The majority of rigid-flex boards consist of multiple layers of flexible circuit substrates connected to each other, either internally or externally. The connection depends on the design of the application. The substrates are designed to remain in a constant flux state and they are formed to make a flexed curve when being installed or manufactured.
The rigid-flex PCB manufacturing process is long and involves a lot of labor when you compare with the traditional rigid board fabrication. This process involves steps that should be carried out with utmost accuracy. When you mishandle or misplace the flexible components, it will affect the durability of the final assembly.
The first thing to do is to prepare and clean the laminate. The laminate consists of a copper layer with or without an adhesive coating. It should be cleaned thoroughly before the fabrication process. The pre-cleaning is important because when you purchase the copper from vendors, it contains anti-tarnish features that give them the oxidation protection. This coating hinders the rigid-flex PCB manufacturing and hence its removal using the following process.
Immerse the copper in an acid solution
Then treat the coil with Sodium Persulphate to micro-etched it
Finally, coat the coil using oxidation agents to prevent oxidation.
After cleaning, the next thing to do is to generate a circuit pattern, then laminate the preparation. This is achieved using two methods.
• Screen Printing. Here, the required circuit deposits are generated on the surface of the laminate.
• Photo Imaging. This method is the oldest but still commonly used. A dry photoresist film that has the desired circuitry is placed close to ensure contact with the laminate. Later, the assembly is exposed to UV light that transfers the pattern to the laminate.
Rigid-flex manufacturers then dip the laminate in an etch bath or spray it with an etchant solution of both sides to give a good result.
Drilling requires vias, pads, and holes. A laser drill may be used to create ultra-small holes.
This is the most crucial step in the manufacturing process and should be done with care. After the specified holes are drilled, copper is deposited in them and they are plated chemically. This process is done to form layers of electrical interconnection.
It's important to always cover both the top and bottom side of a flex circuit by applying a cover coat. This provides a complete protection to the circuit from solvents, harsh weather or chemicals. A cover lay is engraved on the surface using screen printing and then exposed to UV light.
The cutting process is also crucial and should be carried out carefully. Some manufacturers choose the hydraulic process when producing high volume rigid-flex PCBs. For smaller productions, use the blanking knife.
This is the final stage. The boards undergo severe electrical testing for continuity and circuit performance. Various testing methods like flying probe and grid are used. The quality of PCBs determines the integrity of the electrical assembly. For this reason, the manufacturing process should be maintained at a high level.