Flex & Rigid-Flex Circuit Assembly Pre-Bake Specifications
The need for an assembly pre-bake cycle is an industry accepted standard and a byproduct of the hydroscopic properties of flexible circuit materials. This property is independent of which material manufacturer is used. Entrapped moisture may flash to steam during assembly and out-gas with considerable negative results.
Applying a Pre-Bake cycle and eliminating retained moisture will improve assembly yields, product reliability and prevent delamination and blistering.
Flex Circuit & Component Preparation
Source - "Flexible Circuit Technology, Third Edition" by Joseph Fjelstad
As the principle elements of a flex circuit assembly, the flex circuit and components should be properly prepared. One thing that most plastic components and flex circuits have in common is that they take up moisture. This means that they must be protected from humid environments. Otherwise, they must be pre-baked to prevent explosive out gassing of trapped moisture and the creation of defective conditions such as cracked components or blistering and delamination of the coverlayer.
DuPont Pyralux® Flex materials Baking Recommendations Prior to Reflow
Source - From the Pyralux® Technical Manual page 5.23
We recommend that boards made with Pyralux ® Flex materials are baked prior to exposure to solder processes (e.g. solder leveling and reflow). Boards are generally baked at 250 °F (121 °C) from two to ten hours, * depending on the board thickness and design. Baking removes any moisture that may have been absorbed during processing. Polyimide films absorb moisture quickly; therefore, soldering and reflow should be done within 30 minutes after baking.
Vacuum ovens are also used to remove water. Lower temperatures, such as 150-175 °F (65-80 °C) can be used. This method also reduces the oxidation of the exposed copper pads.
Boards should be baked prior to soldering by hand, wave, IR and Vapor Phase soldering. This bake is typically done at 250 °F (121 °C) for two to ten hours, * depending on the board thickness and design.
Note: Moisture Absorption
Kapton® NH: 2.8%
Pyralux® LF: 1.8%
Pyralux® RF: 1.8%
* Times may vary based on type of materials in board, layer count, % copper ground planes, size of board, room/area conditions (%RH) etc...