Components to Flexible PCBs
Flex PCBs are flexible circuit boards that offer many benefits to the engineer. These include easy assembly, reduced size and weight, and the ability to accommodate a variety of shapes and sizes in the final device. Nevertheless, a flexible printed circuit must be designed with care to ensure that it is durable enough for its end use application. The following flexible circuit engineering tips can help with this task.
First and foremost, the design of a flexible printed circuit must take into account its bending capabilities and the maximum radius within which it can be bent without damaging the copper pattern. This will determine the number of layers, copper type, and metallization scheme that will be used. Secondly, the designer must decide whether the flex circuit will be dynamic or static. If the flex circuit will be permanently bent, the design can be a little more simple as it is possible to reduce the number of layers and the copper thickness while maintaining the same flexibility. Similarly, if the flex circuit is going to be used in a dynamic device where it will regularly fold or bend, then the metallization and the number of layers can be increased to allow for this.
As with rigid PCBs, there are many different types of adhesive materials to choose from for a flex circuit. Acrylic adhesives, epoxy adhesives, and pressure-sensitive adhesives (PSA) are just a few of the available options. Each type of adhesive has its own advantages and disadvantages depending on the application. For example, PSAs are easy to use and can stick directly to the flex circuit. However, they can be more prone to peeling and delamination when exposed to heat.
Tips for Attaching Components to Flexible PCBs
Stiffeners are a common addition to flex circuits. They can be used to create rigid sections in the circuit or to protect components or connectors that are attached to the flex area. They can be made of metals, FR4, or polyimide and are thermally bonded with the flex circuit using an adhesive material.
When soldering a flex circuit, it is important to remember that the joints are very delicate and need to be treated with great care. The soldering process involves high temperatures and the resulting vapor can damage the circuit. As such, it is essential to minimize the amount of vapor that is released during soldering by using a low soldering temperature and applying a layer of flux before the joint is heated.
The most effective way to prevent this is by creating a small gap between the flex and the rigid board during the manufacturing process. This will not only allow the vapor to escape but will also help to ensure that the soldering is performed correctly and to the highest standard.
Another method for joining a rigid board to a flex circuit is by attaching them with a solder sandwich. This involves placing a pair of pads on each rigid and flex board. These are then tinned and assembled by pulling a solder wire up between them, which will form a connection and keep the two boards together.