Every PCB do have a storing life once it is made and packed. We should get the board baked before sending it to SMT line if it has been kept over standard storing period, otherwise there is a potential risk of PCB defects.
We do have an industy standard of PCB storing time and the temperature of PCB baking.
PCB storing control
Unpack and warehousing
PCBs is vacuum sealed and manufacture date within two months can be used for assebly directly
PCBs manufactured within two months must be labelled with unpacking date;
PCBs manufactured within two months must be used in five days in surface mount.
PCB baking
PCBs made within 2 months and unpacked over 5 days, must be baked at 120+/-5 celsius degree for one hour;
PCBs made over 2 months ago, must be baked one hour at 120+/-5 celsius degree before SMT process
PCBs made between 2-6 months ago, must be baked 2 hours at 120+/-5 celsius degree before SMT process
PCBs made between 6-12 months ago, must be baked 4 hours at 120+/-5 celsius degree before SMT process
PCBs baked must be used within 5 days, re-baking one hour is required if not fully used one time
PCBs made over one year ago, must be baked 4 hours at 120+/-5 celsius degree and solder coated ago at PCB manufacturer before SMT process
Factors
The storing time and baking temperature is not only subjected to PCB factory manufacturing capability and technology, also to the regions.
For PCBs made with OSP and ENIG surface finishing technology, can be stored properly 6 months after vacuum sealed. For PCBs made with OSP surface finishing do not suggest baking.
In the south region of China, we require the PCBs be used in full within 24hours (ideally in 8 hours) after unpacking due to the high humidity level in the air. For the PCBs needed baking treatment, longer time is needed too. While in the North area, with relatively dry weather, PCBs can be stored in good condition for longer period and baking time is shorter than in the South.