Nearly all Flex and Rigid-Flex boards are constructed using a polyimide material. This material is naturally hydroscopic, which means that it easily and readily absorbs moisture. It is therefore strongly recommended that flex and flex-rigid boards be baked to reduce the moisture content prior to any assembly process that requires soldering. Risks such as delamination, inner layer separation or cracking of the through hole copper walls may occur if the pcb’s are not baked prior to being exposed to the assembly process.

General Guideline for Baking

Hot Air Solder Levelling (HASL), Lead-Free HASL and Electroless Nickel Immersion Gold (ENIG)

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RIGID AND RIGID/FLEX PCB’S THAT USE POLYIMIDE MATERIALS

All PCB thicknesses – Bake for a minimum of 6 hours at a temperature of 135° C.

Assembly and soldering should take place within a period of 8 hours.

Immersion Tin, Immersion Silver and OSP

Due to the sensitivity of these surface finishes, a normal baking process is not recommended. In a case where baking is necessary, a trial process may need to be applied using a lower baking temperature followed by solderability tests to confirm viability.

In Conclusion

In all the above cases, baking should take place in a clean oven to prevent any contamination from an external source. The boards should also be placed inside the oven in such a way that the air can circulate freely around the boards during the baking process.

The baking process itself can be seen as a form of “advanced ageing”, this may in turn influence the solderability of the pcb, the relevant temperatures are therefore a recommendation only.

Relative to the above, these suggestions/recommendations serve as guidelines only, there can therefore be no liability to the pcb supplier themselves should there be any negative effect to the pcb’s because of a baking process.