The central core layer of the material in the SEMI-FLEX pool consists of a 100 µm thin layer of FR4, which has on both sides a 35 µm thick layer of copper foil. This is how the semi-flexible part is put together. The circuit boards – with this flexible part in between – continue to build on this core, which therefore continues through the entire circuit board. On the inner two layers comes a double layer of pre-preg with a total thickness of 126 µm. This is followed by a 0.51 mm thick layer of FR4 material that has a 18 µm thick layer of copper foil. These outer layers are processed in exactly the same way as an 'ordinary' 4-layer circuit board. The illustration shows this assembly. This symmetrical construction results in a stable and robust circuit board.


When laying-out the circuit board the designer needs to take into account the following characteristics of the SEMI-FLEX pool:
  • The minimum bend radius is 5 mm
  • The maximum bend angle is 180°
  • To determine the required length of the SEMI-FLEX part, you use the following formula:

2 × pi × radius × (angle/360),

for example, with a radius of 5 mm and an angle of 180°: 2×pi×5×180/360 = 15.7 mm.
 
  • The copper thickness of the semi-flex part is 35 µm
  • In the semi-flex part there are no holes, slots or cut-outs permitted
  • The minimum radius of corners in the semi-flex part is 1.00 mm
  • It is best to plan to have copper on both sides of the semi-flexible part and surround the print traces with a polygon.
  • The final finish of the non-flexible part needs to be Che Ni/Au selective (ENIG)
  • The solder mask is green
  • Screen print, slots and cut-outs are the same as for the STANDARD pool
  • Permitted advanced production techniques are peel-off mask, PTH on board edge, round edge plating, copper up to board edge. Carbon pads and edge connector gold, available from the non-pooling options.
  • UL-marking is not yet available
  • Because of its rigidity it is preferable to supply the PCB in a panel

The ordering of a circuit board using the SEMI-FLEX pool is the same as ordering a board using any of the other services from Eurocircuits. Here too you have access to the extensive range of features of the PCB Visualizer, the PCB Checker and the direct assistance from a knowledgeable technical member of staff from the team at Eurocircuits. (also see our review of the STANDARD pool, the RF pool or the IMS pool).