What is SEMI-FLEX?

SEMI-FLEX is a flex-rigid PCB, typically 4 layers, made entirely of FR4. Unlike conventional flex-rigid PCBs, the flexing element is not polyimide 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 polyimide, 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

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 1.00 mm

Corner where a narrow semi-flex area joins rigid area

Minimum radius 1.00 mm


Use copper in both sides


Use copper pour around tracks where possible

RIGID area

Material (rigid and semi-flex cores)

Isola PR370HR

Number of copper layers


Board thickness

1.6 mm (see build map)

Semi-flex core thickness


Outer layer start copper foil

18 µm

Highest pattern class

7 – 125µm track/gap

Minimum distance between hole edge (TOOLSIZE) and any Flex area


Surface finish

Che Ni/Au (ENIG), Che Ag






Break-routing or scoring

Slots and cut-outs (milling)

As STANDARD pool – in rigid area only

Advanced technologies allowed

Peel-off mask, PTH on board edge, round edge plating, copper up to board edge. Plus carbon pads and edge connector gold as non-pooling options.

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

NAKED proto

Fast proof-of-concept board, No soldermask and no silkscreen

PCB proto

1, 2 or 5 PCBs in 2, 3, 5 or 7 working days


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

BINDI pool

Medium volume boards from our new plant in India

RF pool

Isola I-TERA and Rogers 4000 series material


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


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