Secondly, the purpose of baking PCB before drilling is actually for dehumidification. The baking before PCB drilling is in order to remove the moisture in the board and reduce the internal stress; PCB during solder mask and silkscreen process also need baking. Before vacuum packaging, there is also a process of compressing and baking, which is also to remove moisture, and the warpage of PCB can be greatly improved.
The baking mentioned above can eliminate the internal stress of the PCB board, which is to stabilize the size of the PCB. Its most significant advantage is that it can dry the moisture in the pad after baking, strengthen the welding effect, and reduce the pseudo welding and repair rate. However, baking will cause change in the color of the PCB board, which will affect the appearance.
Usually, the baking temperature is 100-120°C, and the baking time is about 2H. Please pay attention to don't bake for too long! If PCB is exposed to the air, it needs to be used up within a day, otherwise oxidation is prone to occur. Of course, this is not absolute. It also depends on the PCB supplier's production capacity. Some ENIG, HASL PCBs have a relatively longer storage time. The following will give you the PCB baking instructions for your reference:
1. If the PCB is unpacked for more than 5 days within 2 months of the manufacturing date, please bake it at 120 ±5°C for 1 hour before SMT assembly;
2. If the PCB is over 2 months from the manufacturing date, please bake it at 120 ±5°C for 1 hour before SMT assembly;
3. If the PCB is 2 to 6 months from the manufacturing date, please bake it at 120 ±5°C for 2 hours before SMT assembly;
4. If the PCB is over 6 months to 1 year from the manufacturing date, please bake it at 120 ±5°C for 4 hours before SMT assembly;
5. The baked PCB must be used up within 5 days (put into IR reflow), and if can not be used up within 5days, the remaining PCBs will need to be baked for another hour before SMT assembly;
6. If the PCB is more than one year from the manufacturing date, please bake it at 120 ±5°C for 4 hours before SMT assembly. Better is send it to the PCB factory to re-make surface treatment before SMT assembly.