One of the most common problems when a Flex Printed Circuit or Flex PCB is exposed to rapid temperature excursions during soldering reflow is delamination.
The main cause is moisture absorption, which may occur in any of these constructions:
- Adhesive based structures.
- Outer cover (adhesively bonded polyimide covercoats, film, and liquid solder masks) of single and double sided adhesiveless laminates.
- External bonded stiffeners.
- Three Layer and up multilayer construction.
- Rigid flex.
Pica came up with the following solutions to address the problem:
- Change the design to facilitate moisture escape using circuit channels to create a path.
- Add outer surface ports (e.g. openings and holes) at locations to reduce path lengths for moisture to travel.
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Bake the Polyimide Flex PCBs in the following manner:
- Single & double sided (1 & 2 copper layers) — Recommended 2 to 3 hours at 105 to 115 degree C.
- Three copper layer polyimide circuits — Recommended 3 to 4 hours at 105 to 115 degree C.
- Four layer and above Polyimide circuits — Recommended a minimum of 4 hours at 105 to 115 degree C.
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Rigid Flex Circuits
- Up to 1.0 mm Thickness: Minimum 2 hours at 120 degree C
- 1.0 to 1.8 mm Thickness: Minimum 4 hours at 120 degree C
- 1.8 to 4.0 mm Thickness: Minimum 6 hours at 120 degree C
Please be warned: Oven must be clean to avoid outgassing of contaminating materials which could deposit on the surface of the board. Make sure to place the circuits in the oven to allow air circulation freely around the circuit board.
After baking the Flex PCB, you must respect the following guidelines*:
- Do not exceed 8 hours of stage at 50%relative humidity.
- Store in vacuum or inside an oven at 35ºC
- Storage in a sealed bag with a desiccant.
Finally, please remember that exposure to the bake out temperatures can have negative effects on solderability. Solder or ENIG (Electroless Nickel Immersion Gold) surface finishes are very resistant to oxidation while Immersion Tin, Immersion Silver and OSP are not. Baking in a vacuum at lower temperatures can reduce solderability impact.
* Dwell or hold time between baking and soldering is dependent upon storage conditions.
