special requirements for soldering Flexible pcb design

Soldering is the process of joining together a conductive metal and non-metallic parts with solder. It creates reliable electrical and mechanical connections and helps protect the metal surfaces from corrosion. Soldering requires a high temperature to melt the metals and then a rapid cooling phase to set them. Often the melted metal is cooled with water or air. This reduces the heat exposure time and minimizes thermal stress.

In the case of flexible pcb design, it is important to use appropriate temperatures and soldering methods. If the board is too hot, it may suffer from delamination – where the layers separate and cause abrupt damage to the circuitry. This can result in loss of functionality or even failure of the entire assembly. To prevent this, you should use an insulating material between the copper and the dielectric, such as polyimide with adhesive or FR-4.

The insulating material should be thick enough to reduce the chance of stress concentration at the joints, and it should not interfere with the flex board’s conductivity. It is also recommended to use a low-stress joint design, which reduces the number of joints in an area and increases the distance between them. This minimizes the chance of solder dripping onto other components. If the design calls for a joint in an area with tight bends, use a circular section with a radius greater than 1.5 mm to minimize the risk of tearing the flex substrate material at the corner.

Are there any special requirements for soldering Flexible pcb design?

Using the same soldering methods used for rigid boards is usually not recommended for flex PCBs, as they require higher temperature profiles. These are usually above the limit that a flexible PCB can tolerate, and they can cause the copper to burn or blister. Additionally, a long reflow time can cause the flex circuit to become unstable, and this is not conducive to achieving reliable electrical and mechanical connections.

Some manufacturers are now offering a glue-like dispensing option for flex PCBs, which can be applied to the ends of the flex circuit and then pressed against the end of the rigid board. This makes it easier to hand solder the joints without damaging them, and can also help to improve the longevity of the assembly. However, this method adds a step to the production process and must be used carefully to ensure that the dispensing mechanism does not leak.

It is also important to consider how a flex PCB will be handled in production and assembly. Ideally, it should be kept free of discontinuities such as vias or cutouts / slits. This reduces the chances of stress and cracking when the PCB is bent, and it should be sized to allow for some adhesive “squeeze out” around pads. IPC recommends that pads and coverlay openings be sized to provide 360deg wetting around vias for high-reliability flex designs and 270deg wetting for moderate reliability designs. It is also recommended to use teardrop (pad fillets) shaped vias, as this will further reduce the likelihood of stress concentration spots.