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:

COST:

  • 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

Time:

  • 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

 SEMI-FLEX area

 Minimum bend radius

 5 mm

 Maximum bend angle

 180°

 Maximum number of bends

 5

 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

 Recommended

 Use copper on both sides

 Recommended

 Use copper pour around tracks where possible

 RIGID area

 Required

 Yes – At each end of the Semi-Flex Material

 Minimum size

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

 Material (rigid and semi-flex cores)

 Isola PR370HR

 Number of copper layers

 4

 Board thickness

 1.6 mm (see build map)

 Semi-flex core thickness

 100µm

 Outer layer start copper foil

 18 µm

 Highest pattern class

 7 – 125µm track/gap

 Surface finish

 Che Ni/Au selective (ENIG)

 Soldermask

 Green

 Legend

 As STANDARD pool

 Profile

 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

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STANDARD pool

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Isola I-TERA and Rogers 4000 series material

SEMI-FLEX pool

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

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