The following is an introduction to PCBA technology: What are Moisture Sensitive Devices?
Moisture Sensitive Devices (MSDs) refer to electronic components that are particularly sensitive to moisture. “Moisture” is often referred to as “humidity,” which indicates the amount of “water vapor” present in the atmosphere surrounding the components. This environment typically refers to the area near the floor or the surroundings the parts encounter after being removed from vacuum-sealed moisture-proof packaging.
It is important to note that liquid or solid water should not be considered when measuring humidity! Humans can perceive humidity; for instance, in a high-humidity environment, one may feel a sticky sensation before it rains, as if the entire body is damp.
How does humidity impact electronic components? The molecular clusters of water vapor are smaller than those of liquid or solid water, allowing them to penetrate the gaps in humidity-sensitive devices. If these components, which contain moisture, are subjected to the heat of a reflow oven, one can imagine the consequences. Many of us have boiled water and witnessed how steam can lift a pot lid. This illustrates the influence of heat on water vapor, which expands rapidly when heated, generating significant pressure. Historically, steam-powered trains relied on steam to drive their locomotion, underscoring the immense power of steam.
Back to the topic, if the interior of electronic components contains water vapor and is rapidly heated beyond the boiling point of water, the typical lead-free reflow temperature can reach approximately 250˚C. This depends on the humidity levels within the electronic components. Before the heated water vapor can escape through any gaps in the components, the most severe outcome may resemble an explosion from within. A less severe outcome could compromise the internal structure, while more severe cases may result in delamination or cracking.
Given the potential dangers that water vapor poses to electronic components, is it necessary to control moisture levels? Absolutely. Therefore, industrial standards (IPC) include guidelines for moisture-sensitive devices (MSD) and define moisture sensitivity levels (Moisture Sensitive Levels).
Is it true that all electronic components undergoing reflow are at risk of damage from water vapor? If we address just the “popcorn” issue, Shenzhen Grand Power would argue that this is not entirely the case. As previously mentioned, there must be gaps in electronic components for water vapor to penetrate, and these gaps should not be excessively large. If the gaps are sufficiently large, the expanding water vapor can escape smoothly, mitigating the risk. Consequently, only components with small gaps are likely to encounter problems.
Which components have small gaps? Packaged parts, particularly ICs, are the primary concern. ICs are typically enclosed using upper and lower molds or stacked layers, creating joints where gaps can form. Generally, when we refer to humidity-sensitive components, we are discussing these IC parts. BGA components may be even more problematic since the chip must be mounted on the PCB surface, leading to issues related to the coefficient of expansion.
Furthermore, most ICs utilize gold or copper wires for signal transmission. These small wires cannot withstand the force generated by moisture expansion. Engineers at Shenzhen Grand Power have previously noted that many ICs contain holes due to packaging process constraints. If moisture is not adequately controlled, it can easily accumulate in these holes. When moisture expands rapidly, it can lead to significant issues.
Currently, the PCB industry defines “moisture-sensitive parts” primarily in terms of delamination and “popcorn” effects on packaged components. There are two main industrial standards regulating humidity-sensitive components and their sensitivity levels: Classification of humidity/reflow sensitivity of non-hermetic solid-phase surface mount components. This standard helps PCB manufacturers evaluate and classify their components based on moisture sensitivity levels. It essentially defines the test humidity for different sensitivity levels and exposure conditions in the workshop, which are then verified in the reflow soldering furnace.
The second standard outlines how moisture-sensitive components should be handled—transported, stored, and packaged—to prevent dampness, which could compromise reliability after reflow soldering. It also specifies baking conditions to restore moisture-sensitive components to a dry state.
With this information, you should have a clearer understanding of “moisture-sensitive parts.” However, you might wonder why many components, beyond just ICs, often require vacuum drying packaging for humidity control. The reason is that moisture not only contributes to delamination from thermal expansion but also accelerates the oxidation of metal coatings on component leads, leading to issues such as solder rejection. Additionally, certain plastic materials, like PA (nylon), can absorb moisture, resulting in problems like embrittlement, deformation, and discoloration when subjected to high temperatures.
Due to the lack of uniform humidity control conditions for these electronic components, they cannot be regulated by a single standard. Instead, their moisture-proof requirements must be defined on a case-by-case basis. For most current electronic components, aside from the risks associated with ICs, the potential for explosion and delamination extends to other parts, which may vary individually.
