Polyimide PCB

The PCBA industry continues to experience significant growth, with increasing interest in various types of circuit boards. However, according to Google search results, there are many people looking for “Polyimide Rigid PCB” on the Internet. Unfortunately, you don’t seem to find the answer you’re looking for, as this is not a valid combination.

Polyimide, often referred to as Pi, is a type of PCB material that is used as a substrate for flexible PCB. So named because it can be folded or bent without damaging the circuitry. Polyimide is a polymer material known for its excellent thermal stability, electrical insulation properties, and mechanical strength. The development of polyimide-based circuit boards dates back to 1908, with bulk production beginning in 1955. This substrate material is typically available in colors such as yellow, orange, and amber. The term “polyimide” itself is a combination of “poly” referring to polymers and “imide” referring to the monomers used in the material. Now let’s discuss polyimide PCB comprehensively with FS Technology, including material properties, types and applications.

Polyimide Material Type

2nd Generation Polyimide

Also known as pure polyimide, it is considered to be the simplest polyimide flexible PCB but offers excellent flexibility, thermal stability, and chemical resistance. These boards are manufactured with no need to use brominated flame retardants, which enhance their stability and heat resistance at high temperatures. Their high flexibility allows them to maintain performance in environments of vibration and motion, and are often considered the materials of choice for electrical and communications equipment.

3rd Generation Polyimide

The third-generation polyimide is not considered an upgraded version, but rather a material developed to meet specific requirements. Compared with the second generation, the third generation adds the addition of flame retardants, giving it improved flame-retardant properties. This makes it a preferred choice in safety-focused industries such as medical and aviation. But it is important to note that a disadvantage of third-generation polyimide PCB laminate is its reduced flexibility, which is why PCB manufacturers often find it easier to produce.

Low Flow Polyimide

This material, with its lower viscosity during production, offers unique advantages in PCB manufacturing. This material allows for precise placement in limited areas, making it ideal for complex and intricate circuit designs. PCBs made from low flow polyimide exhibit flame retardant properties and excellent chemical resistance, making them suitable for applications where fire and harsh environments are concerns. Unlike other flexible PCB materials, low flow polyimide has a rigid structure, enabling it to withstand high loads. It is worth noting that the cost of this material is relatively high, so careful consideration is required when selecting it for use.

Filled Polyimide

Adding fillers to polyimide boards enhances their mechanical properties such as chemical resistance, stiffness, and strength. Commonly used fillers include mica, glass fiber, and carbon fibers. Glass fiber is the most commonly used filler due to its cost-effectiveness and high strength and stiffness. On the other hand, carbon fiber, although more expensive than glass fiber, offers superior stiffness and strength properties. In the manufacturing of high-performance filled polyimide boards, mica, a natural mineral, is used. These boards find applications in industries such as semiconductors, where chemical resistance is required, as well as in the aviation industry.

Comparison of Polyimide PCB Material Properties

We can source materials from various material manufacturers, but there are slight differences between them, the most famous being Kapton HM and Isola P95.

Kapton HM: Produced by DuPont, the main advantages are toughness, transparency, flexibility and resistance to high temperature, radiation and chemicals.
Isola P95: Produced by Isola, known for its high thermal stability and good dielectric properties, it is often used in high temperature or high voltage applications.

Below is a polyimide PCB material data sheet comparison:

Properties	Kapton HM	Isola P95
Density	1.42 g/cm<sup>3</sup>	1.43 g/cm<sup>3</sup>
Glass transition temperature (Tg)	428°C (802°F)	260°C (500°F)
Tensile strength	1000 psi (6.9 MPa)	800 psi (5.5 MPa)
Elongation at break	200%	150%
Dielectric constant	3.3	3.4
Dielectric strength	4000 V/mil	4000 V/mil
Moisture absorption	0.02%	0.02%
Flame retardant	UL 94 V-0	UL 94 HB
Chemical resistance	Excellent	Excellent
Temperature range	-269°C to 400°C (-452°F to 752°F)	-269°C to 260°C (-452°F to 500°F)

