A large number of electronic parts are integrated into modern cars, and the number of electronic control systems often exceeds 250. In daily driving, we can clearly see that electronic control systems are everywhere. They are not only under the fenders, near the various power control units of the car body, but also widely distributed in the cockpit and around the steering wheel. These systems are essential to the normal operation of the car, covering many aspects from power systems, comfort control to safety assistance.

As far as automotive electronic technology is concerned, the structure of its electrical and electronic equipment is very complex. In order to ensure its performance and safety, these electronic systems need to follow strict technical specifications and undergo high-intensity stress testing and reliability testing. In particular, since cars face extreme environmental conditions such as high temperature, low temperature, humidity, vibration and dust during driving, all electronic systems must be able to work stably in these harsh environments. Therefore, the design and technical requirements of automotive electronic systems must have higher standards and stronger reliability to ensure that the vehicle does not fail during long-term use.

Unlike ordinary electronic equipment, the technical specifications of automotive electronic systems pay special attention to the balance between low cost and high reliability. To achieve this goal, designers usually need more stringent requirements than ordinary rigid PCBs (printed circuit boards). Especially in terms of interconnection, the interconnection design between PCBs needs to be precise and efficient to ensure that each circuit board can be stably connected to other devices without external interference. In addition, many electronic systems in the vehicle need to be connected to external peripherals, and these connections are usually achieved through ordinary cables, wires, ribbon cables, jumpers and various connectors. In order to avoid failures in electronic systems, all these connecting components must be strictly tested and screened to ensure that they can maintain stable electrical performance under high voltage, high temperature and vibration environments.

However, in actual applications, low-quality solder joints and connectors are still one of the common causes of failures in automotive electronic systems. Unqualified solder joint quality or unreasonable connector design may cause electrical failures, thereby affecting the stability of the electronic system of the entire vehicle. Therefore, it is necessary to strictly control the welding quality, connector selection and installation to ensure that every link can meet the highest quality standards. Only through these measures can the automotive electronic system be guaranteed to maintain efficient and stable operation during long-term use, ensuring the safety and driving experience of the driver.

Advantages of Flex-Rigid PCBs applied to automobiles

In order to successfully solve the problems mentioned in the first paragraph, flexible rigid PCBs have been adopted to reduce the number of connectors and solder joints for more than 15 years. As flexible rigid PCBs are applied to automotive systems, the following advantages can be adopted.

l Obviously improve product quality and reliability

When flexible rigid PCBs are applied to automobiles, connectors and solder joints can be reduced, which can reduce the potential risk of causing electrical failures. The performance and reliability of automotive electronic control systems will be proportional to the reduction of connectors and solder joints.

l Cost reduction due to shrinking manufacturing steps

With the application of flexible rigid PCBs, the soldering of ribbon cables and assembly connectors will be cut, thereby reducing costs. After all, the implementation of all manufacturing processes is expensive.

l Maintenance simplification and elimination

Flex-rigid PCBs used in automobiles are composed of two or more rigid materials and one or more flexible materials, and the rigid parts are connected to each other through the application of flexible materials. Each rigid-flex circuit can be accurately packaged in a smaller package, which can eliminate a lot of management and maintenance.

l Designer and assembly freedom improvement

Flex-rigid circuit designers are only responsible for rigid circuit board layout. For the flexible part, they only need to guide the connection, and can be freely fixed, slinged or staked, which greatly simplifies the design and assembly.

So far, two types of flexible-rigid PCBs are currently available on the market:

a. Semi-flexible PCB. The flexible part of the semi-flexible PCB is made of thin FR-4 material, which is particularly suitable for assembly with only a few flexibility. In addition, the semi-flexible PCB leads to low cost.

b. Multi-flexible PCB. Multi-layer flexible PCB is made of polyimide (PI) material, which can meet the applications requiring dynamic flexibility. Since the PI layer can be extended to the inner rigid part of the flexible-rigid PCB, the multi-flexible circuit board is more suitable for applications requiring gradual dynamic flexibility.

Multi-flexible PCB

The flexible part of the flexible-rigid PCB is made of flexible PI copper foil material, which belongs to the category of multi-flexible PCB. Multi-flexible PCB belongs to a kind of traditional flexible-rigid PCB, which has been used for more than thirty years. Multi-flexible PCB has a hybrid structure stacked by rigid substrate material and flexible substrate material, and the interconnection between electrical conductors is achieved by plated through holes, which will pass through the rigid and flexible materials. Figure 1 below shows the structure of a double-layer flexible circuit board.

According to Figure 1, it can be concluded that the flexible substrate material depends on the ordinary PI copper foil material, which is not just laid in the flexible part, but also covers all the rigid parts. However, it is equivalent to lay some PI copper foil structures in the selective section. Once the flexible PI copper foil is used for the selective part, the manufacturing complexity will increase, and this method is generally rarely used.

When it comes to multi-layer flexible PCBs, because of the relatively high CTE (coefficient of thermal expansion) along the Z-axis bonding direction, the adhesive may cause mechanical damage to the plated through-holes during stress testing or thermal shock testing. Therefore, when the automotive PCB requires higher thermal reliability, the use of flexible substrate materials and cover layers within the rigid part must be avoided, because plated through-holes can usually be used in the rigid part.

In addition, the temperature reliability issues of ordinary FR4 adhesives and no-flow prepregs must be considered because FR4 prepreg is also a substrate with high CTE. The Tg of the no-flow prepreg of ordinary FR4 is 105°C, which is about 30°C lower than that of the conventional FR4 prepreg.

In addition to the FR4 material used as a rigid substrate material, almost any type of rigid material is suitable for multi-flex PCBs, including high-Tg materials, halogen-free materials and even high-frequency materials.

Most flexible materials used for flexible-rigid PCBs use PI with adhesive or PI without adhesive, which works better. However, PEN and PET materials can also be used for simple and asymmetric flexible-rigid circuit board structures. LCP (liquid crystal polymer) materials can be regarded as the best flexible materials without adhesive, with high reliability design and high-speed signal transmission design. It is recommended to bake them before application to eliminate moisture due to the high moisture absorption of PI. However, multi-flex PCBs that use LCP as substrate material do not require baking.

As far as flexible-rigid PCBs are concerned, multi-flex circuits can provide some flexibility layers can be used simultaneously. Since the complex interconnection of the circuit is designed integrally, it can be manufactured repeatedly, which is more advantageous than cable and wire connection. Therefore, characteristic impedance controlled signal transmission line design can be realized to replace coaxial cables.

Semi-flexible PCB

Semi-flexible PCBs cannot achieve continuous flexibility. In fact, in many applications, the flexible part of the flex-rigid PCB has only some flexibility, such as during assembly, rework and maintenance. Therefore, expensive flexible materials such as PI are not necessary for such applications and bendable materials are used, which is sufficient. In addition, the cost can be reduced. Semi-flexible PCBs can utilize traditional substrate materials for multi-layer lamination, thus avoiding different materials from being laminated together with minimal internal thermal stress. In order to obtain flexible materials, the best method lies in making the traditional FR4 substrate material flexible enough to bend. Of course, another method is to selectively reduce the thickness of the flexible part.

Semi-flexible PCBs are PCBs manufactured by adhering to the same manufacturing technology as traditional double-sided PCBs and multi-layers. Thinning of the flexible part can be done by milling. In addition, semi-flexible PCBs are manufactured by adhering to similar manufacturing technology as traditional PCBs, except for the addition of flexible manufacturing.