Do you know why PCBs must undergo baking before passing through the reflow furnace after exceeding their shelf life? The primary objective of PCB baking is to eliminate moisture, especially water vapor present in or absorbed from the environment, as certain PCB materials are prone to forming water molecules. PCBs, over time, may also absorb moisture from the surroundings. Moisture is a significant factor contributing to PCB delamination or popping.
When PCBs are exposed to temperatures exceeding 100°C, such as in reflow and wave soldering processes, water turns into vapor, rapidly expanding its volume. The faster the heat application, the quicker the steam expansion, with higher temperatures resulting in greater water vapor volume. If the vapor cannot escape immediately, it poses a risk of bulging the PCB, particularly in the vulnerable Z direction.
This can lead to VIA breakage, layer separation, or visible issues like blistering and plate bursting. Even without apparent signs, internal damage may occur, causing instability in electrical products, CAF issues, and eventual product failure. The PCB baking process involves removing the original packaging before placing it in the oven.
The baking temperature typically exceeds 100°C, with a common industry standard of 120±5°C to ensure effective moisture elimination. Baking time varies based on PCB thickness and size, and for larger or thinner PCBs, weight should be applied after baking to prevent bending.
PCB storage conditions determine the baking requirements: – If sealed within 2 months of the manufacturing date and stored in a controlled environment for more than 5 days, bake at 120±5°C for 1 hour before going online. – For storage between 2 to 6 months, bake for 2 hours. – For storage between 6 to 12 months, bake for 4 hours. – PCBs stored for over 12 months are not recommended due to aging adhesive forces in multilayer boards. If usage is unavoidable, bake for 6 hours at 120±5°C, conduct pre-production trials, and continue production only if solder-related issues are absent.
ENIG-treated PCBs have a shelf life of 12 months, after which surface treatment may fail, affecting reliability. All baked PCBs must be used within 5 days, and unprocessed ones should undergo an additional 1-hour bake at 120±5°C before use. When baking large PCBs, stack them flat with a recommended maximum of 30 pieces per stack to prevent bending. Small to medium-sized PCBs can be stacked flat (max 40 pieces per stack) or upright without quantity restrictions. After 10 minutes of baking, open the oven to cool down the PCBs.
