Moisture in PCBs

Introduction

Moisture is a phenomenon that negatively impacts the reliability of electronic devices. It is one of the contributing factors to issues such as corrosion, electrochemical migration (dendrites), CAF (Conductive Anodic Filament), and others. Surface-mount (SMD) electronic components can be moisture-sensitive, which is defined by the Moisture Sensitivity Level (MSL) of the components.

Moisture (in the context of elevated air humidity), however, can also have a beneficial aspect in the electronics industry—it reduces the accumulation of electrostatic charges, thereby helping to minimize Electrostatic Discharge (ESD) risks.

This article discusses the impact of moisture on printed circuit boards (PCBs), the mechanisms of moisture ingress, its effects, and recommended handling practices for PCBs. As always in our articles, we also address relevant industry standards in this area.

Curious? Let's dive in!

Moisture Ingress Mechanism in PCBs

PCBs have a diverse structure and utilize various materials. A large portion of the PCB base is composed of glass fibers and epoxy resins. In fact, the most commonly used material known as FR-4 consists of multiple layers of glass fiber bonded together with epoxy resin.

During the PCB manufacturing process, these materials undergo numerous chemical treatments (such as etching, coating, and drying) and mechanical processes (drilling, cutting, and milling). This results in the formation of micropores, voids, and microcracks in the laminate structure, which increases its moisture absorption capacity.

FR-4 is a hygroscopic material—it absorbs moisture. Typical moisture absorption levels for FR-4 laminates are around 0.1–0.2%, but there are also materials with higher absorption rates of approximately 0.5-0.6%. Under more challenging environmental conditions, moisture accumulation can be even more significant.

There are several mechanisms by which moisture penetrates into the laminate:[1]

• Bulk diffusion. This is the primary mechanism responsible for increased moisture content in laminates. Water molecules move within the material from areas of higher moisture concentration to areas of lower concentration (i.e., along the concentration gradient). This phenomenon is influenced by the surface topology of the resin and molecular polarity, which affect the "attraction" of water molecules.
• Capillary action. The presence of voids and cracks within the material (in the resin) facilitates the ingress of water molecules.
• Wicking. Voids and cracks at the interfaces of materials—often resulting from manufacturing processes—contribute to increased moisture migration.

PCB Moisture Ingress - When?

Moisture ingress can occur during the following:

• PCB manufacturing process. During PCB production, a variety of chemical processes are used in which the laminate comes into contact with water or water vapor. In some stages, the moist laminate may not be properly dried, causing moisture to become "trapped" within the structure. For example, during the layer bonding process (lamination with prepreg), materials should be thoroughly dried beforehand—otherwise, residual moisture will be sealed between the layers. This trapped moisture may later evaporate rapidly during soldering and cause delamination. Manufacturers can use the IPC-TM-650 2.6.27 method to assess whether a PCB is subject to moisture-induced delamination.[2]
• PCB transportation and storage. During transportation and storage, PCBs should be protected from exposure to moisture. Common causes of moisture ingress at this stage include the use of improper packaging materials, leaky or torn packaging, or failure to reseal or properly protect packaging after opening.
• Assembly. Moisture absorption during the assembly phase is relatively rare. However, it can occur if the production process is multi-stage—for example, after soldering, semi-finished products may sit for several days before further assembly or be shipped to another facility without adequate environmental protection. This can lead to moisture condensation on the PCB surface, and in some cases, this moisture may become trapped (e.g., a damp module enclosed in a housing or potted with resin).
• Operation. Moisture can also penetrate the laminate during device operation. Phenomena such as condensation or prolonged exposure to high-humidity environments are typical sources of PCB moisture. Depending on the operating conditions and design requirements, various forms of moisture protection are applied—such as sealed enclosures, silicone or epoxy potting, and other moisture mitigation techniques.

Effects of Moisture

Moisture negatively impacts PCB quality by degrading both its mechanical and electrical properties. The most significant effects include:

• Risk of blistering and delamination. Rapid expansion of water vapor (i.e., trapped moisture) during thermal processes such as reflow soldering can rupture the bonds between laminate layers.
• Increased risk of CAF. Moisture is one of the contributing factors that increases the likelihood of Conductive Anodic Filament (CAF) formation.
• Changes in dielectric properties. Moisture can increase the dielectric constant (Dk) and dissipation factor (Df), potentially reducing signal switching speed and degrading signal integrity.
• Reduction in glass transition temperature (Tg), which increases susceptibility to mechanical stress—especially in vias and through-holes (THT).
• Accelerated oxidation of copper surfaces, which can negatively affect solderability and lead to defects such as nonwetting or dewetting.

PCB Moisture Protection

• Protective packaging. Newly manufactured PCBs should be packaged immediately after production in a way that protects them from moisture exposure. Proper packaging includes the use of an appropriate MBB (Moisture Barrier Bag), a desiccant pack (silica gel), and a HIC (Humidity Indicator Card). Packaging guidelines are clearly defined in IPC-1602.[2]
• Proper storage. Properly packaged PCBs should be stored in dry environments. The packaging shall remain hermetically sealed. When correctly packed, an MBB should effectively protect the PCB for up to 12 months at 40°C and <90% RH.[2]

PCB Baking

In electronics, the process of drying PCBs is commonly referred to as baking. While the term may suggest a high-temperature process (as in a culinary context), in this case, it specifically refers to the controlled removal of moisture - essentially drying in 105-125°C.

The oven used for this purpose should be clean (free from contaminants from other processes) and convection-based (with forced air circulation). The ideal solution would be an oven that allows drying in a nitrogen atmosphere or under vacuum (vacuum oven), although such equipment is rarely available.

Drying PCBs helps remove excess moisture absorbed in the laminate, but the process also has some drawbacks—most notably, it increases the oxidation of solderable surfaces, which reduces solderability, especially for finishes such as Chemical Tin and OSP.

The drying temperature should remain below the Tg (glass transition temperature) of the laminate. It should also not exceed 125°C (and for OSP finishes, a maximum of 105°C) to avoid degrading solderability.[2]

IPC-1602 provides guidelines for drying temperature and duration in section 3.4.4, along with important caveats. The typical temperature range is 105-125°C, and the drying time is between 4 and 6 hours. However, it is recommended to minimize thermal exposure as much as possible—a better approach may be to begin drying at 105°C for 1 hour and evaluate the results, rather than exposing the entire batch of laminates to 125°C for 6 hours, which could significantly impair solderability. The outcome greatly depends on PCB thickness, number of layers, and surface finish.

It is important to remember that drying reduces solderability, and finishes like Chemical Tin and OSP are particularly sensitive to excessive or prolonged baking.

Standards

• IPC-1602 Standard for Printed Board Handling and Storage. The primary standard defining requirements and recommendations for PCB handling, packaging, storage, and drying.
• IPC-TM-650: 2.6.2, 2.6.3, 2.6.27, 2.6.28. Test methods for measuring moisture in laminates. These documents are developed by the IPC organization.

Summary

Moisture in PCBs is a significant concern for the electronics industry. Excess moisture can lead to laminate delamination, CAF (Conductive Anodic Filament), electrochemical migration, and overall degradation of dielectric properties.

Applying proper practices at every stage—PCB manufacturing, packaging, transportation, storage, and assembly (soldering)—is essential for minimizing moisture content. For guidelines on packaging, storage, and potential drying of PCBs, we recommend following IPC-1602.