Printed circuit board is delicate electronic components that require careful protection against moisture to prevent potential failures. Moisture exposure during production, storage, or transportation can significantly increase the risk of failure. The substrates used in circuit boards can absorb moisture during storage or production, leading to corrosion or electrical shorts caused by condensation inside sealed enclosures and temperature/humidity issues. To ensure effective PCB moisture protection, it is crucial to consider the usage environment during the design phase and select appropriate substrates and conformal coatings for necessary protection. In this article, TechSparks will provide a detailed step-by-step guide on safeguarding circuit boards from moisture, starting from understanding the hazards posed by moisture to the circuitry.
Threat of Moisture to Circuit Board
While there are numerous waterproof electronic products available in everyday life, it is essential to acknowledge that water remains an excellent conductor. It is widely known that pure water at a temperature of 25 degrees Celsius exhibits a resistivity of approximately 1.8 x 10^7 Ω-m, surpassing that of most conductors. When water comes into contact with a circuit board, the presence of current leads to its ionization, resulting in the inclusion of dissolved salts, chlorides, sulfates, and carbonates. These inorganic salts enhance water’s conductivity, thereby leading to common PCB problems such as short circuits and corrosion.
- Short circuit: One of the most serious failure problems. Moisture present on the PCB’s surface infiltrates the gaps between the lead elements or circuits, effectively transforming them into current conductors. As a result, the current deviates from its intended circuit path, leading to system failures. In the case of high-power electrical appliances, this can cause an overload of electronic current, potentially resulting in the complete burnout of the affected components.
- Corrosion: It is a common occurrence during the operation of electronic devices. When power is supplied, moisture can initiate chemical reactions with the metal components on the PCB, accelerating the formation of oxides and leading to solder joint damage. Moreover, specific applications may necessitate prolonged exposure to humidity, creating an environment favorable for the growth of mold and bacteria. This further deteriorates the PCB’s condition and presents a potential health hazard to individuals who come into contact with it.
- Reduced performance: In situations where electronic applications exhibit poor contact or unexplained failures during operation, water ingress is highly likely to be the underlying cause. When a chemical reaction takes place, the current-carrying conductors can develop a protective oxide film, resulting in increased resistance. Moreover, moisture tends to attract airborne dust, leading to issues such as compromised contact and diminished electrical performance.
Safeguarding PCB Against Moisture During Manufacturing
Bare Board Manufacturing
The complexity of the PCB manufacturing process is evident, and as the number of inner layers increases, the manufacturing steps and challenges become more demanding. It is imperative to maintain a dry environment throughout the entire process.
- Controlled environment lamination: Lamination is a critical process in the formation of multilayer PCB board. During this stage, the manufacturing facility must carefully regulate the temperature and employ desiccants or other methods to absorb moisture from the air. Additionally, personnel involved in the lamination process should wear protective gloves to prevent contamination.
- Moisture-free prepregs: Upon receiving prepregs, it is essential to inspect them for moisture. If moisture is detected, baking the prepregs can help alleviate the issue. Otherwise, this step can be skipped.
- Mesh copper plane: Incorporating mesh copper planes facilitates stronger connections within the board and hinders moisture movement between layers. However, it is important to note that these planes may have an impact on the electrical performance of the board. Therefore, the potential influence range must be carefully considered when utilizing them.
PCBA is the process of assembling components onto a bare PCB. This process involves both SMT and THT, and it goes through wave soldering and reflow soldering. These processes create strong solder joints between the solder paste and the component pins through high temperatures. Consequently, the assembly process, being exposed to high temperatures, mitigates the potential impact of moisture on the PCB.
- Bare board baking: After production, the PCB bare board needs to undergo cleaning and assembly. If moisture is present on the board’s surface, it can lead to cracks due to the high temperatures in the soldering machine. Therefore, it is necessary to dry the board before assembly.
- Thermoplastic overmolding: This technique is employed in the manufacturing of electronics to create outer cladding or encapsulation layers. It involves heating thermoplastic materials to a plastic state, forming them into the desired shape and structure on the product, and allowing them to cure and hold their shape once cooled.
Preventing Moisture to Circuit Boards During Storage and Transportation
Conformal Coatings and Potting
Conformal coating and potting are two widely employed techniques utilized by PCB manufacturers. Once the production process is complete, a protective film is applied to the PCB’s surface to mitigate the risks of scratches or moisture damage during long-distance transportation. Depending on customer requirements, various materials such as acrylic, polyurethane, or silicone can be utilized. In essence, conformal coating can be likened to applying a layer of adhesive, while PCB potting is akin to encapsulating the board in a protective resin. It is important to note that prior to using these boards, the coating or resin layer needs to be removed, and the boards cannot be utilized directly in their coated state.
Desiccant and Moisture Absorbent Board
The utilization of desiccants and moisture-absorbing boards follows a similar principle to using moisture-absorbing tape in wardrobes. These methods aim to absorb moisture from the air, creating a drier storage environment for PCB. Desiccants typically consist of silica gel beads, and packaging bags often include a humidity indicator card. This card assists customers or users in determining whether the desiccant has been compromised or exposed to excessively high humidity during transportation or storage. Hygroscopic panels serve as an effective alternative in environments with high humidity, such as during shipping, as they are specifically designed to address such challenging issues.
In cases of bulk orders or when boards are not immediately utilized in electronics following production due to specific circumstances, it is essential to ensure that proper storage conditions are maintained to prevent moisture-related damage. It is advisable to employ dehumidifiers in the warehouse environment. These devices effectively extract moisture from the air, safeguarding the PCBs from prolonged exposure to humidity.
Safeguarding Internal Circuits Against Moisture in Electronic Applications
In certain electronic applications, it is challenging to avoid exposure to humid environments, particularly in outdoor settings. Therefore, it becomes necessary to incorporate moisture-proof design strategies into the circuit design. Firstly, when selecting substrate materials, it is advisable to opt for glass fiber reinforced materials or ceramics with minimal water absorption, instead of paper-based materials. Additionally, the use of waterproof connectors and seals when choosing components is also an effective consideration.
More advanced measures involve transforming embedded systems into intelligent micro-heaters. This entails the installation of humidity and temperature sensors, as well as carefully selected heating elements within the enclosure. A simple PID control system can be employed to regulate the air temperature based on measurements of temperature, relative humidity, and pressure. However, caution must be exercised to prevent overheating of the board and its components. Placing the heating element on the outer edge of the enclosure and selecting an appropriate power output are crucial factors to consider. It is also recommended to utilize a temperature sensor to monitor the temperature inside the enclosure and prevent any potential overheating issues.
When confronted with unavoidable humid environments, it becomes crucial to conduct comprehensive humidity and PCB testing. To begin with, it is necessary to anticipate the humidity levels in the application environment and utilize this as a benchmark to evaluate the severity of the harsh conditions that the circuit will encounter. Subsequently, the moisture resistance of the PCB should be tested. Methods such as subjecting the board to humidity and temperature cycles within an electronic environment can be employed to assess its performance under extreme conditions. Ultimately, the results from both tests should be combined to draw a conclusion regarding the circuit board’s ability to withstand the anticipated environment.
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