SEMI-FLEX are flex to install boards

SEMI-FLEX is a flex to install PCB, typically 4 layers, made entirely of FR4. Unlike conventional flex-rigid PCBs, the flexing element is not polyamide but a thin FR4 core with two layers of copper specially treated to flex without cracking. A less robust solution uses a depth-milled layer rather than a core.

Why use SEMI-FLEX?

SEMI-FLEX is flex to install. Unlike polyamide, the FR4 core is not capable of continuous flexing. However it will bend a limited number of times (typically 5 – see the technical specification below) at a controlled radius and to any angle. This makes it an ideal solution where you need to mount two PCBs in a unit at an angle to each other. Instead of using connectors and cables or a composite flex-rigid PCB, you can design a single FR4 SEMI-FLEX PCB which can be safely bent a sufficient number of times to allow installation and subsequent maintenance as needed.

The benefits of using SEMI-FLEX are:


  • No need for connectors or cables
  • Less time needed for assembly
  • SEMI-FLEX is entirely FR4, so no need for boards made with expensive polyimide

Product reliability:

  • Fewer solder-joints


  • Faster design time
  • Faster procurement
  • Faster build

Eurocircuits’ SEMI-FLEX pool

Eurocircuits’ 4-layer SEMI-FLEX PCBs 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 pre-preg (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 pre-preg before pressing. We can then rout the outer layers with any minor variation in cutter depth being absorbed by the void.

The core build makes it easier to design a main board with up and down sections or a final Z-configuration.

SEMI-FLEX boards are preferably supplied in a panel to avoid any damage during transport or assembly.

SEMI-FLEX technical specification


 Minimum bend radius

 5 mm

 Maximum bend angle


 Maximum number of bends


 SEMI-FLEX length calculator

 (2 x pi x bend radius) x (bend angle/360)

For example, bending 5 times over an angle of 180 degree with a radius   of 5mm requires a minimum length of the semi-flex part of 15,7mm

 Copper thickness on semi-flex core

 35µm high ductility copper

 Highest pattern class

 Class 6 – 150µm track/gap (to accommodate the bending)

 Drill holes

 Not allowed

 Slots and cut-outs

 Not allowed

 Corners in semi-flex area profile

 Minimum radius 1.00mm

 Corner where a narrow semi-flex area joins rigid area

 Minimum radius 1.00mm


 Use copper on both sides


 Use copper pour around tracks where possible

 RIGID area


 Yes – At each end of the Semi-Flex Material

 Minimum size

Width of Semi-Flex Material or 5mm x 5mm whichever is the greater

 Material (rigid and semi-flex cores)

FR4 Improved High Tg

Isola FR406N

 Number of copper layers


 Board thickness

 1.55mm (see build map)

 Semi-flex core thickness


 Outer layer start copper foil


Min. track width/spacing


Min. Finished Hole Size

 PTH 0.100mm

 NPTH 0.200mm

 Surface finish

Pooled – Lead-free for best price, ENIG, LF HAL

Semi-Pooled – IM Ag






 Break-routing or V-cut

 Slots and cut-outs (milling)

Tool sizes – 0.5, 1.2, & 2.0mm – in rigid area only

 Advanced technologies allowed

Pooled – Peelable mask, PTH on board edge, Round edge plating, Copper up to board edge, Heatsink paste, Via filling

Semi-Pooled –  Edge connector gold

Non-Pooled – Carbon pads

 Advanced technologies not allowed

Blind & Buried vias

 UL marking

Not yet available

 Delivery format

Preferably delivered in a panel for safe handling

Explore all PCB Pooling services from Eurocircuits

PCB proto

1, 2 or 5 PCB’s in 2 or 3 working days


FR-4 lead-free soldering compatible material up to 16 layers

RF pool

Isola I-TERA and Rogers 4000 series material


4 layer, 100 micron FR4 core with two specially treated copper layers

IMS pool

Single layer Insulated Metal Substrate PCBs (ALU)

BINDI pool

Medium volume boards from our new plant in India