Flexible circuits are found everywhere: cell phones, tablets, flat screen tvs, cameras, rockets, satellites – anywhere electrical connections are required in three or more axes; where the assembly is required to flex during operation; where weight is an issue; and, as devices get smaller and smaller - where board thickness or space is a driving factor. They are most often connected using hot bar reflow soldering which utilizes a thermode heating element and allows for a more repeatable and consistent joint quality than traditional hand soldering, and are commonly made of two layers of polyimide encapsulating copper traces. Copper trace thickness generally ranges from 0.0007 – 0.004 inches, and thickness of the flex is between 0.001 – 0.0047 inches.
The three most common flex circuit termination designs for successful pulse heated reflow soldering are “exposed lead,” “single-sided,” and “open-windowed.” Choose the one that best suits your design and manufacturing needs:
- “Exposed lead”- both sides of the polyimide material are removed, leaving the traces free of insulation. This allows the thermode (hot bar) to contact the traces directly, conducting heat to the parts and creating the bond. This design tolerates some excess solder on the pads, as there are open areas for the solder to flow into. Exposed leads can be easily bent or damaged; handle carefully!
- “Single-sided” - polyimide is removed on one side only. Heat is conducted from the thermode, through the polyimide, to the exposed traces underneath. The polyimide thickness in the joint area can be no more than 0.002” to enable proper heat conduction without burning. This design is not tolerant of excess solder on the pads, as there is little room for excess to flow. This design is well suited to fine pitch applications.
- “Open windowed” – as it sounds: joint area is exposed in a ‘window’ in the polyimide. This design is a little more durable and tolerant of harsher handling. The traces are exposed, so thermal transfer is good and excess solder has space to flow into. Thermode sizing is critical, however, as it must fit into the window.
Hot bar reflow soldering of flexible circuits is a stable and well controlled process. The process window can be made substantially wider by a joint design that promotes easy and equal heat distribution, accommodates the flow of solder, and can compensate for variations in process.