How To Deal With The Expired PCB Correctly

Do you know “why PCB must be baked before SMT is passed through the resolder furnace after it has expired beyond the shelf life”?

The main purpose of PCB baking is to remove moisture and moisture and remove the water vapor contained in or absorbed from the outside world because the material used in some PCB itself is easy to form water molecules.

In addition, after PCB is produced and placed for a period of time, it will also have the chance to absorb the water vapor in the environment. And water is one of the main killers of PCB popping or delamination. This is because when the PCB is placed in an environment with a temperature over 100℃, such as the backward furnace, wave welding furnace, hot air flatting, or hand welding process, the water will turn into water vapor, and then rapidly expand its volume.

The faster the heat is applied to the PCB, the faster the steam expansion will be. The higher the temperature, the greater the volume of water vapor; When the water vapor cannot escape from the PCB immediately, there is a good chance of bulging the PCB.

In particular, the Z direction of PCB is the most vulnerable. Sometimes the VIA between layers of PCB may be broken, and sometimes the separation between layers of PCB may be caused. Even the appearance of PCB can be seen as blistering, expansion, plate bursting, and other phenomena.

Sometimes, even if the PCB can not see the above phenomenon, but in fact, it has an internal injury, over time, but will cause the function of electrical products is unstable, or CAF and other problems, and eventually cause the product failure.

Analysis of the real cause of PCB bursting plate and countermeasures

The baking procedure of PCB is actually quite troublesome. During baking, the original package must be removed before it is put into the oven. Then, the temperature of baking should be over 100℃, but the temperature should not be too high so that the excessive expansion of water vapor during baking will explode the PCB instead.

Generally, the baking temperature of PCB is mostly set at 120±5℃ in the industry to ensure that the water vapor can really be eliminated from the PCB body before the SMT line plate is welded through the backward furnace.

Baking time varies with the thickness and size of PCB, and for thinner or larger PCB, heavyweight should be used to press the board after baking, in order to reduce or avoid PCB bending deformation caused by stress release during baking and cooling.

Because once the PCB is deformed and bent, there will be a deviation or uneven thickness problem when SMT printing solder paste, which will lead to a large number of welding short circuit or empty welding and other adverse events in the back welding.

PCB baking condition setting

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

If the PCB is sealed well within 2 months of the manufacturing date, and placed in a 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.
PCB storage more than the manufacturing date of 2 ~ 6 months, before the online need to bake at 120±5℃ for 2 hours.
PCB storage over the manufacturing date of 6 ~ 12 months, before the online need to bake at 120±5℃ for 4 hours.
If the PCB is stored for more than 12 months, it is basically not recommended to use it, because the adhesive force of the multi-layer board will age with time. In the future, there may be quality problems such as unstable product function, which will increase the chance of market repair. If you have to use it, it is recommended to bake for 6 hours at 120±5℃ first, a large number of pre-production trial printing solder paste put into production to determine that there is no solder problem before continuing production. Another is not recommended for too long the PCB because of its surface treatment with time and gradually will also fail, in ENIG, the shelf life is 12 months, after this time, depending on its heavy gold layer thickness, if the thickness thinner, the nickel layer may be because the diffusion and appear in the gold layer and oxide formation, affect the reliability, cannot inadvertent.

All the baked PCBs must be used up within 5 days, and the unprocessed PCB must be baked again at 120±5℃ for another 1 hour before going online.

Stacking mode of PCB during baking

When baking large size PCB, it should be placed flat and stacked. It is recommended that the maximum number of one stack should not be more than 30 pieces. Large size PCB is not recommended to stand baking, easy to bend.
When baking small and medium-sized PCB, it can be placed flat and stacked, the maximum number of one stack is recommended not to exceed 40 pieces, or it can be adopted upright type, the number is not limited. After 10 minutes of baking, it is necessary to open the oven to take out the PCB and let it cool down.