The main purpose of PCB baking is to dehumidify and remove moisture, and to remove the moisture contained in the PCB or absorbed from the outside, because some materials used in the PCB itself easily form water molecules.

In addition, after the PCB is produced and placed for a period of time, there is a chance to absorb moisture in the environment, and water is one of the main killers of PCB popcorn or delamination.

Because when the PCB is placed in an environment where the temperature exceeds 100°C, such as reflow oven, wave soldering oven, hot air leveling or hand soldering, the water will turn into water vapor and then rapidly expand its volume.

When the speed of heating the PCB is faster, the water vapor will expand faster; when the temperature is higher, the volume of water vapor will be larger; when the water vapor cannot escape from the PCB immediately, there is a good chance to expand the PCB .

In particular, the Z direction of the PCB is the most fragile. Sometimes the vias between the layers of the PCB may be broken, and sometimes it may cause the separation of the layers of the PCB. Even more serious, even the appearance of the PCB can be seen. Phenomenon such as blistering, swelling, and explosion;

Sometimes even if the above phenomena are not visible on the outside of the PCB, it is actually internally injured. Over time, it will cause unstable functions of electrical products, or CAF and other problems, and eventually cause product failure.

Analysis of the true cause of PCB explosion and preventive measures
The PCB baking procedure is actually quite troublesome. During baking, the original packaging must be removed before it can be put in the oven, and then the temperature must be over 100℃ for baking, but the temperature should not be too high to avoid the baking period. Excessive expansion of water vapor will burst the PCB.

Generally, the PCB baking temperature in the industry is mostly set at 120±5°C to ensure that the moisture can really be eliminated from the PCB body before it can be soldered on the SMT line to the reflow furnace.

The baking time varies with the thickness and size of the PCB. For thinner or larger PCBs, you have to press the board with a heavy object after baking. This is to reduce or avoid PCB The tragic occurrence of PCB bending deformation due to stress release during cooling after baking.

Because once the PCB is deformed and bent, there will be offset or uneven thickness when printing solder paste in SMT, which will cause a large number of solder short circuits or empty soldering defects during subsequent reflow.

At present, the industry generally sets the conditions and time for PCB baking as follows:

1. The PCB is well sealed within 2 months of the manufacturing date. After unpacking, it is placed in a temperature and humidity controlled environment (≦30℃/60%RH, according to IPC-1601) for more than 5 days before going online. Bake at 120±5℃ for 1 hour.

2. The PCB is stored for 2-6 months beyond the manufacturing date, and it must be baked at 120±5℃ for 2 hours before going online.

3. The PCB is stored for 6-12 months beyond the manufacturing date, and it must be baked at 120±5°C for 4 hours before going online.

4. PCB is stored for more than 12 months from the manufacturing date. Basically, it is not recommended to use it, because the adhesive force of the multilayer board will age over time, and quality problems such as unstable product functions may occur in the future, which will increase the market for repairs. In addition, the production process also has risks such as plate explosion and poor tin eating. If you have to use it, it is recommended to bake it at 120±5°C for 6 hours. Before mass production, first try to print a few pieces of solder paste and make sure that there is no solderability problem before continuing production.

Another reason is that it is not recommended to use PCBs that have been stored for too long because their surface treatment will gradually fail over time. For ENIG, the shelf life of the industry is 12 months. After this time limit, it depends on the gold deposit. The thickness depends on the thickness. If the thickness is thinner, the nickel layer may appear on the gold layer due to diffusion and form oxidation, which affects the reliability.

5. All PCBs that have been baked must be used up within 5 days, and unprocessed PCBs must be baked at 120±5°C for another 1 hour before going online.