it is not usually the limiting factor in a soldering process, and the decomposition temperature is well above the reflow peak. That allows a flex circuit to be assembled with the same processes as a rigid board.

The limit comes from the adhesive, not from the film. An adhesive based construction uses an acrylic or an epoxy layer whose temperature rating is much lower, and that layer sets the maximum temperature the assembly can see. An adhesiveless construction removes that limit by casting or laminating the copper directly onto the film.

Mechanical and Flex Properties

Polyimide has a high tensile strength and a high elongation, which is why a thin film can survive repeated bending. The flex life depends on the thickness, the copper type and the position of the copper within the stack rather than on the film alone.

Rolled annealed copper is preferred for dynamic bending because its grain structure is elongated in the direction of rolling, and its ductility survives the forming better than an electrodeposited foil. The coverlay adds its own contribution: a coverlay with an adhesive layer may crack before the conductor does.

Electrical Properties and Moisture

The dielectric constant of polyimide is around 3.4 and its loss is moderate, which is adequate for many high speed interfaces but not for the lowest loss applications, where a fluoropolymer or a ceramic filled material is used.

Polyimide absorbs moisture, and the absorbed water raises both the dielectric constant and the loss. In a humid environment that changes the impedance of a controlled line, so the effect is accounted for in a design where the tolerance is tight or where the product operates outdoors.

Adhesiveless Versus Adhesive Constructions

An adhesiveless construction has a thinner total stack and a higher temperature rating, and it is the standard choice for a flex that has to be soldered or that has to survive a demanding flex life. It costs more than the adhesive based equivalent.

An adhesive based construction is cheaper and easier to handle, and it is adequate for a static flex that sees no soldering and limited temperature. The choice is usually made from the assembly process rather than from the electrical requirement.

Processing Differences

Polyimide is drilled and routed with parameters different from those used for FR4, and it absorbs moisture from the air, so a bake before lamination or before reflow is common. The material also moves when it is processed, and the artwork is compensated for that movement.

The lamination of a flex stack requires a temperature and pressure cycle that cures the adhesive without distorting the film, and the process window is narrower than for a rigid laminate. The fabricator’s experience with the material matters as much as the equipment.

Reliability and Applications

The failure modes of a polyimide flex are a crack in the conductor at a bend, delamination between the coverlay and the base film and a broken plated through hole where a rigid section meets a flexible one. Each of them is addressed by design rather than by material choice alone.

The material appears in camera modules, medical catheters, wearable devices, aerospace harnesses and anywhere the circuit has to bend or fit into a curved space. Its combination of temperature rating, toughness and thinness is difficult to replace.

Selection Rules

Choose the thickness from the flex life and the impedance requirement, choose the construction from the assembly temperature, and choose the copper from the bending requirement. Where the loss has to be very low, a fluoropolymer or a ceramic filled material is a better base film and the polyimide is used only where its mechanical properties are needed.

gopcb produces flex and rigid-flex circuits in polyimide, in both adhesive and adhesiveless constructions, with the coverlay and stiffener options and the bend documentation that a dynamic application requires.

Handling and Storage of Polyimide Film

Polyimide takes up water from the air, and the moisture absorption of a film that has been stored in an unconditioned room is enough to cause blisters during lamination or during reflow. The standard practice is to bake the material before a high temperature step and to keep it in sealed packaging with desiccant between operations. The bake is specified by the material supplier in terms of temperature and time, and a hotter or longer bake damages the film rather than drying it further.

Dimensionally, the film also moves as it takes up and releases water. A panel that is patterned wet and then dried will not have the same dimensions as one processed dry, so the compensation applied to the artwork accounts for the moisture state of the material at the point of exposure. Where a design has very tight registration, this is one of the reasons a polyimide flex is more difficult to hold than a rigid board.

Comparing Polyimide with Other Base Films

Polyester is cheaper and has a lower dielectric constant, but its temperature rating is far below a soldering process, so it is used for membrane switches and simple static flexes that are never reflowed. Liquid crystal polymer offers a very low moisture absorption and a low loss, but it is more expensive and its processing is less widely established.

Polyimide sits between them, and the choice is usually driven by the assembly temperature. Where the circuit is soldered, polyimide is the practical answer. Where it is not, and the electrical requirement is modest, a cheaper film may serve the product better, provided the mechanical specification for the flex life is met by the alternative and the design rules are adjusted for its different behaviour.

FAQ

Is polyimide the same as Kapton? Kapton is one trade name for a polyimide film. The material family is polyimide, and several suppliers produce films with slightly different properties.

Can polyimide be soldered? The film can, and adhesiveless constructions are rated for reflow. An adhesive based construction is limited by the adhesive, which has a much lower temperature rating.

Why does a flex circuit change impedance in humid conditions? Because polyimide absorbs moisture, which raises the dielectric constant and the loss. The effect is accounted for where the tolerance is tight.

Related reading: FPC outline design, rigid-flex PCB design, FPC stiffener comparison, and high temperature PCB materials.