Once a PCBA comes off the assembly line, it could be subjected to any number of extreme environmental factors that lead to failure of the end product. One of these factors leading to premature failure is exposure to moisture, which can lead to formation of solid salts and metal filaments. These filaments can create shorts that lead to failure of the board, requiring cleaning or complete replacement of the PCBA.
Preventing exposure to moisture in the end product might seem obvious, but it can also occur during fabrication and assembly. Just like exposure to moisture during operation can lead to failure, uptake of moisture during storage or manufacturing can also create a risk of failure. There are some design decisions that can help prevent moisture exposure and uptake in the end product, and your manufacturer will need to properly handle base materials to prevent moisture uptake. We’ll look at some steps that can be taken in this article.
Why Protect a Circuit Board From Moisture?
PCBs need to be protected from moisture prior to assembly, before they have sensitive electronic components mounted on them, as well as after assembly, when they will be placed into operation. With exposure to moisture, it is possible that the electronic components themselves fail, and this is probably the most common view of moisture-induced failure in a PCBA.
In reality, the board itself can fail from excessive moisture exposure during storage, fabrication, assembly, or during regular operation in an end product. Once the board is deployed in its intended environment, measures taken at the assembly level are the only ways to prevent failure. To begin the discussion, let’s look at the causes of moisture exposure during manufacturing.
Improper Storage and Handling
PCB base materials are built with a few key components:
- A resin that becomes rigid during the lamination and curing process
- A curing agent that assists resin curing during board buildup
- A glass fiber weave that is impregnated with the above materials
The resin will be the major component in PCB base materials. Resins used in PCBs are hygroscopic, meaning they can absorb and trap water from humid air or liquid water that makes its way onto the surface of the base materials.
PCB laminate construction.
Due to the hygroscopic nature of PCB base materials, they must be stored in a dry environment prior to processing, and they should be handled in a humidity-controlled facility to prevent further moisture uptake during processing.. It is also common to pre-bake materials in vacuum just above the boiling point of water to remove any residual moisture from the PCB laminate. These steps can help prevent delamination and conductive anodic filamentation (CAF), which are two common board-level failure mechanisms related to moisture exposure.
The PCBA and the final assembly also need to be protected from moisture. There are three things designers can do to help protect the PCBA from moisture exposure:
Storage and Packaging With Desiccants
Have you ever opened a box with an electronic device, and you find that the box contains a small packet of silica gel beads? That silica gel is a desiccant. Its job is to prevent moisture adsorption on electronics by directly absorbing water from humid air that might infiltrate the device package. Desiccant packets can be included in device packaging during storage or during shipping to the end destination. Those same silica gel packets are placed in vacuum-sealed packages of chips to prevent moisture adsorption, particularly if those chips are moisture-sensitive.
If there is a risk of moisture-induced failure in the PCBA, then the device should be packaged with a silica gel to prevent moisture uptake by the board and components. Also, make sure to include a moisture indicator card in the PCBA packaging. This allows the customer/user to see if the desiccant has failed or if there was extreme moisture exposure during shipping or storage.
Use a Conformal Coating
There are multiple conformal coating materials that provide significant protection against humidity and water uptake. These materials are strong enough to prevent electrochemical growth of conductive salts or filaments between exposed metals on surface layers, which normally occurs on the leads between components. The conformal coating also protects the components themselves by providing a complete shield against moisture uptake.
Certain conformal coatings can provide additional protection against noxious or corrosive substances that would otherwise damage exposed conductors or components. Conformal coatings can also strongly absorb electromagnetic radiation at radio frequencies, acting as an EMI shielding barrier. If conformal coatings are only desired on certain portions of the PCB, such as along the board edge or on specific components, these materials can be applied through a mask.
Design an IP-rated Enclosure and Use IP-rated Connectors
Even if the device is handled perfectly during manufacturing and a conformal coating was applied to the PCBA, moisture can still enter the device through the connectors and enclosure. Ingress protection (IP) ratings are used to categorize different electronic products in terms of their resistance to solid or liquid ingress. An enclosure can be designed to a high IP rating, and there are commercially available connectors that have waterproofing ratings.
The mating connectors for these M12 cables are available in IP-6X rated versions. These can be directly mounted to a PCB as needed.
The rating system used to designate moisture and solid ingress protection is written as “IP-XY”, where X and Y are numbers. The typical water resistance rating for an electronic assembly will be IP-6X, where X is 4 or larger. Make sure to design to these ratings if you require the end product to experience moisture exposure during operation.
Design to Prevent Moisture
Although you will need to rely on your manufacturer to handle boards properly during manufacturing to prevent early moisture exposure, there are design choices you can make to help protect circuit boards from moisture. Consider the design options listed above if your board will be placed in an operating environment where humidity is a concern. To ensure true water resistance or complete waterproofing, multiple steps will need to be taken to build a rugged assembly that can resist moisture.
When you’ve finished your PCB layout and it’s time to specify any assembly requirements that will help protect a circuit board from moisture, use OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.