Flexible Printed Circuit Board Layer Stackup

As electronics continue to evolve towards lightweight designs, the utilization of flexible PCB has become prevalent in various applications, ranging from simple to highly complex systems. While single-layer structures may suffice for certain straightforward applications, multilayer boards with densely packed circuits are essential for more intricate applications in fields such as medical and automotive industries. Within the realm of multilayer circuit design, it becomes necessary to consider the concept of stacking, which encompasses the arrangement and positioning of various layers and interlayer structures, including conductive layers, insulating layers, and specialized layers. A well-designed stackup approach can effectively reduce board area, lower costs, and enhance reliability and stability. In this article, TechSparks aims to provide valuable insights and guidance for optimizing flex PCB stackup, thereby aiding your circuit design endeavors.

Table of Contents

Flex PCB Layer Stackup

Single layer flexible PCB

Single Layer Flex PCB

Single-layer PCB is the most basic circuit structure and does not possess a stacked configuration. However, it is important to mention them here for the purpose of clarification. Single-layer circuits consist of a single chemically etched conductive pattern layer while stacking refers to the inclusion of insulating layers to provide electrical isolation between the layers. Although single-layer flex PCBs may not offer the same level of electrical performance as their multi-layer counterparts, they are cost-effective and straightforward to manufacture.

Double layer flexible PCB

Double-sided flexible PCB

Compared with traditional rigid circuits, flexible PCB manufacturing is more difficult, and the double-layer circuit structure strikes a balance between flexibility and functionality. It involves etching conductive patterns on both sides of an insulating base film and connecting the patterns on both sides using metallized holes to form conductive paths, thus providing greater design and application flexibility. The cover film is used to protect the circuit and indicate the placement of components. In a 2 layer flex PCB stackup, two flexible substrates are bonded together in both X and Y directions to create a double-layer structure. Each layer can have separate signal and power layers within the stack-up.

Multi-layer flexible PCB

Multi-Layer Flexible PCB​

Multilayer FPCB involves the stacking of four or more layers of single or double-sided flexible circuits to create metallized holes. By incorporating vias, conductive paths are established between the different layers, enabling the creation of more intricate circuit structures. The process for creating via holes includes the drilling of the base material and copper foil, cleaning, and electroplating with a predetermined thickness of copper. This method distinguishes a multi-layer flex PCB from a single-layer flex PCB as it permits the copper foils in each layer to be interconnected. Moreover, the subsequent manufacturing technology for multi-layer flexible PCB is nearly identical to that of 2-layer flexible PCB, thereby ensuring the maximum achievable electrical performance.

Flexible PCB Layer Stackup Design Elements

Hierarchy and Hierarchy Order

In electronic engineering, the selection of layers and layer order is a crucial factor in designing multilayer flexible circuit board stackup as it significantly impacts their performance and cost. A hierarchical structure refers to the internal layer configuration of the circuit board. Each layer of a multilayer circuit board includes one or more circuit layers and substrate layers. The hierarchical order is the sequence of these inner layers. To optimize the circuit stack up design, the outermost layer is typically a copper-clad layer that provides excellent electromagnetic shielding and mechanical strength. The ground plane layer is next in sequence, providing a ground reference for the circuit. This is followed by the power plane layer, which supplies the circuit with power. The signal layer is the final layer, which is utilized to arrange signal lines. In addition, for applications requiring advanced functionality, additional inner layers may be necessary to facilitate more complex circuit layouts and interconnections.

Thermal Management and Thermal Design

Multilayer flexible PCBs, due to their unique structure, have a greater impact on thermal management and thermal design than single-layer flex PCBs. Heat generated by high power applications can be transferred between different layers, creating hot spots. To address this issue, several measures can be taken, such as installing heat sinks or heat dissipation materials, optimizing thermal simulation techniques, arranging the stacking order and layout of the board, and using materials and components with high thermal conductivity. The stacking structure of a multilayer flex circuit can also affect heat dissipation. Placing layers with higher heat-generating components closer to the heat sink layer can improve heat dissipation. Therefore, careful consideration of the stacking structure and layout of the board is crucial in ensuring effective thermal management and design.

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