1. Flexibility and Reliability of Flexible Circuits
Currently, there are four types of flexible circuits: single-sided, double-sided, multilayer flexible boards, and rigid-flex boards.
2. Single-sided FPC is the most cost-effective option for applications that do not require high electrical performance. It features a single layer of chemically etched conductive patterns on a flexible insulating substrate, which is typically a rolled copper foil. The insulating substrate can be made from polyimide, polyethylene terephthalate, aramid cellulose ester, or polyvinyl chloride.
3. Double-sided flexible boards have conductive patterns etched on both sides of the insulating base film. Metallized holes connect these patterns to form conductive paths, ensuring flexibility and meeting design requirements. Cover films protect the wires and indicate component placement.
4. Multilayer flexible boards are created by laminating three or more layers of single-sided or double-sided flexible circuits. Conductive paths between layers are formed through drilled and electroplated metallized holes, eliminating the need for complex soldering processes. These boards offer superior reliability, better thermal conductivity, and enhanced assembly performance. Design considerations must balance assembly size, number of layers, and flexibility.
4. The traditional rigid-flex board consists of rigid and flexible substrates selectively laminated together. This structure is compact, and the metallization hole L forms a conductive connection. When components are present on both sides of a printed board, a rigid-flex board is a suitable choice. However, if all components are on one side, opting for a double-sided flexible board with an FR4 reinforced layer on the back will be more economical.
5. The flexible circuit with a mixed structure is a type of multilayer board that utilizes different metals for its conductive layers. An 8-layer board, for example, uses FR-4 for the inner layers and polyimide for the outer layers. Leads extend from three different directions of the main board, with each lead made from a distinct metal: constantan alloy, copper, and gold. This hybrid structure is mainly employed in low-temperature conditions where the interplay between electrical signal conversion, heat dissipation, and electrical performance is critical, making it the only feasible solution. Evaluation of the best performance-to-price ratio can be done by considering the convenience of internal connection design and the total cost.
2. The economy of flexible circuits
For simple circuit designs with a compact volume and suitable space, traditional internal connection methods are generally more cost-effective. However, if the circuit is complex, handles many signals, or has special electrical or mechanical performance requirements, flexible circuits become a better design choice. When the size and performance requirements exceed the capacity of rigid circuits, flexible assembly methods are the most economical. For example, a 12mil pad with 5mil through holes and a flexible circuit with 3mil lines and spacing can be fabricated on a film. Therefore, mounting the chip directly on the film is more reliable, as these films do not contain flame retardants that could cause ion drilling contamination. These films can be protective and cured at higher temperatures to achieve a higher glass transition temperature. Flexible materials save costs compared to rigid materials primarily by eliminating the need for connectors.
High-cost raw materials significantly contribute to the high price of flexible circuits. The cost of raw materials varies considerably; for instance, the raw materials used in the lowest-cost polyester flexible circuits are 1.5 times more expensive than those for rigid circuits, while high-performance polyimide flexible circuits can be four times or more expensive. Additionally, the flexibility of the material complicates automation in the manufacturing process, leading to reduced output and a higher likelihood of defects such as peeling flexible accessories or broken lines. Such issues are more prevalent when the design is not well-suited to the application. Under high stress from bending or forming, it may be necessary to use reinforcing materials. Despite the high raw material costs and manufacturing challenges, the foldable, bendable, and multi-layer splicing capabilities of flexible circuits can reduce overall assembly size and material usage, thereby lowering the total assembly cost.
The flexible circuit industry is experiencing rapid but modest development. The polymer thick film method offers an efficient and cost-effective production process by selectively screen-printing conductive polymer inks on inexpensive flexible substrates, such as PET. Polymer thick film conductors use silk-screened metal fillers or carbon powder fillers. This method is clean, uses lead-free SMT adhesive, and avoids etching. Due to its additive technology and low substrate cost, the polymer thick film circuit is approximately one-tenth the price of copper polyimide film circuits and one-half to one-third the price of rigid circuit boards. It is particularly suitable for control panels in devices and for converting components, switches, and lighting devices on PCB motherboards into polymer thick film circuits. This approach not only reduces costs but also lowers energy consumption.
Generally, flexible circuits are indeed more expensive than rigid circuits. Manufacturing flexible boards often involves dealing with parameters outside tolerance ranges, and the flexibility of the materials adds to the manufacturing difficulty.
3. The cost of flexible circuits
Despite the aforementioned cost factors, the price of flexible assemblies is declining and approaching that of traditional rigid circuits. This is due to the introduction of newer materials, improved production processes, and structural changes. Current structures offer better thermal stability and fewer material mismatches. Some new materials allow for the production of more precise lines with thinner copper layers, making components lighter and more suitable for small spaces. Previously, copper foil was adhered to adhesive-coated media using a rolling process. Nowadays, copper foil can be directly formed on the medium without adhesive, enabling the production of very thin copper layers and precise lines. Removing certain adhesives can also improve the flame-retardant properties of the flexible circuit, expediting UL certification and further reducing costs. Additionally, flexible circuit board solder masks and other surface coatings help reduce assembly costs.
In the coming years, smaller, more complex, and costlier flexible circuits will necessitate novel assembly methods, including hybrid flex circuits. The challenge for the flexible circuit industry is to leverage its technological advantages to keep pace with advancements in computing, remote communications, consumer demand, and active markets. Furthermore, FPC will play a significant role in lead-free initiatives.
