Flex Circuit Board Rework And Repair
Flexible circuits are used in various applications where conductors are connected to each other,these conductor interconnections must be bendable or be able to remain bent for long periods of use. In the past, this interconnect technology was implemented using wire interconnections.
There are many kinds of flexible circuits:
One is a two-way access flexible circuit, which is a single-sided flexible circuit, the purpose of making this circuit is to connect conductive materials from both sides of the flexible circuit.
The second is a double-sided flexible circuit, which a circuit with two conductive layers, two conductive layers are located on both sides of the base layer in the circuit. For your specific requirements, you can form trace patterns on both sides of the substrate sheet, traces on both sides can be interconnected by copper plated through holes.
The third type is a multilayer flexible circuit, which combine several single-sided or double-sided circuits with complex interconnections, shielding and surface mount technology are often used in multi-layer designs.
The fourth is the rigid-flexible circuit, combines the advantages of both rigid printed circuit boards and flexible circuits, the circuit is usually interconnected by plated through holes between rigid and flexible circuits.
Flexible circuit has many advantages. One of the main benefits of flexible components is the ability to implement virtually error-free cabling, replacing labor-intensive manual cabling, In addition, unlike rigid circuits, flexible circuits can also be designed into complex three-dimensional structures,because you can bend them into various shapes. As the name implies, materials used in flexible circuits can bend back and forth numerous times, this means that they can be used for highly repetitive applications, for example on a print head. Flex circuits are very good substitutes for rigid circuit boards and wires when it comes to considering the weight of the product,as its dielectric materials and conductor lines are very thin.
In the past few years, the demand for the flexible circuit industry has been growing, now the annual output value of the flexible circuit industry reaches 10 billion U.S. dollars, and the annual growth rate reaches 7% – 10%.
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 (to repair physical damage on flexible circuits) have not kept synchronization.
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 Dupont materials or other basic flexible materials is a challenge to rework. Although their flexibility is their advantage in application. In order to keep the assembly flat, it must be taped to keep it flat. In some cases, making a vacuum fixture for the rework of a flexible circuit is a more expensive approach. The vacuum structure of the fixture 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 cannot 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. In 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 caused by rework. At this time, reworking a large amount of waste products is a more attractive economic option.
From the 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 soldering a flexible circuit board, the heating time to the liquidus temperature is shorter than the rigid circuit board.This speeds up the replacement process of the rework process. In addition, this reduces the temperature of the air from the hot air system required for welding by several times, and the possibility of hot air causing component damage is relatively small. The high temperature resistance of flexible materials, high temperature resistant polyimides, makes the process window for the flex circuit rework process large.
According to industry standards for repairing PCBs, IPC 7711/21 repairs and modifies printed circuit boards and electronic components to 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 heading “Board Type” in the upper right corner of the process documentation. There is even a flexible circuit-specific conductor repair standard in this standard. Various processes covering repair conductors on flexible circuits in step 7.1.1.
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 idea to use the rigid circuit board as the basis to find the best rework method for flexible circuits.