Circuit Design Tips: PCB Moisture Protection for Humid Environments

October 18, 2017 CircuitStudio

 

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 of to a family of five lizards. If your electronics 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.  

How to Protect Electronics From Humidity

High humidity represents the amount of water vapor in the air and relative humidity is a value that quantifies it.  

A common problem caused by moisture 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.

 

I’ve encountered a couple of these situations in my line of work. One was a faulty emergency phone where a colony of ants decided to build a nest in electronic devices on the PCB. Scraping off the nests revealed irreparable damage of corroded printed circuit boardstracks. Another incident, which I’d rather forget, was when I went to investigate a damaged power management component and found a burnt lizard stuck to.

 

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

 

 

How To Prevent Absolute Humidity From Damaging Embedded Systems.

Obviously, these are situations we would all like to avoid. Here are some best practices for doing this.

 

1. Conformal Coating And Enclosure

Of course, the easiest solution for keeping your electronics safe from moisture is to apply the conformal coating and place it into an enclosure. It gives a decent protection on the PCB as the exposed copper and components are coated with materials like acrylic, urethanes, and silicone. The downside of this passive approach is that rework on the PCB can be difficult, as you’ll need to strip of the coating before the components can be removed and reapplying them before the PCB is re-installed.

 

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 can help in reducing the temperature and humidity. 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 with your PCB can help in reducing the moisture content in the air. There is a reason why vitamin C came with a pack of silica gel. However, silica gel is only effective as a moisture absorbent below 60°C.

 

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 levels 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. The intelligent proportional–integral–derivative (PID) algorithm can be applied to ensure that the humidity and the temperature of the air can be continuously regulated in an efficient manner. This will greatly reduce any chance of water droplets from forming.

 

Hot heating element
Turning Up The Heat Lowers Relative Humidity

 

 

Needless to say, I’ve had no problems of burnt lizards or ant colonies after turning my embedded system into a mini heater. But it’s prudent to ensure that the heated temperature does not exceed the maximum operating temperature of your components on the PCB. You can easily add the maximum allowed temperature on each components using 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 they affect your circuit boards? Talk to an expert at Altium.  

 

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