prototyping PCB flex circuits

The basic construction of a flex circuit is similar to that of a rigid PCB. A flexible insulating substrate, usually a polyimide (PI) film, is used as the base, with copper being pre-laminated to it. Conductors on the surface of a flex circuit are protected by an additional layer called a coverlay, which also acts as a solder mask. The coverlay protects the conductive surface of the circuit from corrosion and damage during manufacturing, and may be printed using photo-imageable or screen-printed solder mask ink. For a more robust flex circuit, manufacturers sometimes laminate a section of rigid board to the flex.

When designing a flex circuit, the design engineer must consider how the circuit will be used and determine what type of bending it will undergo during operation. This helps avoid damage to the traces, which could result in short circuits or even a complete break in the copper. The designer must also ensure that the flex circuit can bend within a specified radius without suffering any stress or strain on the copper, which can cause failure. To reduce the possibility of stress, it is a good idea to stagger the copper trace layers in multi-layer circuits, and always use a large radius when bending the flex circuit to prevent damage to the copper.

The copper on a pcb flex is plated the same way as it is on a rigid PCB, with nickel and gold being used to create the contact layers. For high-speed signal transmission, it is essential that the thickness of the copper is not reduced. However, the thickness can be impacted by bending, so it is important to have an understanding of how much a flex circuit can bend before making any design changes.

What are the steps involved in prototyping PCB flex circuits

For more flexibility, a flex circuit can be made with thinner copper layers. However, this can impact the EMI shielding ability of the circuit. In most cases, a design engineer will need to compromise between flexibility and EMI shielding.

To reduce the overall cost of a flex circuit, manufacturers may add adhesive tape to selected areas on the circuit to allow it to be glued to a surface or stick to something else. The tape will increase the cost slightly, but it will provide a strong and durable bond to any surface or object that the circuit is attached to.

The last step in prototyping a PCB flex is to cut it to size, which is done using a blanking knife and a routed slot on a rigid backing board, or in high-volume production by a CNC machine. The process can take a few weeks, although quick turnarounds are often available for urgent orders. An electrical test is done on the flex circuit to make sure that it works as intended, and all components have been properly installed. It is also a good idea to run a thermal test to verify that the flex circuit can operate at the temperature it will be used in.