Circuit Design Tips: PCB Moisture Protection for Humid Environments

Moisture is the enemy of electronic devices, including integrated circuits, specialized sensing elements, and PCBs. Whether or not a PCB has moisture-sensitive components, the PCB itself and any exposed conductors can be impacted by excess humidity and moisture in the operating environment, storage environment, or factory floor. So to make sure your design does not experience failures from moisture exposure, a combination of design choices, proper storage, and handling on the factory floor are needed.

If you want to know how to protect electronics from humidity, you can jump to one of the following sections to see our design tips.

How Excess Humidity Can Affect Your PCB

Humidity refers to the amount of water vapor in the air and is quantified in terms of relative humidity. Unless you are in the desert, almost any area on the planet will be humid, and water can condense on cool surfaces. When the environment has higher humidity levels, more water can condense on cool surfaces causing humidity damage.

In an electronic device, one potential problem in a humid environment is the condensation of water droplets on electronics, particularly exposed conductors in PCBs, on component packages, or on PCB layer stack materials. This excess moisture can cause humidity damage to your electronic device. It's important to know how to protect electronics from humidity to help ensure the longevity of your devices. In a PCB or other electronic device, the goal of preventing damage from humidity is two-fold:

• Keeping humid air from sensitive electronic components
• Preventing any condensed water from creating short circuits

Here’s how humidity can affect your electronics during operation:

1. Short Circuits

Extreme amounts of moisture on exposed conductors on a PCB can lead to a short circuit. Anyone who has accidentally poured water on their laptop has likely watched in horror as their computer screen goes blank. Water is a conductor, and a current surge in a device during a short circuit can cause an entire board section to fail, or it can completely burn out a component.

2. Corrosion

When exposed to condensation,  excess moisture, water, and any dissolved salts on a PCB can cause exposed conductors to corrode. For example, water-soluble flux residues are reactive and can participate in electrochemical reactions that lead to corrosion. Exposed metal in a PCB can corrode in several ways:

• Atmospheric: When exposed to moist air, a reaction can occur where metal ions bond with oxygen atoms, forming an oxide. These oxides are insulators, which slightly increases the resistance of an exposed conductor. Oxides are also mechanically weak and will fracture easily.
• Electrolytic filamentation: When water on exposed metal contains some dissolved electrolytes, dendrite structures can start growing on the surface as an electric current flows through the solution. In addition to PCB conformal coating (see below), exposed metal should be thoroughly cleaned before plating, including removal of flux residues.
• Galvanic: Galvanic corrosion occurs between dissimilar metals in the presence of a dissolved salt. Unlike electrolytic filamentation, this will occur regardless of the presence of an electric current.
• Fretting: This type of corrosion occurs when solder-plated switches are closed. When the switch is closed, the surface oxide layer can be removed via friction. If there is any water on the exposed metal, the exposed metal will oxidize. Corrosion builds up after a significant period of time.

These corrosion mechanisms are prevented or slowed using certain surface platings on exposed traces (ENIG, ENIPEG, Ni-Au, etc.).

Assembly Options to Protect Electronics From Humidity

With the problems that can arise from humidity in a printed circuit board, there are some simple steps that can be taken to prevent humidity from damaging sensitive components. 

1. Conformal Coating

The easiest solution for keeping an assembly safe from moisture is to apply a conformal coating to the board. This provides decent environmental protection as long as the coating is not porous and has cured completely. The idea is to coat the PCB and the exposed copper; some representative materials include acrylic coating, urethane conformal coating, and silicone conformal coating. The downside of this passive approach is that rework on the PCB can be difficult as it requires stripping off the coating before the components can be removed. The coating would need to be reapplied after rework is completed.

Using the right conformal coating material can provide other benefits beyond environmental protection. Electromagnetically absorbing coatings can also help reduce EMI from a noisy board at high MHz frequencies. Such radiated EMI typically occurs on the PDN in a board with insufficient coupling.

