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:
- 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). Acid Brushes can be used to apply the flux; the 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. The soldering tip should be held on the pad/component lead while applying the solder. A solder joint should be formed in about 3-5 seconds. Obviously you want to minimize the dwell time. Too long of a dwell time can cause the 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