Rigid-Flex PCB Fabrication
The quality of rigid-flex circuit boards is crucial to the performance and reliability of the end product. The smallest design or manufacturing flaw can have a significant impact on the final device. Therefore, PCB manufacturers are extremely careful from the planning and designing phases through material selection, fabrication, and testing to ensure that they maintain a high level of quality throughout the process.
One of the key aspects of rigid flex pcb fabrication is drilling the holes needed for interconnection between layers as well as component placement. This is accomplished by creating different via types including blind vias, buried vias, and through-hole vias. While mechanical drills can be used to create these holes, laser drilling is an optimal choice for this type of application because it offers superior precision.
Unlike mechanical drill bits that cut the board by friction, laser systems vaporize the material to form the hole. This is also known as laser ablation and guarantees exceptional accuracy even when working with very tiny holes. The vaporization process also eliminates the possibility of leaving a ragged edge on the feature. Additionally, laser systems can process more production panels simultaneously than mechanical drilling machines.
Laser Drilling in Rigid-Flex PCB Fabrication
Rigid-flex PCBs are made from a combination of flexible and rigid sections of circuit board materials such as polyimide and FR-4 style materials. Typically, the flexible section will be adhered to a rigid backing board of MDF, plywood, or thick plastic such as teflon. The flex section is then routed into the rigid sections and copper is plated onto both surfaces. Finally, the flex section is affixed to the plated copper with an adhesive or through a photoimageable solder mask (the same technique as for rigid sections) in order to protect the underlying surface from soldering.
Before the etching and routing processes are performed, the copper layer on the top and bottom of the rigid-flex circuit board is covered with an additional layer of material called a coverlay. This is usually an additional piece of polyimide film with a printed mask. In some cases, there will be components mounted on the flex sections and these sections will need to have their own solder mask layer.
In most cases, the covers will be affixed to the flex section with an adhesive in order to protect the sensitive conductive layers during handling and assembly. The flex section will then be routed into the covers with the desired pattern. This is followed by etching the bare copper and the applied metallization to expose the pads for component mounting.
Laser drilling is required for several reasons when fabricating rigid-flex circuits. First, it is the preferred method for creating vias in flex circuits. The small diameter of blind vias allows for more room to place components and improves the density of the circuit layout. It is also ideal for burying vias in the flex area where it can be difficult to access with traditional mechanical routing tools.
Another advantage of using lasers for these types of applications is that they are able to work on the variety of different material layers found in rigid-flex PCBs. For example, some flex circuit materials such as ductile polyimide have very thin copper layers which are not easily routed with a mechanical tool. This can lead to impedance discontinuities in vertical transitions which can be minimized by backdrilling these holes.
