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WorkingBear has noticed that many engineers or managers involved in Surface Mount Technology (SMT) have a strong passion for “PCB baking,” but their understanding of the concept might not be very clear. If you browse through discussions on PCB and SMT related forums, you’ll often find people treating “PCB baking” as a magical solution to struggle the soldering and quality issues. They believe that pre-baking PCBs before SMT can improve solderability, wetting performance, and the height of solder fillet. But is that really the case?
WorkingBear wants to remind those with this viewpoint that the primary purpose of “PCB baking” is to remove moisture and prevent the PCB from popcorn effect and de-lamination during reflow. However, if PCBs are baked for too long, it can lead to premature oxidation of the metal surface finished and result in poor solder wetting, achieving the opposite effect.
When it comes to PCB surface finishes like OSP (Organic Solderability Preservative) coating, WorkingBear don’t recommend pre-baking them before soldering. OSP is made of organic materials, and high temperatures can damage it, causing it to shrink and curl. Baking OSP-coated PCBs at high temperatures can lead to cracks in the coating, exposing the copper underneath. Once the copper is exposed, it starts to oxidize, which makes soldering difficult.
For PCBs with ENIG (Electroless Nickel Immersion Gold) surface finished, the gold layer usually protects the nickel layer below it from quick oxidation. However, since gold is expensive, the ENIG coating is often thin. Pre-baking ENIG-coated PCBs before soldering may not speed up nickel oxidation, but it can accelerate the diffusion process between the nickel and gold layers. Diffusion means that gold and nickel atoms can penetrate each other’s layers. Although this process happens naturally at room temperature, heating can make it faster. If the nickel diffuses to the surface and gets exposed to air, it will oxidize, affecting the soldering process. Generally, baking ENIG PCBs doesn’t have a significant impact on solderability, but it’s still not recommended to expose them to high temperatures for too long.
For PCBs with HASL (Hot Air Solder Leveling) or ImSn (Immersion Tin) surface finished, the Intermetallic Compound (IMC) layer forms early in the PCB manufacturing process, even before reflow soldering. Baking these PCBs at high temperatures can make the already formed IMC layer thicker and may change it from a good-quality chemical compound of Cu6Sn5 to a weaker one of Cu3Sn. A thick IMC layer is not good for strong solder joints; it’s like having too much cement between bricks, making it fragile and prone to breaking.
So, the idea that pre-baking PCBs before SMT assembly helps soldering is not true. Pre-baking PCBs does remove moisture and prevent PCB from popcorn effect and de-lamination , but it won’t improve soldering and may even harm it.
If your components or PCBs show only minor oxidation, and you’ve ensured that they are moisture-free, you could consider slightly increasing the reflow temperature. However, be careful to avoid any quality problems caused by higher temperatures. Metal oxidation creates a barrier, but enough heat can still form the Intermetallic Compound. Another option is using flux with better deoxidation properties. Just remember that stronger deoxidation often means more acidity (lower pH value). If you try using a bathroom cleaner on oxidized surfaces and then solder, it may work initially, but over time, the cleaned areas may corrode.
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