Do you know "why PCB must be baked before SMT can be returned to the welding furnace when the expiration date exceeds the preservation period"?
The main purpose of PCB baking is to remove moisture and moisture, to remove the moisture contained in PCB or absorbed from the outside world, because some PCB itself uses the material is easy to form water molecules.
In addition, PCBS that are produced for a period of time have a chance to absorb moisture into the environment, and water is one of the main culprits for creating popcorn or delamination. Because when the PCB is placed in the temperature of more than 100℃ environment, such as back welding furnace, wave welding furnace, hot air flat or hand welding process, water will turn into water vapor, and then rapidly expand its volume.
The faster the PCB is heated, the faster the water vapor expands. The higher the temperature, the greater the volume of water vapor; When water vapor cannot escape from the PCB immediately, it has a good chance to inflate the PCB.
In particular, the Z direction of PCB is the most vulnerable, sometimes it may break the via between the layers of PCB, and sometimes it may cause the separation between the layers of PCB, and more serious even the appearance of PCB can be seen foaming, expansion, explosion and other phenomena;
Sometimes, even if the PCB can not see the above phenomena on the outside, it has been internally damaged. As time goes by, the function of electrical products will be unstable, or CAF and other problems will occur, and eventually result in product failure.
Analysis of the real cause of PCB board explosion and countermeasures
PCB baking process is actually quite troublesome, baking must be removed after the original packaging in the oven, and then to use more than 100 degrees Celsius temperature to bake, but the temperature can not be too high, lest the excessive expansion of water vapor during the baking instead of PCB to burst.
Generally, the temperature of PCB baking in the industry is mostly set at 120±5℃, so as to ensure that the moisture can really be eliminated from the PCB body, before the SMT line to hit the plate through the back welding furnace welding.
Baking time varies with the thickness and size of PCB, and for relatively thin or large size PCB must be pressed with heavy weights after baking, in order to reduce or avoid PCB bending deformation due to stress release during cooling after baking.
Once the PCB is deformed and bent, there will be deviation or uneven thickness when SMT solder paste is printed, which will cause a lot of welding short circuit or air welding in the back welding.
PCB baking condition setting
At present, the industry generally sets the conditions and time for PCB baking as follows:
1. If the PCB is well sealed within 2 months from the manufacturing date and placed in temperature and humidity controlled environment (≦30℃/60%RH, according to IPC-1601) for more than 5 days after unpacking, it shall be baked at 120±5℃ for 1 hour before going online.
2. PCB stored for 2 ~ 6 months beyond the manufacturing date shall be baked for 2 hours at 120±5℃ before going online.
3. If the PCB has been stored for 6-12 months beyond the manufacturing date, it should be baked for 4 hours at 120±5℃ before going online.
4, PCB stored more than 12 months after the manufacturing date, basically not recommended to use, because the adhesive force of multilayer board will be aging with time, may occur in the future product function instability and other quality problems, increase the probability of market repair, and the production process also has the risk of board explosion and bad tin eating. If not used, it is recommended to bake for 6 hours at 120±5℃ first, a large number of pre-test printing solder paste production several pieces to determine no solder problems before continuing to production.
Another is not recommended to use PCB stored for too long because its surface treatment will gradually fail with the passage of time, to ENIG, the industry preservation period is 12 months, after this aging, depending on the thickness of the gold layer, if the thickness is thin, the nickel layer may appear in the gold layer due to diffusion and oxidation, affecting the reliability, not inadvertently.
5. All baked PCB must be used within 5 days, and unprocessed PCB must be baked at 120±5℃ for 1 hour before going online.
The way PCBS are stacked when baking
1. When large-size PCB is baking, it is recommended that the maximum number of a stack should not exceed 30 pieces. After baking, open the oven to take out PCB and lay it flat to cool it. Large size PCB is not recommended upright baking, easy plate bending.
2, small and medium-sized PCB baking, can use flat stacked, a maximum number of a stack is recommended not to exceed 40 pieces, can also be adopted upright, the number is not limited, after baking 10 minutes to open the oven to take out PCB flat place to cool it, after baking need to pressure anti-plate bending tool.