Rigid-flex PCB assembly
Rigid-flex PCB is a new type of printed circuit board with the adaptability of a flexible PCB and the durability of a rigid PCB.. Among all types of PCBs, the
combination of rigid PCB and flexible PCB is the most resistant to harsh environmental applications, so it is favored by industrial control, consumer electronics,
medical, and military equipment manufacturers.
Just as the name implies, a rigid-flex PCB is a standard rigid board attached to a flexible
PCB during the PCB assembly process. The flexible portion is usually used to connect
the main rigid PCB to other boards or another device in post-production, or to satisfy the
shape or space constraints.
The difference in the rigid-flex PCB assembly process
The SMT process of rigid-flex PCB assembly is the same with the rigid PCB and the flexible PCB because they all need to pass the solder paste soldering process, component
placement, and reflow soldering. However, there are some unique points should be considered for rigid-flex PCBs. It will prevent great trouble if these additional requirements can be
fulfilled in the assembly process.
- Solder paste process
- SMT components placement
- Reflow process
As the same with the rigid PCB, the solder paste is applied to the rigid-flex board through the operation of stencil and solder paste printer. But workers are plagued by
the size and vulnerability of the board in the process. Unlike rigid boards, the flexible board’s surface is not smooth, which means it needs to be held in place by
fixtures and positioning holes. Besides, flexible materials are not stable in size and will extend or fold with the changes in temperature and humidity. For this reason,
rigid-flex PCB assembly often requires smaller carriers than rigid board.
In the current trend of miniaturization of SMT components, small components will cause some problems during reflow soldering. If the rigid-flex PCB is tiny, the
extend or fold will not be a significant problem, but it will lead to smaller SMT fixtures or additional mark points. And the lack of overall flatness of the fixture will also
cause displacement in the SMT process, so SMT fixture is one of the factors to keep the surface flat in PCB assembly.
The flexible part of the rigid-flex PCB must be dried before reflow soldering, which is an apparent difference in the assembly process between flexible PCB and rigid
PCB. In addition to the dimensional instability of flexible materials, they are also relatively hygroscopic, and they absorb water like sponges. Once the flexible part of
the board has absorbed moisture, it must stop the reflow soldering process. Therefore, flexible boards need to be pre-baked before using, and the preheated baking
must be completed quickly within one hour. If it is not baked in time, it needs to be stored in a dry or nitrogen storage.