Soldering in Rigid Flex PCB

Rigid Flex PCB

The soldering process of rigid flex pcb requires special consideration. This is because the flex circuits are often thinner than the rigid PCBs and therefore can be more susceptible to thermal and mechanical stresses. To address this, a fixture must be constructed to hold the flex section in place so that it does not deform during the pre-soak and reflow of the solder. This may involve adding local tooling pins to accept the ends of mechanical clips, or it may take advantage of mounting holes in the flex circuit. Alternatively, a local layer of PSA (pressure sensitive adhesive) can act as a temporary mechanical strain relief.

Soldering of rigid-flex pcb can be performed manually or with a semi-automated hot bar soldering system. Which connection method is chosen will depend on the expected volume of parts to be assembled. In addition, the choice of connection method should align with manufacturing capabilities and processes. For example, the use of a conductive adhesive may require specialized equipment and processes, while a soldered connection might be better suited to high-current connections or signal contacts that need compliance.

One big DFM (design for manufacturing) consideration is deciding whether the flex circuits are to be folded during assembly and then left in a static position – such as in a handheld ultrasound device – or will be continuously bending and moving, like a flexible battery cell or motor. This will dictate the amount of flex and copper layers, the type of copper (RA or ED) and so on. If flex sections are going to be repeatedly folding and bending, it is recommended that they are given stiffeners to reduce stress concentrations near the solder joints and connector pads. This will also improve the longevity of the flex section as it will be less likely to break or fail due to fatigue.

Soldering in Rigid Flex PCB

Another important DFM consideration is the selection of a suitable reflow profile. The reflow profile is the temperature and duration at which the board is exposed to a soldering iron. This profile is important because it will determine the onset of any delaminations or bubbles that might occur in the flex circuit. Generally, reflow profiles are designed to be fast and uniform for all flex circuits in a production run, so that the entire flex circuit is treated the same way.

The reflow profile should also be optimized for the thickness of the flex circuit and the corresponding pad size and pitch. In addition, the reflow profile should consider any potential contaminants in the reflow environment, such as air and water vapour.

During the soldering process, it is also important to minimize the exposure of the flex circuits to moisture. This is achieved by ensuring that the bake out process takes long enough to evaporate any entrapped moisture within the dielectric film. If this is not done correctly, the moisture will boil at the soldering temperature, leading to violent delamination or, in milder cases, small blisters that can grow into more serious defects later on.