Semiflex PCB: the low-cost alternative to rigidflex
Semiflex is considered a low-cost alternative to rigid-flex technology. The absence of flex foils lowers the price, but also reduces the bending properties. The movable areas here are created by deep milling in the FR4 PCB, the remaining thickness is only about 150 µm. The PCB is therefore bendable in this area together with the tracks placed on it and the flex varnish or cover foil applied over the tracks. In contrast to the rigid-flex technology, the bending radius must be at least 5 mm. The Semiflex structure is well suited for static bending stress during assembly and installation. In the simplest case, part of the PCB can be bent after assembly and fixed in this position during installation in the enclosure.
An example of this technology is the newly developed test bench of the company LSP Innovative Automotive Systems for the automotive sector. This test bench is used for the electromotive actuation of the 16 gas exchange valves of a research combustion engine. For assembly and space reasons, the control unit of four valves of a cylinder is combined into a U-shaped module. Test bench measurement signals are acquired in the central part of the board and must be distributed to the lateral wings. Since a solution with connectors was too large for the available installation space, Semiflex technology was chosen as the most suitable and cost-effective PCB technology.
When flat, the Semiflex PCB measures 413 mm x 276 mm and consists of six layers of 70 µm copper each. The areas to be bent by 90° during assembly have a length of 8.5 mm and the total width of all bending areas is 426 mm. A total of 195 conductive tracks are routed over the four semi-flexible areas. Bent twice by 90°, the PCB only requires a volume of 175 mm x 67 mm x 276 mm when assembled. The current conduction to the power components and even the heat dissipation could be realised with embedded copper profiles on the same PCB.