In order to prevent problems in the assembly process caused by humidity in the flex material we recommend the following baking procedure:
Please notice that all flexible materials absorb more moisture than FR-4, and need to be baked before soldering to avoid blow out problems and de-lamination.
Place the boards horizontally side by side and bake in an oven at 1050C for a minimum of 1 hour. Then store in a desiccation cabinet or process within 1-3 hours (depending on the humidity of the environment). Baking times to remove moisture may be increased depending on the Copper Density and Areas of Ground Plane, which reduce the time for moisture to escape. Also the storage conditions and time of storage can affect the level of moisture uptake and hence the time required to remove it.
If kept in a humid environment after baking, the flexible material absorbs significant amounts of moisture within the first 15-30 minutes, depending on the material's specific moisture absorption rate, and must after that be baked again.
Storage in a desiccation cabinet or low humidity environment to prevent moisture re-absorption is recommended.
Flex Rigid PCBs:
Flex-Rigid PCBs needs extended baking compared with pure flexible PCBs.
For the Flex-Rigid, moisture is slower to be removed, as the Boards are generally thicker and if internal layers have a high Copper Density or large ground planes then the moisture must exit through the edges or directly from the flex material. The boards should be kept flat during the baking operation and it is recommended that they are kept separated from each other with spacers if stacked on top of one another inside the oven.
Bake the boards in an oven at 105-120 0C for 3-12 hours, depending on the Flex-rigid construction, the size of the Rigid part and how much humidity that needs to be extracted.
If packed and stored in a controlled environment, and kept in the original sealed bag up to assembly, the baking time can be reduced.
The amount of baking required can be found by checking the moisture content. This can be done by comparing the weight before and after baking. It is always recommended to Bake the product but time may be reduced to a minimum by baking a sample within the oven having weighed the panel before baking and
monitoring how long it takes in the oven until its weight no longer falls, i.e. it reaches steady weight. Then as a rule of thumb take this time +10% as the baking time for the batch.
Baking can affect solderability
It should be noted that baking of Printed Circuits for extended times and in atmospheres containing Oxygen may affect the solder-ability. Baking in an inert atmosphere such as Nitrogen or in vacuum is therefore strongly recommended, and always required if the board has a sensitive surface finish such as Immersion Gold. Vacuum Baking has the benefit of also speeding up the moisture removal process and reducing time required.
These instructions are provided as guidance only and the product design and storage can influence how the product performs in practise. It is therefore always advisable to check that all the moisture has been removed and the solder-ability in not affected on a representative sample before processing the batch.
Note for normal rigid PCBs, if no recommendations are made on the package by the supplier please check that the package are without openings and that the HIC shows normal humidity(<30%)