Will PCB expire? Should I bake it after it expires?

Do you know why PCB must be baked before SMT reflow oven after it expires and exceeds the shelf life?

The main purpose of PCB baking is to remove moisture and remove water vapor 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 water vapor 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 particularly 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 may be blistering, swelling, or cracking.

Sometimes, even if the above phenomena are not visible on the PCB surface, it is actually damaged internally. As time goes by, it will cause the function of the electrical product to be unstable, or CAF problems may occur, eventually causing the product to fail.

1.Analysis of the real cause of PCB cracking and prevention measures

The PCB baking procedure is actually quite troublesome. During 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 to avoid excessive expansion of water vapor during baking and bursting the PCB.
Generally, the industry sets the temperature for PCB baking at 120±5℃ to ensure that the water vapor can really 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, and for thinner or larger PCBs, heavy objects 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.

2.PCB baking condition setting

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 also risks such as board explosion and poor tinning during the production process. If you have to use it, it is recommended to bake it at 120±5℃ for 6 hours. Before mass production, try printing solder paste and put it into production for a few pieces to make sure there is no soldering 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 become ineffective 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 thin, the nickel layer may appear in the gold layer due to diffusion and form oxidation, affecting the reliability, so be careful.
5. All baked PCBs must be used within 5 days, and unfinished PCBs must be re-baked at 120±5℃ for another hour before going online.

3.Stacking method when baking PCB

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 PCB and lay it flat to cool it down. After baking, the board bending prevention fixture should be pressed. It is not recommended to bake large-size PCBs upright, as they are easy to bend.
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 PCB and lay it flat to cool it down. After baking, the board bending prevention fixture should be pressed.
   Precautions when baking PCBs
3. The baking temperature should not exceed the Tg point of the PCB, and it is generally required not to exceed 125℃. The Tg point of some lead-containing PCBs in the early days was relatively low, and the Tg of lead-free PCBs is mostly above 150℃.
4. The PCBs after baking should be used up as soon as possible. If they are not used up, they should be vacuum-packed again as soon as possible. If they are exposed to the workshop for too long, they must be baked again.
5. Remember to install exhaust drying equipment in the oven, otherwise the steam from the oven will remain in the oven and increase its relative humidity, which is not conducive to PCB dehumidification.
6. From the perspective of quality, the fresher the PCB solder, the better the quality after the oven. Expired PCBs will still have certain quality risks even if they are baked before use.

4.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, 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 speed.
   Too high a temperature or too fast a vaporization speed will easily cause the 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 to before.
2. Whether the PCB needs to be baked should depend on whether its packaging is damp, that is, to observe whether the HIC (Humidity Indicator Card) in its vacuum packaging has shown that it is damp. If the packaging is good and the HIC does not indicate dampness, it can actually be put online directly without baking.
3. When baking PCB, it is recommended to use “vertical” and interval baking, because this can achieve the maximum effect of hot air convection, and moisture 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 the “anti-board bending fixture” on the top of the board, because the process of cooling from high heat to cooling of general objects is more likely to absorb moisture, but rapid cooling may cause board bending, which requires a balance.
   Disadvantages of PCB baking and matters that need to be considered
5. 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.
6. Baking may affect the formation of IMC, especially for boards with HASL (spray tin) and ImSn (chemical tin, immersion tin) surface treatment, because the IMC layer (copper-tin compound) is actually 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 IMC, causing reliability problems.