Polyimide Flexible PCB Manufacturing

Manufacturing Process

Raw Material Preparation: Preparing the material used for fabrication according to the project requirements is the first step. This involves a polyimide film polymer layer with a coating of copper, which facilitates current flow. The copper layer and polyimide film are bonded together using resin.
Lamination: The substrate materials are laminated together using a press-lamination process. This involves placing the polyimide film on top of the copper foil and applying heat and pressure using a laminator. The heat melts the resin, which acts as a bonding agent, and the pressure ensures that all the layers are securely bonded together. This process is typically completed within a few minutes.
Drilling: To connect the components, the manufacturer drills holes in the laminate using a CNC machine or laser drilling equipment, based on a blueprint of the circuit pattern in the design file indicating component locations and traces. Different from traditional PCB materials, polyimide circuit boards are more brittle, so extra attention should be paid to the drill type and parameters.
Silkscreen Printing: During silkscreen printing, circuit patterns are created on the board. A layer of ink is applied to the board, and a stencil is used to form the circuit pattern. The ink used is typically made of a photosensitive material. The circuit pattern is etched into a thin sheet of metal or plastic, which serves as the stencil. The board is placed in a silkscreen printer, and the ink is applied. The stencil is then pressed firmly onto the ink, transferring the circuit pattern onto the board.
Etching: The circuit pattern is revealed by dissolving the ink using a chemical solution. This solution is made up of solvents and acids. When applied to the polyimide flex PCB, the ink is dissolved, leaving behind the desired circuit pattern.
Plating: Copper is applied to the exposed circuit design. A layer of copper is applied to the board, and through a plating process, the copper is deposited onto the circuit pattern using an electric current. The plating bath contains a solution with copper ions, which are deposited into the circuit pattern due to the flow of current.
Testing: The board undergoes PCB testing to check its compatibility with requirements. Different test instruments, such as thermal shock testers, electrical testers, and tools for visual inspection, are used for this purpose.
Assembly: Components are connected to the PCB either manually or using a pick-and-place device. Soldering is commonly employed to mount the components onto the board. A reflow oven or soldering iron is used to melt the solder. The PCB assembly process typically takes place in a clean environment to prevent contamination.

Latest Techniques

Semi-Additive Process: Creating circuit patterns by combining additive and subtractive techniques allows for the creation of smaller, thinner copper traces, improving board performance.
Modified Semi-Additive Process: An improved version of the semi-additive process that provides thinner traces and smaller dimensions for industries such as aerospace that require high-performance circuits.
Electrodeposition Polyimide Process: By electrochemically depositing uniform copper layers on polyimide substrates, these copper layers help improve circuit board reliability.
Laser Direct Imaging: For some more precise circuit designs, which are difficult to achieve with traditional processes, a laser can be used to directly expose the photoresist on the polyimide substrate.

Why Polyimide PCB

Features

Dimensional Stability: One of the key advantages of polyimide PCB is their ability to maintain their structure and dimensions even when subjected to temperature changes. They exhibit little to no change in structure within a temperature range of -100 °C to 100 °C, making them highly suitable for microelectronics applications.
Flexibility: Different materials can be selected according to project needs to obtain flame retardancy, rigidity and chemical corrosion resistance.
High Dielectric Strength: This means they can withstand high voltage conditions without the risk of electrical breakdown.
High Cost: It’s important to note that the use of polyimide PCBs may contribute to higher project costs. This can be attributed to the relatively expensive materials, manufacturing and assembly complexities, and the challenge of maintenance associated with this type of PCB.
Limited Options: Polyimide board generally has thinner thickness values compared to rigid PCB, which may not be suitable for applications requiring thicker boards. Furthermore, not all PCB manufacturers offer polyimide boards, except specialized PCBA company like FS Technology that focus on high-end applications.

Common Application

Automotive: Vehicle engine control unit, power supply, sensor and other components;
Medical: X-ray machines, CTR, and diagnostic tools;
Military and Defense: Radar systems, avionics, communications system;
Consumer Electronics: smartphones, wearables tablets, and laptops;
Computer Field: Processors, graphics cards and power modules as well as computer systems, servers and data centers,

Polyimide vs FR4

Features	Polyimide	FR4
Flexibility	A flexible PCB material that enables dynamic or static bending	Common rigid PCB material, not bendable and firm
Thermal stability	High glass transition temperature value is higher, so it has better thermal stability and can withstand temperatures of 250°C or higher	The thermal stability is relatively low, and the resistance to low temperature is excellent, but it may be a little weak in the face of high temperature, usually can withstand the temperature of 130°C to 140°C. When exposed to high temperature for a long time, FR4 materials may become soft, brittle or lose their insulating properties.
Chemical resistance	Chemically inert material with excellent resistance to common solvents, acids, bases, etc.	Although glass fiber reinforced epoxy resin composites have certain mechanical strength, their chemical properties are relatively weak.
Flame retardancy	Fire retardant additives were used during construction	FR4 stands for flame retardant grade
Cost	High	Low