Prevent Delamination in Rigid Flex Circuits

Rigid flex circuits offer the advantages of rigidity and a smaller overall package size, as well as reduced weight and lower assembly costs. These features make them ideal for applications where a high degree of reliability and durability are required. Examples include medical imaging equipment, military missile guidance systems, wearable electronics, and vehicle infotainment systems.

Like rigid PCBs, rigid flex circuits use polyimide materials that can be fabricated into a wide variety of thicknesses and shapes. They also feature copper-plated conductors that provide the necessary electrical performance for the intended application. Rigid flex circuits must be designed with care, though, to avoid delamination, which can lead to failure.

Delamination is most often caused by moisture absorption during solder reflow in the final assembly process. The absorbed moisture can heat quickly, which can cause it to expand and separate the layers of the circuit board. This problem can be prevented by prebaking the flex circuits prior to assembly, which ensures they are 100% moisture-free.

How to Prevent Delamination in Rigid Flex Circuits

Other causes of delamination in flex circuits include incorrect stack-up, oversized traces, and excessive copper. It is important to select an optimal layer stack for your design, which includes a minimum of two copper layers, and proper distribution of dielectric materials. This will help prevent signal coupling and control impedance in flex sections. For maximum mechanical stability, a minimum of a 1 mil thickness is recommended for the copper plating.

In addition to the aforementioned factors, it is important for designers to keep in mind that rigid flex circuits can experience increased vibration and shock compared to their rigid counterparts. In order to mitigate these effects, the design should incorporate adequate flex stiffeners and maintain a tight bend ratio.

Another way to prevent these issues is to perform rigorous DFM (Design for Manufacturability) reviews, and a robust assembly process. This will help to catch any potential problems before they can be realized during manufacturing, which can lead to production delays and cost increases. The construction of a rigid-flex circuit involves combining multiple layers of rigid and flexible substrates, along with conductive traces and components, into a single, integrated unit. The rigid sections typically consist of layers of fiberglass-reinforced epoxy laminates, similar to those used in conventional PCBs. These layers provide structural support and stability to the circuit.

A final way to avoid delamination in rigid flex circuits is to take advantage of the full set of CAD features available with Altium Designer, part of the complete Altium 365 platform. These tools include 3D rendering and simulation that can help you visualize how your flex section will bend, as well as evaluate its fit in the mechanical enclosure.

In the video below, John shows how to use these tools to evaluate a rigid-flex design, and how to stimulate it based on the bending angles needed to fit the PCB into its mechanical enclosure. John also discusses some common DFM mistakes to avoid when designing rigid-flex circuits. To get started with your own design, try out the free version of Altium Designer. Start your free trial today!