Do you know the PCB baking stacking method? What are the precautions for PCB baking?

PCBs are either single-layer or double-layer. If the PCB size is too large and the printed lines are long, the impedance will increase. In order to enhance everyone’s understanding of PCB, this article will introduce PCB, PCB baking stacking methods, PCB baking precautions, and PCB baking suggestions.

1. PCB

PCB (printed circuit board), also known as printed circuit board, is one of the important components of the electronics industry. Almost every electronic device, ranging from electronic watches and calculators to computers, communication electronic equipment, and military weapon systems, as long as there are electronic components such as integrated circuits, printed circuit boards are used to electrically interconnect the components. plate. The printed circuit board is composed of an insulating base plate, connecting wires and pads for assembling and welding electronic components. It has the dual functions of conductive lines and insulating base plates. It can replace complex wiring and realize electrical connections between components in the circuit. It not only simplifies the assembly and welding work of electronic products, reduces the wiring workload in traditional ways, and greatly reduces the labor intensity of workers; it also reduces the size of the entire machine. volume, reduce product costs, and improve the quality and reliability of electronic equipment. Printed circuit boards have good product consistency and can adopt standardized designs, which is conducive to mechanization and automation in the production process. At the same time, the entire printed circuit board that has been assembled and debugged can be used as an independent spare part to facilitate the interchange and maintenance of the entire machine. At present, printed circuit boards have been extremely widely used in the production and manufacturing of electronic products.

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2. PCB baking

(1) Stacking method during PCB baking

a. When baking large-size PCBs, lay them flat and stack them. It is recommended that the maximum number of pieces in a stack should not exceed 30 pieces. Within 10 minutes after baking, you need to open the oven, take out the PCBs, and lay them flat to cool down. After baking, you need to press the anti-board Bend jig. It is not recommended to bake large-size PCBs in an upright position as they are prone to bending.

b. When baking small and medium-sized PCBs, they can be laid flat and stacked. The maximum number of pieces in a stack is recommended not to exceed 40 pieces. They can also be placed upright, with no limit on quantity. The oven must be opened within 10 minutes after baking is completed, and the PCBs must be taken out and laid flat. Let it cool, and then press the anti-bending jig after baking.

(2) Precautions when baking PCB

1. The baking temperature cannot exceed the Tg point of the PCB, and the general requirement is that it cannot exceed 125°C. In the early days, the Tg point of some lead-containing PCBs was relatively low. Nowadays, the Tg points of lead-free PCBs are mostly above 150°C.

2. The baked PCB should be used as soon as possible. If it is not used, it should be re-vacuum packed as soon as possible. If exposed to the workshop for too long, it must be rebaked.

3. Remember to install exhaust drying equipment in the oven, otherwise the water vapor will remain in the oven and increase its relative humidity, which is not conducive to PCB dehumidification.

4. From a quality point of view, the fresher the PCB solder is used, the better the quality will be after it has been baked. Expired PCBs will still have certain quality risks even if they are baked before use.

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(3) Suggestions for PCB baking

a. It is recommended that you only use a temperature of 105±5℃ to bake PCB, because the boiling point of water is 100℃. As long as it exceeds its boiling point, water will turn into water vapor. Because PCB does not contain too many water molecules, it does not require too high a temperature to increase its vaporization speed.

If the temperature is too high or the vaporization speed is too fast, it will easily cause the water vapor to expand rapidly, which is actually detrimental to the quality, especially for multi-layer boards and PCBs with buried holes. 105°C is just above the boiling point of water, and the temperature is not too high. , can dehumidify and reduce the risk of oxidation. Moreover, the temperature control capabilities of current ovens have been much improved than before.

b. Whether the PCB needs to be baked depends on whether its packaging is affected by moisture, that is, whether the HIC (Humidity Indicator Card) in the vacuum package has shown that it has been affected by moisture. If the packaging is good, the HIC does not indicate that it is affected by moisture. It can be put online directly without baking.

c. When baking PCB, it is recommended to use “upright” baking with intervals, because this can maximize the effect of hot air convection, and it is easier for moisture to be baked out of the PCB. However, for large-size PCBs, you may have to consider whether the upright configuration will cause board bending and deformation problems.

d. After the PCB is baked, it is recommended to place it in a dry place and allow it to cool quickly. It is best to press an “anti-board bending jig” on the top of the board, because general objects tend to absorb moisture during the process from a high-heat state to cooling. , but rapid cooling may cause plate bending, which requires a balance.

(4) Disadvantages of PCB baking and things to consider

1. Baking will accelerate the oxidation of the PCB surface coating, and the higher the temperature and the longer the baking time, the more disadvantageous it will be.

2. It is not recommended to bake OSP surface-treated boards at high temperatures because the OSP film will degrade or fail due to high temperatures. If baking is necessary, it is recommended to bake at a temperature of 105±5℃ for no more than 2 hours. It is recommended to use it within 24 hours after baking.

3. Baking may have an impact on the formation of IMC, especially for boards with surface treatment of HASL (spray tin) and ImSn (chemical tin, immersion tin plating), because the IMC layer (copper-tin compound) has actually been formed as early as the PCB stage. Generation, that is, it has been generated before PCB soldering. Baking will increase the thickness of this layer of generated IMC, causing reliability problems.

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