Multilayer flex circuits
More complex flex circuits may need more than two layers of conductors to form the multilayer flex circuit. Interlayer interconnections are often made with plated through holes or other means depending on the features and applications.
The thickness of copper layers may or may not be the same depending on the flexible circuit’s functional requirements. The thickness of the dielectric may also be different for different flexibility and insulation requirements.
For the multilayer build-up, the additional layers of copper-clad substrates are typically on prepreg dielectric (B-staged or yet to be cured dielectric adhesive). They are cut into sizes and shapes that are used for multilayer build-up. The layering lamination is done under pressure and heat that are prepreg material dependent.
COUPER™ prepreg differs from traditional flexible circuit substrates using acrylic-type adhesive. The dielectric is a specialty AIT molecularly engineered B-staged dielectric that when cured is exactly the same as the copper-clad substrate. In addition, the lamination pressure required is less than 15 psi and at a temperature from 125-175°C with a controlled melt-flow using a temperature-controlled calendaring process. The instant melt-flow calendaring process can be done in continuous processing with inline post-curing for dramatically higher productivity in comparison to traditional vacuum hot-press processing.
Rigid-flex circuits and flex circuit with stiffener
Rigid-flex circuits are a hybrid construction flex circuit consisting of rigid and flexible substrates which are laminated together into a single structure. Rigid-flex boards are normally multilayer structures.
Rigid-flex circuits differ from flexible circuits with a stiffener. In this case, a flexible circuit is locally bonded on a stiffener to allow components mounting without suffering from flexing and bending stresses.