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:
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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