Polyimide-free FR-4 printed circuit boards for flex-to-install solutions
AT&S is registering increasing demand for rigid-flexible printed circuit boards. Flex-to-install printed circuit boards in particular are much in demand. These are printed circuit boards that are basically rigid, and which have to be bent or twisted in order to fit into the space available in the casing or appliance.
Up until now, rigid printed circuit boards were combined with flexible polyimide-based ones in these kinds of application. With its 2.5D technology, developed in-house, AT&S offers its customers an attractive alternative at a competitive price using standard FR-4. This new technology is primarily suitable for printed circuit boards needing to be bent up to about 5 times for installation. Where the boards need to be twisted, traditional polyimide is still the recommended solution. The permissible bending radius is highly dependent on the printed circuit board design, e.g. copper thickness, and must be determined from case to case.
Advantages of 2.5D technology over traditional flex-to-install printed circuit boards:
- When using multiple soldering, FR-4 allows the same processing parameters as with rigid printed circuit boards. Polyimide absorbs more humidity in the soldering process, which considerably shortens the processing window. Compared with no-flow prepregs, FR-4 materials also show better filling behaviour, which significantly reduces the risk of delamination.
- Partial plane and depth milling in the bending area is also avoided, which in turn means that no glass fibres need to be milled. This makes for a very smooth surface with very narrow residual tab tolerances in the bending area. Break lines can be avoided, and the maximum residual tab of approximately 16 mm as a consequence of partial plane / depth milling can be increased as needed.
- The price! Polyimide and no-flow prepregs are more costly than FR-4. An additional plus is that 2.5D technology flex-to-install printed circuit boards undergo the normal manufacturing process for rigid printed circuit boards. Additional work stages, such as preparation of no-flow prepregs, are unnecessary.
- HDI printed circuit board technology can be applied to the entire printed circuit board, which means higher packing densities for components, and thinner printed circuit boards.