The presence of moisture can lead to various functional failures on a PCB, depending on which components or conductive paths come into contact with it as diffusion takes hold. Moisture can fester in the epoxy glass, resin or glass interfaces and cracks in a board. Problems commonly associated with moisture include slowed circuit speeds and increased delay times with the functions of a corresponding device. If the problem exceeds a certain limit, the device might simply fail to activate.
Tests have been conducted that show the effects of moisture absorption and desorption in printed circuit boards. In a PCB with plated through-holes of varying density, trapped amounts of moisture have different desorption rates based on the distance between each hole. In PCBs that are heavily saturated, desorption can take hundreds of hours within high-temperature environments.
If a PCB is placed in an environment where the atmospheric moisture pressure exceeds the resistance of the board and its components, moisture can penetrate the PCB. To prevent moisture delamination from taking effect on a PCB, soldering should be performed only with high temperatures of less than 0.1 percent moisture content or with low temperatures of less than 0.2 percent moisture content. High-temperature soldering would hover around 260 degrees Celsius, while low-temperature soldering would be in the ballpark of 230 degrees Celsius.
Detecting and removing moisture
When measurements are made of a PCB’s ability to store electric energy, a change in moisture content can be detected within the board. Capacitance sensors are used in this process. Capacitance levels move in inverse proportion to hole density. If the latter is high, the former is low because there’s less distance between the moisture and the surface but more space for the moisture to escape.
In non-PTH PCBs, capacitance decreases at a more rapid pace. As such, less bake time is needed for these boards to have a low enough moisture level. On PTH boards, there’s less exposed surface room for the moisture to escape.
Due to the inverse effect of copper planes on the desorption process, they should be baked with consideration for their design. On one hand, you can empty moisture more effectively from a board by running the baking process for longer periods, but doing so could reduce the board’s solderability and functional capacity. Consequently, bake time should be measured to avoid these possible side effects.
The process of moisture removal does not always yield predictable results. For example, a pair of identical copper planes could undergo a central flare-up of moisture as the baking gets underway, only to diffuse moments later. If this momentary swell of moisture occurs in an area of the board where delamination is most probable, it could be the unintended side effect of baking.
On some boards, moisture removal is simply not possible once the moisture has diffused through several layers. Therefore, it’s crucial to employ measures to prevent moisture from entering into the board during the initial process of assembly.
How does it work?
One of the most common ways for PCBs to be exposed to moisture is through contact with cold winter air. In cold temperatures, there’s not enough heat within the atmospheric air to absorb moisture. As a result, the moisturized content of the air is released onto cold surfaces. If a surface becomes colder than the air itself, that surface can serve as a magnet for released moisture, or condensate. This process is why windows often get foggy during the wintertime.
Cold items placed in proximity to areas of condensation are also liable to attract moisture from the air. For example, a vase placed along the sill of a foggy window is liable to become damp with droplets of moisture. Any surface that could become colder than the inside air and hold standing water can serve as a magnet, including the surfaces of computer components.
The internal and external surfaces of an inactive computer or peripheral device can easily become cold in the winter months. As the surfaces become colder than the air itself, they attract moisture. With inside components, the problem can be exacerbated if no vent holes are present for the moisture to escape. PCBs, for example, can be situated horizontally inside a computer box, scanner, video player or stereo device. During hours of the day when the house is empty and the heat and electricity are both off despite low outside temperatures, these devices can serve as moisture magnets.
As moisture diffuses onto the surfaces of PCBs and other internal circuitry, the devices can eventually fail to power back on. When a device is left dormant over the winter months and fails to activate come springtime, internalized moisture is sometimes the cause. Since the device itself has been inactive, there has been no internal heat generation within the unit during the span of time in question.
Additional ways that moisture can accumulate on PCBs include the following:
Insufficient packaging: PCBs can also come into contact with moisture when they’re packaged or stored in unsound bags and cabinets. If a board is shipped in an unsound package that doesn’t offer protection from ambient conditions, moisture can seep in under certain conditions. In some cases, moisture will diffuse into the layers of the PCB before it arrives in the hands of the customer.
Assembly: One of the most elusive and frustrating ways that moisture can come into contact with a PCB is when water particulates from the ambient air land on the board during the assembly process. If water lands on the board itself prior to soldering and diffuses through the layers, the board itself can essentially be manufactured with a defect. Even though steps exist to remove moisture, those same steps can also open the board to further exposure.
Baking: Moisture can even swell within a board during the most common process used to combat PCB moisture: baking. While the intent behind baking is to heat PCBs to temperature levels that force moisture out, the internalized moisture molecules often expand momentarily as the temperature rises within the board. While most of the moisture is ultimately driven out of the PCB, the momentary expansion can cause further irreversible diffusion within the board.