Printed circuit board (PCB) manufacturing relies on a coordinated combination of materials and processes to create reliable electronic circuits. Insulating substrates like FR-4 fiberglass-epoxy or specialized polymers provide mechanical strength and electrical isolation, while conductive copper layers are patterned using photoresists and chemical etchants.
Additional materials, including plating metals, solder masks, and surface finishes, enhance conductivity, protect the circuitry, and ensure solderability. Throughout fabrication, wet processes such as chemical etching, plating, and application of liquid coatings are used to precisely form, clean, and finish the circuit boards. These steps, combined with thermal bake cycles, ensure proper material curing, moisture removal, and strong adhesion between layers.
Moisture management is critical because many PCB materials are hygroscopic, including substrates, prepregs, polyimide films, and photoresists. Absorbed moisture can cause delamination, blistering, cracking, or soldering defects during high-temperature processes like reflow soldering or multilayer lamination. To prevent these issues, manufacturers use pre-baking, drying, and controlled curing cycles, and package PCBs in moisture barrier bags with desiccants and humidity indicator cards.
Printed circuit boards packaged with humidity indicator.
Contract manufacturers often pre-bake boards based on Moisture Sensitivity Level (MSL), storage conditions, and component guidelines to ensure structural integrity and reliable performance. Proper storage, handling, and packaging together maintain the quality and longevity of PCBs.
In this blog post, we will discuss moisture sensitivity in PCBs, what the materials are, and what the risk is regarding moisture.
Inclusive Materials
PCB manufacturing relies on a coordinated set of materials that work together to create reliable electronic circuits. The process begins with insulating substrate materials such as fiberglass-reinforced epoxy or specialized polymers, which provide mechanical strength and electrical isolation. Conductive copper layers are then added and precisely patterned using photoresists and chemical etchants.
Additional materials, including plating metals, solder masks, and surface finishes, enhance conductivity, protect the circuitry, and ensure solderability. Throughout fabrication and assembly, various chemicals, inks, and soldering materials are used to refine the circuit board’s structure and functionality, resulting in a durable and fully operational electronic platform.
Processing
During the process of making PCBs, there are many processes that are considered “wet.” Wet-processed materials in PCB manufacturing are those applied or used in liquid chemical stages to modify, clean, or build up the board. These include liquid photoresists for imaging, chemical etchants such as ferric chloride and ammonium persulfate for removing unwanted copper, and desmear and cleaning solutions like permanganate and solvents.
Conductive layers are also formed using wet processes through electroless and electrolytic plating baths for copper, nickel, and gold. Additionally, liquid coatings such as photoimageable solder mask and surface finishes like ENIG, immersion silver, and OSP are applied in chemical baths. Together, these wet materials enable precise pattern transfer, surface preparation, metal deposition, and protective finishing throughout PCB fabrication.
Baking During Manufacturing
Bake cycles in PCB manufacturing are used at multiple stages to remove moisture, cure materials, and improve overall reliability. Initial pre-baking dries the substrate to prevent defects, while photoresist undergoes soft, post-exposure, and hard baking to stabilize and define circuit patterns. The solder mask is then thermally cured to form a durable protective layer, and multilayer circuit boards experience a heat and pressure cycle to bond layers together.
Additional drying or post-plating bakes remove residual moisture after wet processing, and a final bake before assembly helps prevent thermal damage during soldering. Together, these bake steps ensure structural integrity, strong adhesion, and consistent performance of the finished PCB.
Moisture-Prone Materials
Several materials used in PCB manufacturing are moisture-absorbing, which makes careful handling and baking essential. The most common moisture-sensitive materials are polymer-based, including FR-4 fiberglass-epoxy substrates, prepreg layers, polyimide films for flexible circuits, and photoresists. These materials can absorb humidity from the environment, which may negatively affect adhesion, dimensional stability, or pattern quality during fabrication. Even solder masks and organic surface preservatives can absorb moisture before curing, potentially affecting protective coatings and surface finishes. If moisture is not properly managed, it can lead to serious defects during high-temperature processes such as reflow soldering or multilayer lamination.
Common issues include delamination, blistering, and cracking, as absorbed water rapidly vaporizes under heat. To prevent these problems, PCB manufacturers implement pre-bake, drying, and curing cycles, as well as controlled storage conditions, to ensure that moisture-sensitive materials remain stable and the final PCBs are structurally sound and reliable.
Pre-Bake
Contract manufacturers (CMs) should prebake PCBs in certain situations, especially if the boards are moisture-sensitive or have been exposed to ambient humidity during storage or transport. Prebaking drives out absorbed moisture, which is crucial because even small amounts of moisture can cause “popcorning” – delamination or cracks in components – during high-temperature soldering processes like reflow.
Whether prebaking is necessary depends on a few factors:
- Moisture Sensitivity Level of the PCB components: Higher MSL components (e.g., MSL 2–3 or above) often require prebaking if storage conditions exceed recommended limits.
- Storage duration and environment: If PCBs were stored in high-humidity conditions or out of sealed bags for a long period, prebaking is recommended.
- Manufacturer instructions: Follow the board or component datasheets for recommended pre-bake temperature and duration, as excessive heat can damage some components.
In practice, many CMs include a standard prebake step as part of their moisture control plan to ensure reliability and minimize the risk of soldering defects.
How Packaging Helps with Moisture
When dealing with PCBs, moisture is a critical concern because it can lead to corrosion, delamination, or soldering defects during reflow. Proper packaging is essential to protect them. To protect PCBs from moisture, start by drying them if they’ve been exposed, typically by baking at the manufacturer's recommended temperature. Moisture-sensitive components must follow IPC/JEDEC baking guidelines to prevent damage during later soldering.
Once dry, PCBs should be packaged in moisture barrier bags, which are metalized or multi-layer plastic bags that block humidity. Proper sealing, vacuum sealing, or heat sealing is crucial to minimizing air and moisture inside the package. Inside the packaging, include desiccants to absorb residual moisture and humidity indicator cards to monitor conditions. For additional protection, vacuum sealing or nitrogen-flushed packaging can reduce air and moisture further. Store the packaged PCBs in a clean, dry environment, ideally in a controlled dry cabinet with low relative humidity, and label packages with the date, MSL, and handling instructions. These steps collectively prevent moisture-related issues like corrosion, delamination, and soldering defects.
Summary
PCB manufacturing relies on a coordinated combination of materials and processes to create reliable electronic circuits. Insulating substrates like FR-4 fiberglass-epoxy or specialized polymers provide mechanical strength and electrical isolation, while conductive copper layers are patterned using photoresists and chemical etchants.
Additional materials, including plating metals, solder masks, and surface finishes, enhance conductivity, protect the circuitry, and ensure solderability. Proper storage, handling, and packaging together maintain the quality and longevity of PCBs.
Key Takeaways
- Many PCB materials, including FR-4, prepregs, polyimide films, and photoresists, are hygroscopic and can absorb moisture from the environment.
- Excess moisture inside a PCB can lead to serious reliability issues during high-temperature processes, including delamination, blistering, cracking, and soldering defects.
- Bake cycles throughout PCB manufacturing are essential for removing moisture, curing materials properly, and maintaining structural integrity and adhesion.
- Proper packaging methods, including moisture barrier bags, desiccants, humidity indicator cards, and controlled storage conditions, help protect PCBs from moisture-related damage during storage and transport.
- Prebaking PCBs before assembly is often necessary when boards have been exposed to humidity or extended storage conditions, helping prevent popcorning and other reflow-related failures.