2. Silica Gel Desiccants

While not the most elegant solution, placing a pack of silica gel in your PCB packaging can help reduce the moisture content in the air.  This is normally done when packaging a PCB to be shipped to a customer. There is a reason why some vitamins and pills come with a pack of silica gel: the silica gel will easily adsorb water from humid air, thus there will be little or no water available to adsorb on conductors in the PCB.

Unfortunately, silica gel is only effective as a moisture absorbent below 60 °C. Above this temperature, the adsorption equilibrium will be driven back towards the vapor state, and water will begin desorbing from the gel back into the surrounding air.

Other adsorbents can be used instead of silica gel to remove moisture and other trace gases from the surrounding air. Activated alumina is one commercially available porous desiccant that can provide lower moisture capacity at low temperatures. It has a somewhat higher capacity at higher temperatures. Activated carbon is another alternative that is used as an adsorbent for odors and toxic gases in military gas masks, and it can be used to remove corrosive gases and moisture from surrounding air. Phosphorus-containing compounds and metal salts are other options that provide a number of other benefits. If your system will be deployed in a unique environment with corrosive gases and high humidity, you may consider using one of these alternative desiccants to protect your electronics.

3. Enclosure Design

Your enclosure design is another factor that can provide safety from moisture. Fully sealed enclosures typically cannot provide protection from humidity, but they can provide protection from liquids and dust if they have the right IP rating. An IP-rated enclosure can be designed by an experienced mechanical engineer or some IP-rated enclosures can be purchased off-the-shelf.

IP ratings are standardized in IEC-60529 and are read using a two-digit code (such as IP65). The IP rating can be decoded as follows:

IP[Solid debris rating][Moisture rating]

Larger numbers in the code indicate greater protection against solid debris or liquid contaminants. The table below summarizes the broad operating environments for IP-rated products.

An IP-rated enclosure is ineffective without a set of IP-rated connectors for any external interfaces. Most standardized connectors have an IP-rated equivalent specifically designed for marine environments or harsh environments.

Proper Storage and Handling in a Humid Environment

Both PCBs and components have storage and handling practices that should be followed. The storage practices for PCBs and parts are intended to ensure these devices can be pulled from storage and assembled with high yield. PCBs and components can be affected by moisture such that there may be problems soldering parts onto the boards.

The IPC-1602 standard provides guidelines for the handling and storing of bare PCBs and assemblies. If boards will be stored before they pass through assembly, then the conditions in the storage environment need to be regulated. Room temperature storage at a relative humidity near 50% is typical. Bare PCBs should be stored in a dry place, and it is appropriate to keep desiccant and a moisture indicator card in the storage container/cabinet. Note that it is not common to store bare PCBs in a vacuum environment prior to assembly and soldering. To ensure maximum ESD safety and exposure to excess humidity, an ESD-safe moisture barrier bag is typical for storing bare circuit boards, as well as loose components.

Once components are brought to the factory floor, they may require baking to ensure the parts can be properly assembled on the PCB. The J-STD-020F standard defines moisture sensitivity level (MSL) values that classify moisture-sensitive components based on their allowable exposure time to a humid environment (assumed 40% to 60% relative humidity). In total, there are 8 MSL classifications outlined below.

If these limits are exceeded, then an MSL reset is performed by baking the components, typically at a temperature from 100 to 125 °C. Within the assembly environment, the factory floor does not need to be totally dry as some minimal level of humidity (about 30% minimum) is needed to ensure solder paste can flow and wet during soldering.

The dangers of soldering with moisture-compromised ICs are well known, with the following problems being well-known:

• Popcorning, where trapped moisture quickly vaporizes and damages component packaging, leaving bubbles or minuscule cracks
• Difficult solderability, where trapped moisture interferes with the ability of a solder joint to form
• Oxidation, appears as corrosion of the internal metal parts and exposed leads

Make sure to check component datasheets to determine whether a part must be baked. If special assembly procedures are required, or you just want to ensure your assembly house is aware of the presence of MSL components, make sure to include the information in a scope of work (SOW) document, your fabrication/assembly drawings, and any quote from the manufacturer might provide. The same applies to any PCBs that might need a pre-bake before assembly.

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