When you intend PCB fabrication, you need to pre-plan ways of board protection first. These electronics may be highly receptive to excessive moisture. It is mostly because the copper plating and electrical components mounted deteriorate quickly. In the worst-case scenario, unprotected PCB may fail as fast as within two weeks.
Here are our tips and advice on protecting your PCBs from moisture in the main fabrication phases.
Protect PCB During Fabrication
For starters, it is essential not to let moisture in during the manufacturing process.
1. Ensure that Cores and Prepregs are Moisture-Free
You should begin by controlling the moisture on all the sheet components that you use for lamination then. In case these had been stored in an inadequate environment for some time, you may bake them at temperatures slightly above 100°С to dry them gently.
2. Laminate in a Controlled Environment
Once you have your sheet components dried and prepared, you may proceed with lamination. Ensure that the temperature does not fluctuate in the process. Besides, it is a good idea to apply a desiccant drying agent. It will absorb moisture from the air.
An additional measure is to wear protective gloves. These must be changed each lamination cycle. It will prevent dust, dirt, and sweat from entering the lamination environment.
3. Use Meshed Copper Planes
It is not obligatory but a highly advisable measure to put meshed planes between layers. These improve bonding between epoxy layers and increase the bonding strength. It also limits moisture movement between layers.
You should not forget that meshed copper planes alter electrical properties. So pre plan their usage in the designing stage.
4. Apply Surface Finish
It is an absolutely common measure that protects your PCBs in multiple ways. It limits adverse moisture effects, makes boards less vulnerable to mechanical stresses, and enhances electrical properties. It also should be planned in the designing stage.
Your options here are such types of finish:
- Hot Air Solder Level (HASL).
- Lead-Free HASL.
- Organic Solderability Preservative (OSP) finish.
- Immersion Silver (Au).
- Immersion Tin (Sn).
- Electroless Nickel Immersion Gold (ENIG).
- Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG).
- Electrolytic Wire Bondable/ Hard Gold.
Protect PCB During Assembly
Then, your PCBs undergo the assembly phase. This means that all the removable components are mounted onto it. You should take some preventive measures here as well.
5. Baking
While this was optional in the first phase, you definitely need to give your PCBs a bake before assembly. It is so because, during the fabrication, micro drips of fluid congregate onto the PCB surface and can affect electronics mounted on the assembly stage.
6. Apply Coating
Similar to finishing, a coating is the best form of PCB protection. Here you have several options available, but let’s review three of them.
- Acrylic Acid Conformal Coating. This is simply spraying acrylic acid onto PCB surfaces. The best results are achieved in case boards are preheated to about 60-70°С. It’s a cheap but reliable coating that reduces the impact of moisture on electronics.
- Polyurethane Conformal Coating. This one is considered a modified version of the previous coating. It forms a firm film after complete immobilization. Such protection is moisture-proof and has enhanced anti-corrosion properties. The suitable temperature for this coating is in the range of 60-130°С.
- Silicone Electronic Moisture-Proof Plastic. Such a form of coating is the most durable and the simplest in its application. The acceptance temperature is -60-130°С. It means that you can apply in normal conditions without preheating. Such coating has extreme abrasion resistance and moisture-proof properties.
There are other forms of coating, such as Potting, Microencapsulation, Thermoplastic Overmolding, and others. So you can always choose the one that suits your purposes better.
Protect PCB During Packing And Storing
And here are some guidelines on how to ensure that your PCBs are not exposed to moisture during packaging and storing.
7. Store Boards in Hermetically-Sealed Bags
It is absolutely essential to use moisture-barrier bags as the primary packaging material. These can be EDF or electrostatic discharge bags that are hermetically sealed. This way no moisture will impact your boards.
8. Follow IPC Standards
There is a set of specific IPC-1601A guidelines that regulate the process of handling PCBs. These detailed methods of moisture prevention, ways to limit physical stresses, and many more.
You should pay particular attention to the Water Vapor Transmission Rate (WVTR), which helps evaluate the usability of moisture barrier bags. It is also essential to choose sulfur-free materials that do not release corrosive H2S or SO2 substances.