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    Circuit Design Tips: PCB Moisture Protection for Humid Environments

    October 17, 2017
    Circuit Design Tips: PCB Moisture Protection for Humid Environments

    I pride myself on being meticulous when I check my designs. I can spend hours checking schematics, footprints and the proper values of components before finalizing the design. Unfortunately, the same zeal and perfectionism cannot be said with my housekeeping skills. A cleaner would have been horrified by the accumulated dust in the corners of my house.

    It was particularly embarrassing when I forgot to wash my dish tray and it became the home to a family of five lizards. If your PCB is placed in a damp, dark and humid environment, these friendly reptiles are going to be only one of many problems. This is why it is important to design with your board’s environment in mind. Humid environments are particularly challenging, so let’s explore what can go wrong and how you can prevent damaging your electronics.

    Here’s how humidity can affect electronics and reduce the useful lifetime of your system:

    1. Short Circuits
    2. Corrosion

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

    1. Conformal Coating and Enclosure
    2. Suction Fan
    3. Silica Gel
    4. Heating Elements

    How Humidity Can Affect Your PCB

    Humidity refers to the amount of water vapor in 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 cold surfaces. When the environment is more humid, more water can condense on cold surfaces. A common problem caused by humidity is the formation of water droplets on electronics, particularly PCBs, since it corrodes the copper traces. Condensation on a powered PCB can cause short circuits and damage to other components. Besides directly damaging the PCB, humid environment attracts reptiles and insects that can potentially cause short circuits. In a PCB or other electronic device, the goal in 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

    The most common situation that can arise when a PCB is exposed to significant amounts of water is formation of a short circuit. Anyone that has accidentally poured water on their laptop has likely watched in horror as their computer screen goes blank. Water is highly conductive, 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.

    Short circuit, burnt battery cables
    Water Droplets Can Cause Short Circuits

    2. Corrosion

    When exposed to condensation, water on a PCB can cause exposed conductors to corrode. 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 conformal coatings (see below), exposed metal should be thoroughly cleaned before plating.
    • 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 a surface plating on exposed traces (ENIG, ENIPEG, Ni-Au, etc.).

    How to Protect Electronic Devices from Humidity

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

    1. Conformal Coating and Enclosure

    Of course, the easiest solution for keeping your electronics 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, urethanes, and silicone. 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 can provide other benefits beyond environmental protection. Electromagnetically absorbing coatings can 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. Suction Fan

    Some embedded systems are commonly placed inside industrial enclosures, and trapped moisture can be an annoying problem. Installing a fan that sucks the air out from the casing is an inelegant solution, but it can help reduce humidity levels. You can see a similar application in your bathroom’s exhaust fan. 

    3. Silica Gel

    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. There is a reason why vitamin C came 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.

    how to protect electronic devices from humidity silica gel

    Silica gel is a powerful desiccant

    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 provides somewhat lower moisture capacity at low temperatures, but it has 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.

    4. Heating Elements

    Turning your embedded system into an intelligent mini heater can be an effective way to solve moisture problems. This works well for embedded systems that are placed in an industrial enclosure for outdoor applications. I’ve used a heating element to lower the relative humidity and prevent moisturization in vehicle parking machines, where condensation can get really bad in the morning.

    Instead of blindly heating up the air, a humidity and temperature sensor can be installed in the enclosure along with a heating element. A simple proportional–integral–derivative (PID) control system can be used to regulate air temperature based on temperature, relative humidity, and pressure measurements. If the air temperature is too low, you can increase the temperature of nearby air with the heating element. Ideally, this will place the system above the triple point of water in the particular environment. This, in turn. will greatly reduce any chance of water droplets from forming.

    Hot heating element
    Turning Up The Heat Lowers Relative Humidity

    Be careful with this type of control system as you do not want to heat up the board and its components to an unacceptably high temperature. When your components reach a high temperature, they effectively age faster and can fail early. In extreme cases, where the temperature is too high, a component can fail entirely. This technique is best used by placing any heating element on the outer edge of the enclosure so that the element heats any air inflow. This technique is also best used in an environment where the external air temperature and has high relative humidity. If the pressure is already near the triple point, then the heating element can bring the air temperature above the triple point and help prevent condensation.

    You can easily build a control system from open-source software and simple hardware, or you can implement the other important recommendations in your new board, when you use circuit board design software like, Altium’s Circuit Studio, and cross-checking on the BOM list before tuning your heating parameters.

    Have a question about humidity and how it affects your circuit boards? Talk to an expert at Altium.  

     

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