A PCB stackup is the arrangement of copper layers and insulating layers that make up a PCB. It defines the layer count, materials used, and thickness of each layer. Proper stackup design is critical for signal integrity, impedance control, and mechanical reliability.
What materials are used in flex PCB stackups?
Flex PCBs typically use: 1) Polyimide (PI) as the flexible base material, 2) Rolled annealed (RA) or electrodeposited (ED) copper for conductors, 3) Adhesive for bonding layers, 4) Coverlay (polyimide + adhesive film) for protection. Rigid-flex boards also include FR4 and prepreg in the rigid sections.
How do I choose the right layer count?
Layer count depends on: 1) Routing complexity and signal count, 2) Power and ground plane requirements, 3) Impedance control needs, 4) Board size constraints. Start with the minimum layers needed, as more layers increase cost and thickness, which can impact flexibility.
What is the difference between coverlay and solder mask?
Coverlay is a polyimide film with adhesive, applied as a sheet and patterned via laser or mechanical drilling. It's more flexible and durable for flex applications. Solder mask is a liquid coating (LPI) that's screen printed or sprayed. Solder mask cracks when flexed, so coverlay is required for flex areas.