The production process of flexible printed circuit boards (FPC circuit boards) is basically similar to the production process of rigid boards. For some operations, the flexibility of laminates requires different devices and completely different handling methods. Most flexible printed circuit boards use a negative method. However, there are some difficulties in the machining and coaxial processing of flexible laminates, one of the main problems is the treatment of the substrate. Flexible materials are coils of different widths, so the transfer of flexible laminates during etching requires the use of rigid brackets.
In the production process, the handling and cleaning of flexible printed circuits is more important than the handling of rigid plates (Lexin, 1993). Incorrect cleaning or contravention of procedures can lead to failure in the subsequent manufacture of the product, due to the sensitivity of the materials used in the flexible printed circuit, which plays an important role in the manufacturing process. The substrate is subject to mechanical pressures such as waxing, laminating and electroplating, and the copper foil is also susceptible to knocks and dents, while the extension ensures maximum flexibility. Mechanical damage or work hardening of copper foil will reduce the flexibility life of the circuit.
Typical flexible single-sided circuits need to be cleaned at least three times during manufacturing, whereas multi-substrate boards need to be cleaned 3-6 times due to their complexity. In contrast, rigid multi-layer printed circuit boards may require the same number of cleaning times, but the cleaning procedure is different, and more care is needed when cleaning flexible materials. Even when subjected to extremely light pressure during the cleaning process, the spatial stability of the flexible material is affected and can cause the panel to elongate in the z or y direction, depending on the bias of the pressure. Chemical cleaning of flexible printed circuit boards should pay attention to environmental protection. The cleaning process includes an alkaline dye bath, thorough rinsing, micro-etching and final cleaning. Damage to the film material often occurs during panel mounting, when agitated in the pool, when shelves are removed from the pool or no shelves are installed, and when surface tension is broken during pool clearing.
The holes in the flexible circuit board are generally punched, which leads to an increase in processing costs. Drilling is also possible, but this requires special adjustment of drilling parameters to obtain unstained hole walls. After drilling, the drilling dirt is removed in a water cleaner with ultrasonic agitation.
It has been proven that the mass production of flexible boards is cheaper than rigid printed circuit boards. This is because flexible laminates enable manufacturers to produce circuits on a continuous basis, a process that starts with the laminate coil and leads directly to the finished board. In order to manufacture a printed circuit board and etch a continuous machining diagram of a flexible printed circuit board, all production processes are completed in a series of sequentially placed machines. Screen printing may not be part of this continuous transmission process, which causes an interruption in the online process.
In general, due to the limited heat resistance of the substrate, welding in flexible printed circuits is more important. Manual welding requires sufficient experience, so wave soldering should be used if possible. When welding flexible printed circuits, the following matters should be noted:
1) Because polyimide is hygroscopic, the circuit must be baked before welding (for 1h at 250°F).
2) The pad is placed in a large conductor area, such as ground layer, power layer or heat sink, and the heat dissipation area should be reduced. This limits heat loss and makes welding easier.
3) When manually welding the pins in a dense place, try not to continuously weld the adjacent pins, and move the welding back and forth to avoid local overheating.
Information on the design and processing of flexible printed circuits can be obtained from several sources, however the best sources are always producers/suppliers of processed materials and chemicals. Through the information provided by the supplier, coupled with the scientific experience of the processing experts, high-quality flexible printed circuit boards can be produced.