Flex Circuits
Whether in under-the-hood automotive applications or handheld consumer electronics, soldering flex circuits presents a different challenge than mass soldering to hard boards. Nevertheless, it is possible to produce reliable solder joints on flex PCBs with proper fixturing, preconditioning and tooling, careful handling and precise soldering techniques.
One of the main issues is that flex circuits are more flexible than rigid PCBs, and that can place stress on components, especially solder points. For this reason, designers should make sure that flex points are far enough away from the solder pads to prevent stress fatigue. A safe rule is to use a bend radius of 10X the material thickness. In addition, designers should not apply stress to plated vias or solder fillets. The stress on these components can cause them to crack or break, and the stress can be transmitted back through the flex board to the solder point.
In a conventional wave soldering process, the flex circuit is placed in a preconditioning fixture that holds it flat and keeps it from moving during the soldering process. This is especially important for fine pitch and BGA components. A vacuum jig that holds the flex circuit down is often used for this purpose. This is not ideal, however, if the vacuum is directly under some of the component leads because it can cause them to make contact with each other and create an open circuit.
Soldering Considerations For Flex Circuits
Soldering a flex circuit requires special tools and techniques that can be difficult for inexperienced operators to master. For example, the temperature of the soldering iron needs to be kept lower on a flex circuit than a hard board because the copper in the flex is thinner and hotter than a rigid PCB. The temperature of the solder must also be adjusted to account for differences in thermal expansion between the substrate, the flex and the components.
To prevent stress fractures of the flex circuit, a protective potting compound must be applied. This compound is usually a low-temperature epoxy. It helps to protect the solder joints from the abrasive action of hand-soldering and from damage during flexing of the pcb. It also helps to prevent the stress-fractured solder joints from breaking through the flex circuit dielectric.
The other soldering consideration is that a good quality no-clean rosin core solder is required. This type of solder is softer and can accommodate the bending of a flex circuit better than other types of solder. It is also recommended to use a liquid no-clean flux that is activated with a conductive brush. This will improve heat transfer, wetting and prevent oxidation.
In addition to these technical considerations, a careful and thorough cleaning of the flex circuit is essential. This should be done prior to placing any components. In addition, the flex circuit should be stored and transported in an antistatic bag to minimize the effects of static charge. Finally, a good visual inspection is essential to avoid mishandling that could result in traces or pads being lifted from the flex circuit.
