Pre-baking prior to assembly for flex and rigid-flex applications is extremely critical due to the hydroscopic nature of flexible PCB materials. We offer the following suggestions:
- Hand Soldering
- Surfaces to be soldered need to be clean
- Bake circuits at 250°F (121°C) for 1 hour
- Remove enough units from the oven that can be soldered within 2 hours.
- Allow to cool to the touch (if the units are not soldered within 2 hours, they should be re-baked or placed in a desiccant after removing from the oven).
- Apply Flux (a rosin type flux like Kester 185 works well).
- Use Acid Brushes to apply flux; area to be soldered should covered by the flux.
- A soldering iron (something like a MetCal unit set at 700°F) with a 700°F #2 solder tip should apply the right amount of heat.
- Soldering tip held on the pad/component lead while applying the solder.
- A solder joint forms in 3-5 seconds.
- Minimize the dwell time
- Longer dwell time causes solder to "run under" the covercoat or delaminate.
- Remove flux using an appropriate flux remover (a vapor degreaser works well).
- The solder joint should a have a nice uniform fillet with no voids or dewetting.
- Pre-bake for 45 minutes at 250°F
- Rigid-Flex with 2 layer flex
- Pre-bake for 3 hours at 300°F
- Rigid-Flex with Multilayer Flex
- Pre-bake for 4 hours at 325°F