What is PCB Baking in SMT Line | How to Bake PCB to High Quality

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Wed, Feb 24, 2021

Question
What is PCB baking? Before we soldering, why do we need to bake the boards? and how to bake it will be good.

PCB baking is a process before the boards are arranged in the SMT part. This blog will explain why the PCB (printed circuit boards) need to bake and how can we bake it to a high-quality level.


pcb baking machine.jpg


Part 1: Why PCB Popcorn Appears and How to Solve it

The procedure for PCB baking is actually quite troublesome.

When baking, the original packaging must be removed before it can be put into the oven and then baked at a temperature of more than 100℃, but the temperature should not be too high to cause excessive expansion of PCB.

The general industry for PCB baking temperature is mostly set at 120±5℃ conditions to ensure that the moisture can really be eliminated from the PCB before the SMT soldering. Baking time varies with PCB thickness and size.

The thinner or larger size PCB also has to press the board with heavy objects after baking, in order to reduce or avoid PCB bending deformation caused by stress relief during cooling after baking.

Once the PCB is deformed and bent, there will be offset or uneven thickness in the SMT of tin paste, which will lead to a large number of welding short circuit or empty welding in the back soldering.

Part 2: The Setting of PCB Baking Time

1. When a PCB is within 2 months of manufacturing date and sealed well, Unseal and place in a temperature and humidity controlled environment (≦30℃/60% RH).

And according to IPC-1601) for more than five days, Roast 120±5℃ for 1 hour before going online.

2. PCB storage exceeds the manufacturing date by 2~6 months, Roast for 2 hours at 120±5℃ before going online.

3. PCB storage exceeds the manufacturing date by 6~12 months, Roast 120±5℃ for 4 hours before going online.

4. PCB stored more than 12 months after the date of manufacture, Basically not recommended, Because the bonding force of the multilayer will age over time. There may be quality problems such as product instability in the future and increase the chance of market rework, and the production process also has the risk of PCB baking and bad tin waste.

5. All baking PCBs must be used within 5 days, and the unprocessed PCB must be re-baked for 1 hour at 120±5℃ before going SMT production line.

Part 3: The precautions of PCB Baking

1. We recommend using a temperature of 105±5℃ to bake PCB because the boiling point of water is 100℃.

The PCB does not contain too many water molecules, it does not require too high a temperature to increase its gasification rate.

If the temperature is too high or the gasification rate is too fast, it is easy to make the water vapor expand rapidly, which is unfavorable to the quality and quality, especially for the multilayer board and the PCB with buried holes, 105℃ is just above the boiling point of water, and the temperature is not too high.

Not only dehumidification but also reduce the risk of oxidation.

2. Measuring PCB whether need baking or not we can observe its packaging to check if it’s damp, that is, whether the HIC in its vacuum packaging (Humidity indicator Card) has shown moisture, and if the packaging is good, HIC does not indicate that moisture can be directly online without baking.

humidity indicator card.jpg

3. When baking PCB, it is recommended to use "vertical" and interval baking, because this can maximize the effect of hot air convection, and the moisture is easier to be baked out of the PCB. However, for large size PCB, it may be necessary to consider whether the vertical type will cause bending deformation.

4. After baking, we suggest that the PCB be placed in a dry place and let cool quickly. It will be better if can press the “anti-bending fixture” on the top of the board because the process from high heat to cooling is easy to absorb water and gas. But rapid cooling can cause bending.

Part 4: Disadvantage of PCB Baking and Suggestions

1. Baking will accelerate the oxidation of PCB surface coating, and the higher the temperature, the longer the baking is more unfavorable.

2. we don't recommend baking OSP surfaces at high temperatures, Because OSP film will degrade or fail because of high temperature.

If you have to bake, Recommended temperature of 105±5℃, No more than two hours, It is recommended to use up within 24 hours after baking.

PCB baking may affect IMC generation, particularly for HASL( tin spray), ImSn( chemical tin, dip tin) surface treatment panels, Since its IMC layer (Cu - tin compound) was already formed as early as the PCB stage, Before PCB the solder, baking increases the thickness of the IMC, Cause reliability problems.

Conclusion

This article will help engineers, customers, or students who are interested in the PCB production process.

PCBONLINE will make a point of all processes except PCB baking checking, surely we always control the PCB quality in the process of PCB baking strictly, because PCBA production is a complicated proceeding, the high standard of PCB baking can lay a foundation for the subsequent installation of components.