Rigid Flex PCB Technology and Production Process
The rapid innovation in electronic gadgets - from HDTVs to video game consoles and everything in between - is made possible thanks to the emergence of rigid flex PCBs. Even your DSLR camera, laptop, PC, and smartphone make use of this technology.
What is Rigid Flex PCB?
Simply put, rigid flex PCBs are boards that combine the useful properties of flexible and rigid PCBs. It is common for rigid flex PCBs to incorporate multiple layers of flexible boards and rigid boards in application design. These boards provide manufacturers with considerable savings in terms of real estate, weight, and cost savings compared to traditional printed circuit boards.
With that out of the way, let’s give you a cursory review of the manufacturing process involved in making a rigid flexible circuit board.
Rigid Flex PCB Board Build Up
This is the first phase, where the primary focus is preparing and cleaning the laminate. The primary material for the laminate is copper. This material may be adhesive-backed or adhesiveless, but either way, it must be cleaned thoroughly before proceeding with the manufacturing process.
The cleaning is necessary because the coating over copper coils can disrupt the rigid flex PCB manufacturing process, and must be removed. Vendors of copper coils apply the coating for anti-oxidant protection.
The following steps are used to remove the coating:
- The copper coil is kept immersed in an acidic solution
- This is followed by micro-etching the copper coils to compensate for any isotropic losses in the manufacturing process.
- This step is concluded by coating the copper coil using various oxidation agents to prevent oxidation and adhesion.
- Now is also a good time to check the copper surface for imperfections such as pits and dents to ensure that it meets industry standards.
Designing the Circuit Board Pattern
The next step is to design circuit patterns. Circuit pattern design is an art form that requires a good understanding of circuit schematics. It serves as a roadmap for the circuit and contains various symbols to represent different components to be used on the circuit. These components include switches, diodes, nodes, resistors, and more.
Depending on the schematic, rigid flex circuit pattern printing can be a complicated process. The main techniques involved are:
Screen Printing: Screen printing is a popular technique because it allows fabricators to quickly design circuit patterns onto the surface of the board. Total thickness should be in the range of 4 to 50 microns.
Photo Imaging: This is an older technique that predates screen printing. It utilizes a dry photoresist film that is applied in close proximity with the laminate. The laminate is then exposed to ultraviolet radiation to transfer the pattern from the photoresist film to the laminate. The film is removed once the desired circuit pattern is achieved.
Etching the Circuit Pattern
Once the pattern has been generated, the next step is to etch the copper laminate. Etching is a process that uses a liquid or gas etchant to clear unnecessary parts until the desired circuit patterns are left. The two main types of etchants are liquid-phase and plasma-phase, depending on the material that is to be etched. Rigid flex PCB production process involves dipping the laminate in an etch with the etchant solution.
This phase starts by placing the PCB between phenolic fiber boards for support. Two or more registration holes along the edges of the PCB are used to line up various features of the PCB such as the conductive pattern and the drill hole. These holes are used for pads, vias, and other components of the board such as slots.
Fabricators take many precautions to ensure that the drill bit used for the rigid flex PCBs is the right size. If the drill bit is too big, it can damage the components in the circuit board drill. On the other hand, if the drill is too small, then it will be difficult to pass wires through the holes. Most manufacturers use a drill press to ensure increase the accuracy of the drilling holes. A drill press is four times more efficient than hand-held drills.
This is a critical stage of the rigid flex board manufacturing process. The fabricator chemically deposits a thin layer of electroless copper over the surface of the board including the inside of the holes. An electrical connection forms from one side of the rigid flex PCB to the other.
It is important to protect both sides of the rigid flex PCB by applying a coverlay. The fabricator uses a coverlay to mask off all areas of the flexible circuit that should not be soldered. Moreover, the coverlay also protects the circuit from extreme weather, solvents, and harsh chemicals.
The cover lay material of choice is a polyimide film with adhesive. The cover lay material is imprinted on the surface using screen printing, which is cured with UV exposure. Coverlays may be laminated under heat and pressure to ensure proper adhesion.
An alternative to coverlays is covercoat. This is a material that can be applied directly to the surface of the substrate. Fabricators use either type of coating after considering various manufacturing methods, application areas, and materials used. In any case, both cover lay and covercoat ensure the electrical integrity of the rigid flex board.
Blanking or Cutting the Flex Board
Blanking the flex board is an important step that must be performed with accuracy. If the fabricator is producing rigid flex boards in high volume, they may choose a hydraulic punching machine. The tool isn’t used when the production volume is low because of the high costs. A specialized blanking knife may be used instead.
Testing and Verification
Finally, the fabricator tests the rigid flex circuit board against various design specifications such as isolation, continuity, and performance. Different testing methods are used, including the flying and grid probe test methods.
Rigid flex PCBs are playing a vital role in circuit assembly. However, fabricators must exercise quality control measures to ensure that the board performs according to design. Every step of the production process must be performed with caution. This ensures that the board is unmatched in terms of durability and performance.