Overmoulded Flex Circuit

Material Interface

Adhesion between the flexible polymer substrate and the injected thermoplastic is critical for preventing moisture ingress. During the production of an overmoulded flex circuit, the hot melt must melt and fuse with the outer surface of the flexible substrate. Commonly, polyimide substrates are treated with plasma or liquid primers to enhance the chemical bond with the injected resin.

Poor bonding results in gap formation and eventual delamination under thermal cycling.

Mold Protection

Delicate electronic components soldered to the flexible substrate are exposed to extreme pressures and temperatures during the injection phase. The overmoulded flex circuit must be positioned securely within the cavity to prevent the force of the incoming melt from washing away resistors or tearing the copper traces. Low pressure molding techniques using specialized polyamides or hot melts are often selected to protect these sensitive components.

This process keeps the tooling pressure below the mechanical limits of the solder joints.

Part Failure

Incompatible thermal expansion coefficients between the plastic overmold and the metal traces lead to internal stresses during cooling. When the overmoulded flex circuit undergoes thermal cycles in the field, these internal stresses can shear solder connections or fracture the conductive tracks. To avoid this outcome, engineers select resins with low thermal expansion and mold parts with thin, uniform walls.

This careful design reduces the stresses that develop as the different materials shrink at different rates.