Playing in the rain can be fun. And I agree with Gene Kelly that “Singing in The Rain” can be fun. I even recall from Science, as it was called when I was in high school, that rain is an integral part of photosynthesis or the process by which plants create food needed to grow. And we, of course, cannot do without plants for food and oxygen. Nevertheless, there are times when you just do not want to get wet. Say, when you have just spent the better part of the morning dressing to the nines and are clad in a brand new suit. At times like that, rain can ruin a perfectly good day.
Water drops on a PCBA
It does not take a downpour of water to ruin your PCBA. In fact, a few drops or an even smaller amount of moisture can cause havoc on a circuit board. Problems can range from distorted waveforms to complete failure. Obviously, moisture can be a problem during operation, especially if used in environments like industrial protection equipment. It can also be a problem for boards before they are installed. Therefore, it is important that you know what options you have to protect your boards from moisture, such as PCB coating. But before we get to PCB protection methods, let’s take a look at the types of threats to your board that may be due to moisture.
Moisture Threats for Circuit Boards
It is well-known that water is a very good conductor of electricity, right? Actually, pure or deionized water has a resistivity of about 1.8 x 10^7 Ω-m at 25 °C, which makes it a much better insulator than a conductor of electricity. However, the water that is likely to come in contact with a circuit board is ionized and contains dissolved salts, chlorides, sulfates and carbonates. With these inorganic materials, the water can be highly conductive and pose a significant threat to component operation and current flow along traces, by providing alternative paths that can result in shorts and corrosion. Fortunately, there are ways to protect boards from exposure to significant amounts of water; such as sealed enclosures.
A much harder threat to guard against is condensation, which can occur when a surface or the board itself is colder than its environment. Condensation can occur within enclosures and cause moisture on the board or within component packages and should be counted among the considerations for selecting components. In fact, moisture can be introduced or diffused onto board materials and surfaces during fabrication, assembly and packaging or storage. Due to the damage that moisture can promote or facilitate it is imperative that PCB moisture protection methods are applied.
Applying the Best PCB Moisture Protection
Although there are a number of specific techniques or methods, it is more effective to view PCB moisture protection as a comprehensive approach that includes steps to be taken during PCBA manufacturing and afterwards as listed below.
Perform lamination in a controlled environment
During this stage, the temperature should be regulated and a desiccant drying agent should be utilized to absorb moisture from the air. Additionally, personnel should wear protective, clean gloves during each lamination cycle.
Ensure prepregs are moisture-free
If materials have been exposed to an unregulated environment prior to usage, then baking the prepregs can be helpful; otherwise, this step can usually be avoided.
Use meshed copper planes
Meshed copper planes form a stronger bond and prevents moisture movement between layers. It should be noted that these planes can affect the electrical properties of the board, which should be factored into their usage.
Apply surface finishing
It is also important to protect your boards between the fabrication and assembly stages. Choosing the right surface finish is the best way to achieve this and facilitate assembly by providing a good foundation for soldering connections.
Baking after assembly is a common and effective practice for removing moisture; however, high temperatures can cause delamination or cracking if the coefficient of thermal expansion (CTE) for the board is exceeded.
- Conformal coating
Probably, the best form of PCB protection is to apply one of the types of conformal coating. Not only will this coating guard against moisture, but fluids, contaminants and even harmful ultraviolet light.
Potting, which is completely encapsulating a board within an enclosure can also be very effective, provided that the enclosure does include a means for trapped moisture to escape; such as through vents.
As an alternative to full coating or enclosure, the board can be partially covered with epoxy or microencapsulated. When this method is used only certain areas or components are covered.
- Thermoplastic overmolding
For thermoplastic, usually airtight, overmolding can be used to protect the board as well. However, connectors need to be exposed for interconnection with other boards or devices.
Packaging and Storage
Store boards in moisture barrier bags
It is not uncommon for boards to stored and shipped in electrostatic discharge or ESD bags to protect against damage from charge buildup and dissipation. However, for the best protection against moisture, boards should be stored in moisture barrier bags.
Follow IPC Guidelines
Traditionally, guidelines on protecting PCBAs from moisture have not been as prevalent as for other areas; such as quality assurance, performance and reliability. However, IPC-1601A Printed Board Handling and Storage Guidelines, does provide suggestions for packaging and storing boards to minimize the possibility of moisture diffusion.
Failing to apply PCB moisture protection to your boards can lead to board failures and unnecessary costs for repairs or replacements. However, by applying techniques that are listed above for fabrication, assembly and packaging/storage you can greatly reduce the possibility that moisture will threaten the operation and reliability of tour boards.
|Tempo‘s Custom PCB Manufacturing Service
Tempo Automation, the industry leader for fast, high-quality software-accelerated PCBA prototyping and low-volume production, can apply surface finishes, conformal coating or other PCB moisture protection methods to meet your needs.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.