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

In addition, PCBS also have the opportunity to absorb moisture into the environment after they are produced and displayed for a period of time, and water is one of the main culprits of popcorn or delamination. When PCB is placed in an environment with temperature above 100℃, such as backweld furnace, wave furnace, hot air formation or hand welding process, water will turn into steam and 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 in time, 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 layers of PCB, sometimes it may cause the separation between layers of PCB, even the appearance of PCB can be seen bubblings, expansion, burst board and other phenomena;

Sometimes, even if the PCB does not see the above phenomenon on the surface, it is actually internally damaged. Over time, it will cause the function instability of electrical products, or CAF and other problems, and finally lead to product failure.

The real cause analysis and prevention measures of PCB burst board

In fact, the process of PCB baking is quite troublesome. The original packaging must be removed before it is put into the oven, and then the temperature should be over 100℃, but the temperature should not be too high, so that the PCB will burst due to excessive expansion of water vapor during baking.

In general, the PCB baking temperature is usually set at 120±5℃ in the industry to ensure that moisture can really be eliminated from the PCB body before the SMT line can be welded through the back welding furnace.

The baking time varies with the thickness and size of PCB, and for PCB with relatively thin or large size, the board should be pressed with heavy weight after baking, in order to reduce or avoid the tragedy of PCB bending deformation caused by stress release during PCB cooling after baking.

Because once the PCB is deformed and bent, the problem of offset or uneven thickness will occur when the SOLDER paste is printed on SMT, which will lead to a large number of welding short circuit or empty welding and other adverse events.

PCB baking condition setting

At present, the general conditions and time Settings for PCB baking are as follows:

1. PCB shall be well sealed within 2 months from the date of manufacture. If the PCB is unsealed and placed in a temperature-humidity controlled environment (≦30℃/60%RH, according to IPC-1601) for more than 5 days, it shall be baked at 120±5℃ for 1 hour before being put on line.

2. PCB shall be stored for more than 2 ~ 6 months after the manufacturing date, and shall be baked for 2 hours at 120±5℃ before on-line.

3. PCB should be stored for more than 6 ~ 12 months, and roasted for 4 hours at 120±5℃ before it goes online.

4, PCB storage more than 12 months of manufacturing date, basically not recommended to use, because the adhesive force of multilayer board but will be aging with time, product function instability and other quality problems may occur in the future, increase the probability of market repair, and the production process has board explosion and tin eating poor risk. If you have to use, it is recommended to bake at 120±5℃ for 6 hours, a large number of pre-print solder paste into production to ensure 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 failure, 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.

5. All baked PCBS must be used within 5 days, and unprocessed PCBS must be baked at 120±5℃ for another 1 hour before going online.

Stacking of PCB during baking

1. Large size PCB should be placed horizontally and stacked when baking. It is recommended that the maximum number of a stack should not exceed 30 pieces. Vertical baking is not recommended for large size PCB, easy to bend.

2. When baking small and medium-sized PCB, it can be placed horizontally and stacked, the maximum number of a stack is not more than 40 pieces, or it can be used vertically and the number is not limited. After 10 minutes of baking, it is necessary to open the oven and take out the PCB and lay it horizontally to cool it.

Notes for PCB baking

1. The baking temperature shall not exceed the Tg point of PCB, and generally shall not exceed 125℃. In the early stage, the Tg point of some PCB containing lead was relatively low, but now the Tg of most PCB without lead is above 150℃.

2, after baking PCB should be used as soon as possible, if not used, should be re-vacuum packaging as soon as possible. If exposed to the workshop for too long, it must be re-baked.

3, the oven remember to install exhaust drying equipment, otherwise the baked water vapor will be retained in the oven to increase its relative humidity, adverse PCB dehumidification.

4. From the point of view of quality, the fresher the PCB solder is, the better the quality will be after passing through the furnace. The expired PCB will have certain quality risk even if it is used after baking.

Suggestions for PCB baking

1. It is recommended to bake PCB at 105±5℃ as long as the boiling point of water is 100℃. As long as the boiling point is exceeded, water will be turned into steam. Because PCBS don’t contain too many water molecules, they don’t need high temperatures to increase the speed of gasification.

The temperature is too high or the gasification speed is too fast, but it is easy to make the rapid expansion of water vapor, which is actually bad for the quality, especially for the multi-layer board and PCB with buried holes. 105℃ is just higher than the boiling point of water, and the temperature is not too high, which can dehumidify and reduce the risk of oxidation. And today’s oven temperature control ability has been much better than before.

2, WHETHER the PCB needs to be baked, should see whether the packaging is damp, that is, to observe the VACUUM packaging of the HIC (Humidity Indicator Card, Humidity Indicator Card) has shown damp, if the packaging is good, HIC does not indicate damp is actually can be directly online without baking.

3. It is recommended to use “upright” and spaced baking for PCB baking, because only in this way can hot air convection achieve the maximum effect, and water vapor is easier to be baked out of PCB. However, for large size PCBS, it may be necessary to consider whether the vertical type will cause plate bending.

4. It is recommended that PCB be placed in a dry place and cooled quickly after baking. It is best to press the “anti-plate bending fixture” on the top of the board, because the general object is easy to absorb moisture from the hot state to the cooling process, but the rapid cooling may cause the board to bend, which needs to achieve a balance.

Disadvantages of PCB baking and matters needing consideration

1. Baking will accelerate the oxidation of PCB surface coating, and the higher the temperature, the longer the baking is more unfavorable. 2, it is not recommended to do high temperature baking on OSP surface treated board, because OSP film will degrade or fail due to high temperature. If you have to bake, it is recommended to bake at 105±5℃ for no more than 2 hours. It is recommended to use up within 24 hours after baking.

3, baking may affect the generation of IMC, especially for HASL (tin spraying), ImSn (chemical tin, tin dipping) surface treatment of the board, because its IMC layer (copper tin compound) actually as early as in the PCB stage has been generated, that is, before the GENERATION of PCB solder, baking will increase the thickness of this layer has been generated IMC, Cause trust problems.