Introduction to the Three Main Characteristics of Flex Board (FPC)

This Gold Phoenix PCB Knowledge Center article introduces the three main characteristics of flexible printed circuits (FPC): flexibility and reliability, economy, and cost.

1. Flexibility and Reliability of Flex Board

There are five main types of flex boards: single-sided, double-sided, multi-layer flex boards, rigid-flex boards, and hybrid flexible circuits.

① Single-sided flex board is the lowest cost printed board when high electrical performance is not required. When wiring on one side, a single-sided flex board should be selected. It has a layer of chemically etched conductive patterns, and the conductive pattern layer on the flexible insulation substrate surface is rolled copper foil. The insulation substrate can be polyimide, polyethylene terephthalate, aramid fiber ester, or polyvinyl chloride.

② Double-sided flex board has a conductive pattern made by etching one layer on each side of the insulating substrate film. Metallized holes connect the patterns on both sides of the insulating material to form a conductive path, in order to meet the design and functional requirements of flexibility. The covering film can protect single and double-sided wires and indicate the position of the components.

③ Multi-layer flex board is made by laminating three or more layers of single-sided or double-sided flexible circuits together, forming metallized holes through drilling and electroplating, and forming conductive paths between different layers. In this way, there is no need to use complex welding processes. Multilayer circuits have significant advantages in terms of higher reliability, better thermal conductivity, and more convenient assembly performance. When designing the layout, the mutual influence of assembly size, number of layers, and flexibility should be considered.

④ Traditional rigid-flex boards are composed of rigid and flexible substrates selectively pressed together. They are tightly structured, with metallized holes forming conductive connections. If a printed circuit board has components on both sides, a rigid-flex board is a good choice. However, if all the components are on one side, choosing a double-sided flex board and laminating a layer of FR-4 stiffener material on its back will be more economical.

⑤ A hybrid flexible circuit is a type of multi-layer board with conductive layers composed of different metals. An 8-layer board uses FR-4 as the inner layer medium and polyimide as the outer layer medium, with leads extending from three different directions of the motherboard, each made of different metals. Copper alloy, copper, and gold are used as independent leads. This hybrid structure is mostly used for electrical signal conversion and heat conversion, as well as in low-temperature situations where electrical performance is relatively demanding, and may be the only feasible solution.

The best performance-to-price ratio can be evaluated based on the convenience and total cost of the design.

2. The Economy of Flex Board

If the circuit design is relatively simple, the total volume is not large, and the space is suitable, traditional interconnection methods are mostly much cheaper. If the circuit is complex, processes many signals, or has special electrical or mechanical performance requirements, flexible circuits are a good design choice. When the size and performance of an application exceed the capabilities of a rigid circuit, flexible methods are the most economical. Flex boards with 12 mil solder pads, 5 mil through-holes, and 3 mil lines and spacing can be made on a thin film. Therefore, directly mounting chips on thin films is more reliable, since the film does not contain flame retardants that may be sources of ionic contamination. These films may have protective properties and cure at higher temperatures, resulting in higher glass transition temperatures. The reason why flexible materials save costs compared to rigid materials is that they eliminate the need for connectors.

High-cost raw materials are the main reason for the high prices of flex boards. The price difference of raw materials is significant: the cost of raw materials used in the lowest-cost polyester flexible circuit is 1.5 times that of raw materials used in rigid circuits, while high-performance polyimide flexible circuits can reach up to 4 times or higher. At the same time, the flexibility of the material makes it difficult to carry out automated processing during the manufacturing process, resulting in a decrease in output. Defects are prone to occur during the final assembly process, including peeling of flexible attachments and broken traces. When the design is not suitable for the application, such situations are more likely to occur. Under high stress caused by bending or forming, it is often necessary to use stiffeners or stiffener materials. Although the raw material cost is high and manufacturing is more complex, the foldable, bendable, and multi-layer splicing capabilities of flex boards can reduce the overall part size and materials used, resulting in a decrease in overall cost.

The flexible circuit industry is currently in a small but rapidly developing stage. The polymer thick film (PTF) method is an efficient and low-cost production process. This process selectively screen-prints conductive polymer ink on inexpensive flexible substrates. The representative flexible substrate is PET. PTF conductors include screen-printed metal fillers or carbon powder fillers. The PTF method itself is very clean, using lead-free SMT adhesive without etching. Due to its use of additive technology and low substrate cost, PTF circuit is 1/10 the price of copper-polyimide film circuit and 1/2 to 1/3 the price of rigid circuit boards. The PTF method is particularly suitable for control panels of equipment. In mobile phones and other portable products, the PTF method is suitable for converting components, switches, and lighting devices on printed circuit boards into polymer thick film circuits. It not only saves costs but also reduces energy consumption.

Generally speaking, flexible circuits are indeed more expensive than rigid circuits. In many cases, flexible boards face the challenge that many parameters exceed the tolerance range during manufacturing. The difficulty in manufacturing flexible circuits lies in the flexibility of the materials.

3. The Cost of Flex Board

Despite the cost factors mentioned above, the price of flexible assembly is decreasing and becoming closer to traditional rigid circuits. The main reason is the introduction of updated materials, improved production processes, and structural changes. The current structure enhances the thermal stability of the product, with few material mismatches. Some updated materials can produce more precise lines due to the thinner copper layer, making the components lighter and more suitable for fitting into small spaces.

In the past, copper foil was adhered to a medium coated with adhesive using roller pressing technology. Nowadays, copper foil can be directly generated on the medium without using adhesive. These technologies can produce copper layers several micrometers thick, resulting in precision lines with a width of 3 mil or even narrower. After removing certain adhesives, flexible circuits have flame retardant properties. This can accelerate the UL certification process and further reduce costs. Flexible circuit board solder masks and other surface coatings further reduce the cost of flexibility.

In the coming years, smaller, more complex, and more expensive flexible circuits will require more innovative methods, including the addition of hybrid flexible circuits. The challenge for the flexible circuit industry is to leverage its technological advantages to maintain synchronization with computers, remote communication, consumer demand, and active markets. In addition, flexible circuits will play an important role in lead-free operations.