Flexible circuits are used in a variety of applications in which conductors are interconnected. These conductor interconnections must be flexible or be able to maintain a curved state for a long period of time during use. In the past, this kind of interconnection technology was realized by wire interconnection. There are many types of flexible circuits, one is a bidirectionally-accessible flexible circuit, which is a single-sided flexible circuit, and the purpose of this circuit is to make it possible to connect conductive materials from both sides of the flexible circuit. The second type is a double-sided flexible circuit, which is a circuit with two conductive layers. The two conductive layers are located on both sides of the basic layer in the circuit. For your specific requirements, they can be on both sides of the substrate sheet. Trace patterns are formed, and traces on both sides can be connected to each other through copper plated through holes. The third type is a multi-layer flexible circuit, which combines several single-sided circuits or double-sided circuits with complex interconnections. In a multi-layer design, shielding technology and surface mount technology are often used. The fourth type is a rigid-flexible circuit that combines the advantages of both a rigid printed circuit board and a flexible circuit. The circuit is usually interconnected by plated through holes between rigid and flexible circuits.

 

Flexible circuits have many advantages. One of the main benefits of flexible components is that they can implement almost error-free cabling instead of labor-intensive manual cabling. In addition, unlike rigid circuits, flexible circuits can also be designed into complex three-dimensional structures because they can be bent into various shapes. As the name implies, the materials used in flexible circuits can be bent back and forth numerous times, which means that they can be used in highly repetitive applications, such as on print heads. When considering the weight of a product, a flexible circuit is a very good substitute for a rigid circuit board and wire because its dielectric material and conductor lines are very thin.

In the past few years, the demand for the flexible circuit industry has continued to grow. Now, the annual output value of the flexible circuit industry reaches 10 billion U.S. dollars, with an annual growth rate of 7% - 10%.

 

Rigid - Flexible Board Assembly

With the rapid growth in the use of flexible circuits, the rework standards for these types of electronic interconnect circuits (replacement devices still meet the original specifications and functions) and repair standards (repair of physical damage on flexible circuits) have not yet kept pace.

There are some rework challenges from the characteristics of the flexible circuit itself. First, it is difficult to keep the flexible circuit flat during rework. From the perspective of rework, the bendability of Kapton materials or other basic flexible materials is a challenge for rework, although their flexibility is their advantage in the application. In order to keep the assembly flat, it must be taped to keep it flat. In some cases, creating a vacuum fixture for the rework of a flexible circuit is a relatively expensive approach. The vacuum structure of this jig has a great influence on the rework union when placing the fine-pitch elements. If the vacuum is just below the lead of a micro-pitch device, a slight vacuum may "pull" the flexible wire into the hole so that the device does not contact the wire of the flexible circuit, resulting in an electrical "open circuit". For solder paste printing on rework, coplanarity is a challenge when the stencil and the surface to be printed are not coplanar. Therefore, it is often necessary to use a syringe to apply solder paste instead of print coating. Sometimes, flexible materials with conductive epoxy are used in the interconnect device. Although the curing temperature of these materials is much lower than the standard solder reflow temperature, it may mess things up. For this case, as long as the design of the rework process is correct, the limitation on multiple rework is that the marginal cost of the component is much lower than the cost of rework. At this time, reworking a large amount of waste products is a more attractive economic option. .

From a process point of view, the process of reworking flexible circuits has some advantages. The thermal mass of the flexible circuit board is smaller than that of the rigid printed circuit board. When the flexible circuit board is soldered, the heating time to the liquidus temperature is shorter than that of the rigid circuit board. This speeds up the replacement of the rework process. In addition, this reduces the temperature of the air from the hot air system required for welding by a factor of several, and the possibility of hot air causing component damage is relatively small. The high temperature resistance of flexible materials, such as Kapton, Peek, and high-temperature resistant polyimides, makes the process window for the flex circuit rework process large.

 

Torn Rigid - Flexible Circuits

According to the industry standard for repairing PCBs, the provisions of IPC 7711/21 to repair and modify printed circuit boards and electronic components cover the rework and repair processes of various flexible circuits. Each of the processes listed in this standard, based on the suitability of each process for rework or repair of flexible circuits, is preceded by the letter “F” under the “Board Type” section heading in the upper right corner of the process documentation. There is even a flexible circuit-specific conductor repair standard in this standard. Various processes for repair conductors on flexible circuits are covered in step 7.1.1.

Here is an example of the repair of a conductor. Figure 2 shows a torn flexible circuit, which is part of a rigid-flexible circuit board. This circuit board was repaired using the standard process found in IPC 7721 3.5.1. Copper foil jumpers are installed in the conductor connections here, replacing the damaged conductors with them and then welding them together. The results of this repair can be seen in Figure 3.

Figure 3. Reconditioned bottom Kapton material and conductor

 

The rework and maintenance technology of flexible circuit components is continuously developing, and the rework and repair of flexible circuit components still face enormous challenges. Since the electronics assembly industry has long been a rigid circuit board, it is a good reference to search for the best rework method for flexible circuits based on rigid circuit boards.