Currently, there are four types of flexible circuits: single-sided, double-sided, multilayer flexible boards, and rigid-flex boards.
2. Single-sided FPC is the most cost-effective option for applications that do not require high electrical performance. It features a single layer of chemically etched conductive patterns on a flexible insulating substrate, which is typically a rolled copper foil. The insulating substrate can be made from polyimide, polyethylene terephthalate, aramid cellulose ester, or polyvinyl chloride.
3. Double-sided flexible boards have conductive patterns etched on both sides of the insulating base film. Metallized holes connect these patterns to form conductive paths, ensuring flexibility and meeting design requirements. Cover films protect the wires and indicate component placement.
4. Multilayer flexible boards are created by laminating three or more layers of single-sided or double-sided flexible circuits. Conductive paths between layers are formed through drilled and electroplated metallized holes, eliminating the need for complex soldering processes. These boards offer superior reliability, better thermal conductivity, and enhanced assembly performance. Design considerations must balance assembly size, number of layers, and flexibility.
4. The traditional rigid-flex board consists of rigid and flexible substrates selectively laminated together. This structure is compact, and the metallization hole L forms a conductive connection. When components are present on both sides of a printed board, a rigid-flex board is a suitable choice. However, if all components are on one side, opting for a double-sided flexible board with an FR4 reinforced layer on the back will be more economical.
5. The flexible circuit with a mixed structure is a type of multilayer board that utilizes different metals for its conductive layers. An 8-layer board, for example, uses FR-4 for the inner layers and polyimide for the outer layers. Leads extend from three different directions of the main board, with each lead made from a distinct metal: constantan alloy, copper, and gold. This hybrid structure is mainly employed in low-temperature conditions where the interplay between electrical signal conversion, heat dissipation, and electrical performance is critical, making it the only feasible solution. Evaluation of the best performance-to-price ratio can be done by considering the convenience of internal connection design and the total cost.
2. The economy of flexible circuits
For simple circuit designs with a compact volume and suitable space, traditional internal connection methods are generally more cost-effective. However, if the circuit is complex, handles many signals, or has special electrical or mechanical performance requirements, flexible circuits become a better design choice. When the size and performance requirements exceed the capacity of rigid circuits, flexible assembly methods are the most economical. For example, a 12mil pad with 5mil through holes and a flexible circuit with 3mil lines and spacing can be fabricated on a film. Therefore, mounting the chip directly on the film is more reliable, as these films do not contain flame retardants that could cause ion drilling contamination. These films can be protective and cured at higher temperatures to achieve a higher glass transition temperature. Flexible materials save costs compared to rigid materials primarily by eliminating the need for connectors.
High-cost raw materials significantly contribute to the high price of flexible circuits. The cost of raw materials varies considerably; for instance, the raw materials used in the lowest-cost polyester flexible circuits are 1.5 times more expensive than those for rigid circuits, while high-performance polyimide flexible circuits can be four times or more expensive. Additionally, the flexibility of the material complicates automation in the manufacturing process, leading to reduced output and a higher likelihood of defects such as peeling flexible accessories or broken lines. Such issues are more prevalent when the design is not well-suited to the application. Under high stress from bending or forming, it may be necessary to use reinforcing materials. Despite the high raw material costs and manufacturing challenges, the foldable, bendable, and multi-layer splicing capabilities of flexible circuits can reduce overall assembly size and material usage, thereby lowering the total assembly cost.
The flexible circuit industry is experiencing rapid but modest development. The polymer thick film method offers an efficient and cost-effective production process by selectively screen-printing conductive polymer inks on inexpensive flexible substrates, such as PET. Polymer thick film conductors use silk-screened metal fillers or carbon powder fillers. This method is clean, uses lead-free SMT adhesive, and avoids etching. Due to its additive technology and low substrate cost, the polymer thick film circuit is approximately one-tenth the price of copper polyimide film circuits and one-half to one-third the price of rigid circuit boards. It is particularly suitable for control panels in devices and for converting components, switches, and lighting devices on PCB motherboards into polymer thick film circuits. This approach not only reduces costs but also lowers energy consumption.
Generally, flexible circuits are indeed more expensive than rigid circuits. Manufacturing flexible boards often involves dealing with parameters outside tolerance ranges, and the flexibility of the materials adds to the manufacturing difficulty.
3. The cost of flexible circuits
Despite the aforementioned cost factors, the price of flexible assemblies is declining and approaching that of traditional rigid circuits. This is due to the introduction of newer materials, improved production processes, and structural changes. Current structures offer better thermal stability and fewer material mismatches. Some new materials allow for the production of more precise lines with thinner copper layers, making components lighter and more suitable for small spaces. Previously, copper foil was adhered to adhesive-coated media using a rolling process. Nowadays, copper foil can be directly formed on the medium without adhesive, enabling the production of very thin copper layers and precise lines. Removing certain adhesives can also improve the flame-retardant properties of the flexible circuit, expediting UL certification and further reducing costs. Additionally, flexible circuit board solder masks and other surface coatings help reduce assembly costs.
In the coming years, smaller, more complex, and costlier flexible circuits will necessitate novel assembly methods, including hybrid flex circuits. The challenge for the flexible circuit industry is to leverage its technological advantages to keep pace with advancements in computing, remote communications, consumer demand, and active markets. Furthermore, FPC will play a significant role in lead-free initiatives.
