Prevent PCB from expiration and how to deal with it after it expires

Do you know “Why PCB must be baked before SMT reflow oven after it expires beyond the shelf life”?

The main purpose of PCB baking is to remove moisture and remove moisture contained in the PCB or absorbed from the outside, because some PCB materials themselves are easy to form water molecules.

In addition, after the PCB is produced and placed for a period of time, it has the opportunity to absorb moisture in the environment, and water is one of the main culprits causing PCB popcorn or delamination. Because when the PCB is placed in an environment with a temperature exceeding 100℃, such as reflow oven, wave soldering oven, hot air leveling or hand soldering, water will turn into water vapor and then expand its volume rapidly.

The faster the heating speed of the PCB, the faster the water vapor will expand; the higher the temperature, the larger the volume of the water vapor; when the water vapor cannot escape from the PCB immediately, it is very likely to expand the PCB.

The Z direction of the PCB is especially vulnerable.

Sometimes the vias between the layers of the PCB may be pulled apart, and sometimes the layers of the PCB may be separated. In more serious cases, even the PCB surface can be seen to be bubbling, expanding, and bursting.

Sometimes, even if the above phenomena cannot be seen on the surface of the PCB, it is actually already damaged. As time goes by, it will cause the function of the electrical product to be unstable, or CAF and other problems will occur, eventually causing product failure.

Analysis of the true cause of PCB bursting and prevention measures

The PCB baking procedure is actually quite troublesome. When baking, the original packaging must be removed before it can be placed in the oven, and then it must be baked at a temperature of more than 100℃, but the temperature cannot be too high, so as not to cause excessive expansion of water vapor during baking and burst the PCB.
In general, the industry sets the temperature for PCB baking at 120±5℃ to ensure that the water vapor can be eliminated from the PCB body before it can be put on the SMT line and soldered in the reflow oven.

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

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

Setting the conditions for PCB baking

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

1. If the PCB is within 2 months of the manufacturing date and is well sealed, and is placed in a temperature and humidity controlled environment (≦30℃/60%RH, according to IPC-1601) for more than 5 days after unpacking, it needs to be baked at 120±5℃ for 1 hour before going online.
2. If the PCB is stored for more than 2 to 6 months after the manufacturing date, it needs to be baked at 120±5℃ for 2 hours before going online.
3. If the PCB is stored for more than 6 to 12 months after the manufacturing date, it needs to be baked at 120±5℃ for 4 hours before going online.
4. If the PCB is stored for more than 12 months after the manufacturing date, it is basically not recommended to use it, because the bonding strength of the multilayer board will age over time, and quality problems such as unstable product functions may occur in the future, increasing the probability of market repairs, and there are risks such as board explosion and poor tin absorption during the production process. If it is not allowed to be used, it is recommended to bake it at 120±5℃ for 6 hours first, and test print solder paste before mass production to make sure there is no solderability problem before continuing production.
   Another reason why it is not recommended to use PCBs that have been stored for too long is that their surface treatment will gradually fail over time. For ENIG, the industry’s shelf life is 12 months. After this period, it depends on the thickness of the immersion gold layer. If the thickness is thinner, the nickel layer may appear in the gold layer due to diffusion and form oxidation, affecting the reliability, so be careful.
5. All PCBs that have been baked must be used within 5 days. PCBs that have not been processed must be baked again at 120±5℃ for another hour before they are put online.

Stacking method when baking PCBs

1. When baking large-size PCBs, they should be placed flat and stacked. It is recommended that the maximum number of PCBs in a stack should not exceed 30. The oven should be opened within 10 minutes after baking to take out the PCBs and lay them flat to cool them down. After baking, the anti-bending jig should be pressed. It is not recommended to bake large-size PCBs upright, as they are prone to bending.
2. When baking small and medium-sized PCBs, they can be placed flat and stacked. The maximum number of PCBs in a stack should not exceed 40. They can also be placed upright with no limit on the number. The oven should be opened within 10 minutes after baking to take out the PCBs and lay them flat to cool them down. After baking, the anti-bending jig should be pressed.

Precautions when baking PCBs

1. The baking temperature should not exceed the Tg point of the PCB. Generally, it should not exceed 125℃. In the early days, the Tg point of some lead-containing PCBs was relatively low, but now the Tg of lead-free PCBs is mostly above 150℃.
2. The baked PCB should be used up as soon as possible. If it is not used up, it should be vacuum-packed again as soon as possible. If it is exposed to the workshop for too long, it must be re-baked.
3. Remember to install exhaust drying equipment in the oven, otherwise the water vapor baked out will remain in the oven to increase its relative humidity, which is not conducive to PCB dehumidification.
4. From the perspective of quality, the fresher the PCB solder is, the better the quality will be after passing through the oven. Even if the expired PCB is used after baking, there will still be certain quality risks.

Suggestions for PCB baking

1. It is recommended to use a temperature of 105±5℃ to bake the PCB, because the boiling point of water is 100℃. As long as it exceeds its boiling point, the water will turn into water vapor. Because there are not too many water molecules in the PCB, it does not require too high a temperature to increase its vaporization rate.
   Too high a temperature or too fast a vaporization speed will easily cause water vapor to expand rapidly, which is actually not good for quality, especially for multi-layer boards and PCBs with buried holes. 105℃ is just above the boiling point of water, and the temperature is not too high, which can dehumidify and reduce the risk of oxidation. Moreover, the current oven temperature control ability has been improved a lot compared with the past.
2. Whether the PCB needs to be baked should be determined by whether its packaging is damp, that is, to observe whether the HIC (Humidity Indicator Card) in its vacuum packaging has shown dampness. If the packaging is good and the HIC does not indicate dampness, it can actually be put online without baking.
3. When baking the PCB, it is recommended to use “vertical” and interval baking, because this can achieve the maximum effect of hot air convection, and water vapor is easier to be baked out of the PCB. However, for large-sized PCBs, it may be necessary to consider whether the vertical type will cause board bending and deformation.
4. After baking, it is recommended to place the PCB in a dry place and cool it quickly. It is best to press a “board bending prevention fixture” on the top of the board, because the process of cooling from a high-heat state to a general object is easy to absorb moisture, but rapid cooling may cause board bending, which requires a balance.

wWDisadvantages of PCB baking and matters that need to be considered

1. Baking will accelerate the oxidation of the PCB surface coating, and the higher the temperature and the longer the baking, the more disadvantageous it is.
2. It is not recommended to bake the board with OSP surface treatment at high temperature, because the OSP film will degrade or fail due to high temperature. If baking is necessary, it is recommended to use a temperature of 105±5℃ for no more than 2 hours, and it is recommended to use it within 24 hours after baking.
3. Baking may affect the formation of IMC, especially for boards with HASL (spray tin) and ImSn (chemical tin, immersion tin) surface treatment, because its IMC layer (copper-tin compound) has actually been generated as early as the PCB stage, that is, it has been generated before the PCB soldering. Baking will increase the thickness of this layer of
4. generated IMC, causing reliability problems.