An Introduction to Moisture Sensitivity in Printed Circuit Board (PCB)

Introduction:
Moisture is one of the top issues with PCBs and is also one of the leading cause of PCB failure. Moisture can be absorbed during the PCB fabrication process or diffuse into the PCB during storage. Moisture can lead to destabilization in PCBs which can further lead to delamination. Therefore keeping moisture out of your PCB is integral if you wish for a high-performance PCB. Here we will discuss some of the causes, affects and ways to prevent moisture absorption in PCBs.

Effects of Moisture on PCBs:
Moisture can have numerous effects on PCB performance depending on where it is present. It may diffuse into the epoxy glass, resin or glass interfaces and cracks in a board.

Moisture can have the following effects on PCB performance:
• Moisture may lead to slowed down circuit speeds and delays in functioning and if moisture persists, may lead to device failure.
• Moisture absorption and desorption rates vary for different PCBs. Heavily saturated PCBs may take hundreds of hours for desorption in high-temperature environments.
• If atmospheric moisture exceeds the resistance of a PCB and/or its components, it may penetrate the PCB.
• Moisture can affect the thermo-mechanical properties of PCBs.
• In order to prevent moisture delamination, soldering should only be done with high temperatures (260 degrees Celsius) of less than 0.1 percent moisture content or with low temperatures (230 degrees Celsius) of less than 0.2 percent moisture content.

Detection and Removal of Moisture:
During measurements of PCB ability to store electrical energy you can detect a change in moisture content in the board by using capacitance sensors. Capacitance is inversely proportional to hole density. If hole density is high, capacitance is low and there is less distance between the moisture and the surface but more space for moisture to escape.

For non-PTH (Printed Through-Hole) PCBs capacitance decreases at a rapid rate and less bake time is needed to keep minimum moisture levels. For PTH PCBs, there is less surface exposed for moisture to escape.
You can remove moisture from PCBs more effectively by baking for longer periods; however this may reduce the PCBs solderability and decrease its functionality. Therefore, you should PCB design into consideration when deciding bake time.
Keep in mind that moisture can be more difficult to remove as it diffuses deeper past several layers therefore you should opt for prevention methods right during the assembly process.

Causes of Moisture Absorption:
Cold winter air tends to release its moisturized contents onto cold surfaces. If the surface is colder than the air itself it may serve as a magnet for moisture release, therefore PCBs are exposed to moisture. This is because during winter months, the surface of electrical devices tends to become colder than the air and so they attract moisture. In the absence of vent holes this may problem may be aggravated. As moisture diffuses onto PCB surface and circuitry of devices, the devices eventually fail to power on. This is why devices left unused in winter fail to operate in spring.

Some other causes of moisture accumulation on PCBs include:
Inadequate Packaging:PCBs can come in contact with moisture if they are not packaged correctly. By the time, they are shipped and arrive to the customer moisture may have seeped into the layers of the PCB.
Assembly of PCBs: Water may also particulate onto PCBs during the assembly process. If water diffuses into the PCB before soldering the board will become defective.
Baking: Yes, although this process is used to combat PCB moisture, it may also cause moisture to build up inside the PCB. Baking is done to force moisture out of the PCB. However, this may also cause the internal moisture molecules to expand with rise in temperature; this can lead to irreversible diffusion within the PCB.

Prevention of Moisture in PCBs:
In order to prevent moisture diffusion in PCBs there are a number of tips. The lamination process should be conducted in a temperature controlled environment with drying agents to keep out moisture. PCB lamination tends to have a dehydrating effect on the finished product. During lamination a low-pressure vacuum is applied to prevent internal voids that could trap moisture. It is also common practice to bake the prepregs prior to lamination particularly those which have spent a certain length of time in storage.
An effective means of preventing moisture is to use meshed copper plates. They inhibit moisture to diffuse between layers as well as in and out of boards. They also allow for stronger bonding between layers. They do however reduce the boards electrical capacity.

Removal of Moisture from PCBs:
The main method for removing moisture is baking. The heat is used to drive moisture out. The effects are also lasting and thorough. Baking is done during the assembly process and before PCBs are shipped. However baking has its drawbacks. If a PCB has large copper plates, the moisture may swell at first during the baking process and cause increased moisture diffusion which may become more difficult to remove.

Moisture can be kept away from PCBs by keeping PCBs in drying enclosures with temperatures having less than 0.05g/m3 of air vapor. This stops moisture from settling on PCBs with the help of a vacuum effect. This also prevents oxidation on surfaces.
PCBs should be properly stored in dry environments. They should be packaged in MBBs (Moisture Barrier Bags) during shipping. While not in use devices having PCBs should be stored in dry storage units with dehumidified air. You should also try to keep devices active during cold months to generate enough heat to combat moisture,
Another tip to combat moisture is to keep devices in vertical position. Horizontal lying computer boxes will have a lying PCB which could host standing moisture. In computer system the PCBs are situated in a vertical position hence whenever necessary, vertical position is preferred.

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