Eurocircuits’ SEMI-FLEX pool
Eurocircuits’ 4-layer SEMI-FLEX PCB’s use a 100 micron central core for the flex area, with 35 micron high-ductility copper foil on each side.
The stack is then made up on each side with a 510 micron FR4 core and two layers of no-flow prepreg (see the full technical specification below).
An alternative technology is to build a 2- or 4-layer PCB and then depth-mill the material away to leave a thin section as the flex area.
We have adopted the core solution even though it needs more operations and so is a little more expensive. It gives a symmetrical build and so a more stable board, while depth-milling is difficult to control meaning that the flex area may have an irregular thickness.
With a core-build we through-mill (rout) the prepreg before pressing. We can then rout the outer layers with any minor variation in cutter depth being absorbed by the void.