Reliability in Your Flexible Circuit Board
Flex circuit boards are unique in their design and fabrication process. Because of this, they are much more delicate than rigid PCBs, and therefore, they are more susceptible to damage in manufacturing and use in the field. This damage will impact the structural reliability of a flex circuit board, as well as its operational reliability. To ensure that a flex circuit is designed and manufactured to meet regulatory performance specification standards, it’s important that the design be optimized for yield and cost, and validated with a specialized flex PCB DFM tool.
Unlike rigid circuits, a flex circuit is composed of multiple layers. It has a flexible plastic (polyimide is the most common) and copper foil to create its conductors. The underlying polyimide substrate protects the circuit from environmental elements such as moisture and chemicals, while allowing it to stretch and bend without damaging itself or its components. In addition, the underlying material should be strong enough to hold the components in place without stressing solder joints or contact crimps.
A metal foil is the most common conductive element of a flexible circuit board, and copper foils are the best choice for most applications due to their excellent balance of physical and electrical characteristics. However, the foil must be high quality to withstand repeated flexing and bending. Lower quality foils are prone to work-hardening and fatigue, which can cause cracking of the copper foil under stress. To minimize these effects, consider using rolled-annealed copper. It can be more expensive, but it will also flex and bend much easier than un-rolled copper foils.
How to Ensure Reliability in Your Flexible Circuit Board
Another important consideration when designing a flex circuit is how it will be used. Will it be static or dynamic? A static flex circuit will be bent only once, during the installation process, while a dynamic flex circuit will be subjected to many more flexes. If the flex will be exposed to harsh environments, such as vibration or shock, it must be ruggedized with thicker material and stronger connectors.
In order to maintain signal and power integrity in a flex circuit, it’s important to properly route signals in the transition zones between the rigid and flexible sections. This helps avoid sharp transitions in impedance, which can result in noise or EMI. It’s also important to route conductors perpendicular to the bend radius in flex circuits, as this will reduce stresses that can cause trace copper breakage.
It’s also a good idea to minimize vias and holes in the flex section of a circuit, as they can be subject to higher stress and peeling under bending. When these are necessary, make sure that the vias have anchors and teardrop-shaped fillets to prevent them from breaking or fracturing under stress. Also, be sure to use large annular rings around them to increase the strength of the connection and limit stress concentration points. Lastly, it’s a good idea to use plated-through holes with a 10 mil diameter or larger in a flex circuit.
