Mechanical Stability in Flex Circuits
The applications for flex circuits are vast and varied. They are used in a variety of electronic devices including calculators, cell phones, printers and LCD televisions as well as medical instruments like heart monitors and pacemakers. They are also utilized in industrial automation equipment such as robot arms, processing machines and bar code scanners and in aerospace/military applications. Because these circuits are subject to significant mechanical stress, it is important to consider their physical stability. The best way to ensure mechanical stability in a flex circuit is through careful design. In order to do this, you need to be aware of the common mistakes that can lead to a circuit breaking down under mechanical stress.
A common mistake is neglecting the correct bend radius when routing traces. This can lead to stress fractures that will cause physical damage to the traces and eventually lead to failure. To avoid this, it is crucial that you calculate and adhere to the recommended bend radius based on the material thickness and type. A good rule of thumb is to keep the bend radius at least ten times the thickness of the flex circuits.
Another mistake is using excessive copper plating, which can increase the sensitivity of the circuit to thermal expansion. Depending on the temperature conditions, this can result in cracking and loss of conductivity. To avoid this, it is important to use the proper copper layer thickness and type as well as follow a thorough thermal cycling process.
How to Ensure Mechanical Stability in Flex Circuits
It is also critical to be mindful of the location of components and pads in a flex circuit, as they can be vulnerable to lifting during assembly. This can lead to failure and damage of the component and may even disrupt the functionality of the circuit. It is recommended that you keep all solder points, whether they are leaded or lead-free, a minimum of 100 mils away from the flex area, but further is better if real estate allows it.
You should also avoid sharp corners when designing a flex circuit, as these can be susceptible to tearing. It is recommended to use rounded corners, as these will be less likely to tear during the manufacturing process and during flexing.
Stiffeners are used to add rigidity to flex circuits in areas that will be exposed to mechanical stress. These are often made from FR4 and serve the purpose of increasing board strength, preventing bending and protecting components and connectors. However, it is important to choose the right stiffener material based on the environment in which the flex circuit will be used.
Lastly, it is also vital to consult with manufacturers throughout the design process to ensure that the flex circuit will be capable of surviving the intended flexing conditions. They can provide valuable input and advice on the materials, mask types, dielectrics, and stiffeners to select to create a robust rigid-flex PCB with adequate flexibility.