When working with some surface-mount components, particularly Quad Flat No-Lead (QFN) packages, I’ve come across the practice of pre-baking them before soldering onto a prototype PCB. This pre-baking typically involves placing the ICs in an oven for around an hour. Can someone explain the purpose of this process? Is it done to enhance protection against electrostatic discharge (ESD) or is it intended to stimulate the silicon in some way?
During the soldering process, particularly reflow soldering, components are subjected to high temperatures. If moisture is present in the package, it can rapidly vaporize and expand, leading to internal stress. This expansion can cause the package to crack or delaminate, an effect commonly known as the “popcorn effect.” Excess moisture can adversely affect the solderability of components. Moisture can lead to oxidation of leads or pads, which in turn can cause poor solder joints. Components with lower MSL levels might need baking at 125°c for 8 to 12hrs.
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As Steve mentioned, in most cases, components are baked to eliminate any moisture present in their package. Certain components are more susceptible to moisture absorption than others. Once moisture is absorbed, removing it is a time-consuming process, often requiring 24 hours or more in specialized baking machines, some of which operate in a vacuum chamber.
However, for hand-soldered prototypes, there’s typically no need to worry. The component body doesn’t reach temperatures high enough to vaporize internal moisture. Unfortunately, many components that require baking, such as QFNs and BGAs, are not suitable for hand soldering.
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During a recent procurement, I observed a significant improvement in the handling of moisture-sensitive devices by the electronics supplier. The sensitive components were delivered in sealed packaging, accompanied by desiccant packets, moisture-detecting paper, and instructions for baking in case of excessive moisture. I comprehend the rationale behind the gentle baking process prior to reflow soldering in an oven, as it prevents the rapid boiling of trapped water within the component, thus averting damage. However, I would not employ this method for a breadboard.
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I have seen several boards, which had been sitting around for years due to BGA issues, undergo IR reflow using a lead-free (high-temperature) profile with minimal preheat. In these instances, none of these components split or exhibited any damage.
While it is crucial to adhere strictly to the manufacturer’s guidelines for products intended for sensitive applications such as aerospace or medical industries, the same rigor may not be necessary for early engineering prototypes that will remain in-house and are not part of the ISO quality system.
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Certain QFN packages that are hand-soldered, socketed, or placed in solderless breadboards, do not need to be baked. Baking QFN packages is necessary if they will undergo wave or reflow soldering. During reflow soldering, any moisture in the QFN package can turn into steam when heated, potentially damaging the package as the steam expands. Therefore, if you are mounting the parts using reflow soldering or heating them to near soldering temperatures, they should be gently baked beforehand to dry them out.
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If you believe that the only concern with moisture is popcorning during the reflow process, you’re missing a larger issue. More frequently than causing popcorning, moisture can lead to “walking wounded” chips if they aren’t baked before reflow. This results in a higher rate of early failures and a lower overall fit rate for your product.
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