PCB Baking

This Gold Phoenix PCB Knowledge Center article explains PCB baking conditions, stacking methods, and precautions to help maintain PCB quality before assembly.

PCB Baking Conditions

1. If the PCBs are well sealed within 2 months of the manufacturing date and placed in a temperature‑ and humidity‑controlled environment (≦30℃/60%RH, according to IPC‑1601) after being unpacked for more than 5 days, they should be baked at 120±5℃ for 1 hour before assembly.

2. If the PCBs are more than 2 to 6 months from the manufacturing date, they should be baked at 120±5℃ for 2 hours before assembly.

3. If the PCBs are more than 6 to 12 months from the manufacturing date, they should be baked at 120±5℃ for 4 hours before assembly.

4. If the PCBs are more than 12 months from the manufacturing date, they are generally not recommended for use because the adhesive in multilayer boards will age over time, which may cause product function instability and other quality problems. If they must be used, it is recommended to bake them at 120±5℃ for 6 hours. Before mass production, samples should be assembled and tested to check whether they are qualified.

5. All baked PCBs must be used within 5 days. Any leftover PCBs must be re‑baked at 120±5℃ for 1 hour before assembly.

Stacking of PCBs When Baking

1. When baking large‑size PCBs, it is recommended that the maximum number of PCBs in a stack be no more than 30 pieces. After baking, open the oven, take out the PCBs, place them flat to cool, and press them with an anti‑bending fixture. Upright baking is not recommended for large‑size PCBs because it may cause board bending and warping.

2. When baking small‑ and medium‑sized PCBs, they can be placed flat in stacks, with a maximum of 40 pieces per stack. They can also be placed upright, with no limit on the number. After baking, open the oven, take out the PCBs, place them flat to cool, and press them with an anti‑bending fixture.

Precautions for PCB Baking

1. The baking temperature must not exceed the Tg point of the PCB and generally must not exceed 125℃.

2. Baked PCBs should be used as soon as possible. If they are not used, they need to be re‑vacuum packaged. If the exposure time is too long, they must be re‑baked.

3. The oven should be equipped with ventilation and drying equipment. Otherwise, the water vapor released during baking will remain in the oven and increase the relative humidity, which is not conducive to PCB dehumidification.

4. Whether a PCB needs to be baked depends on whether its packaging is damp. Observe the HIC (Humidity Indicator Card) in the vacuum packaging. If the packaging is in good condition and the HIC does not indicate moisture, the PCB can be used directly for assembly.

5. Baking will accelerate the oxidation of the PCB surface coating, and higher temperatures and longer baking times are more unfavorable.

6. It is not recommended to bake OSP surface boards at high temperature because the OSP film will degrade or fail due to high temperature.

7. Baking may have an impact on the generation of IMC (copper‑tin compound), especially for HASL and ImSn surface boards. Because their IMC layers have already been generated before PCB soldering, baking will increase the thickness of the IMC layer, causing reliability problems.