Soldering in Rigid Flex PCBs
Rigid flex is a combination of rigid areas and flexible circuits, resulting in an electrically and mechanically robust PCB. It is ideal for high-speed designs and is often used in the medical industry for devices like pacemakers, handheld monitors, imaging equipment, and drug delivery systems. It is also widely used in the aerospace industry for flight controls, radar equipment, radio communication systems, and sensors. It is also employed in environmental and climatic testing systems.
In addition to the above-mentioned considerations, there are some special issues to be considered when working with a rigid flex pcb. Generally, the soldering process must be modified to accommodate the flexibility of the circuit. These include the use of different temperature profiles and avoiding applying too much heat to the flex area. This will help avoid the occurrence of blistering and delamination.
Another issue that must be taken into account when designing a rigid flex pcb is the fact that the copper foil layer of the flexible section will be exposed on the bottom side. It is important to keep in mind that the surface of the flex circuit will be susceptible to corrosion from chemicals, including cleaning solutions and solvents, so it must be protected. A good way to do this is by using an anti-corrosion coating, which can be applied by spraying or brushing.
The Significance of Soldering in Rigid Flex PCBs
When working with a rigid flex circuit, it is essential to ensure that the flex sections remain fixed during pre-soak and reflow soldering. This can be achieved by fabricating a fixture that will hold the flex sections in place and prevent them from deforming during these processes. This can be done in a number of ways, such as by attaching the flex section to an external support structure or by making use of mounting holes on the board.
It is also important to take into account the fact that a flex circuit can experience significant stress while it is being manipulated. This stress can cause fatigue and breakage of solder joints. To prevent this, it is a good idea to design a flex circuit with anchoring stubs and reduced coverlay openings.
To reduce the risk of failure, it is a good idea to use low-temperature potting compounds. These are more durable than high-temperature ones, which can break solder joints from flexing and bending.
In terms of design, it is important to make sure that the flex circuit is not too close to the component pads. This can lead to shorting between the copper and the component. In order to prevent this, a minimum clearance of 20 mils should be allowed between the flex circuit and the pad. It is also a good idea to limit the number of vias in a flex circuit, as these can cause stress in the area around them.
It is also a good idea to use an anti-reflective (AR) finish on the flex circuits. This will help to reduce the amount of light that is reflected from the surface, which can cause defects during inspection and testing.