1. Process Background
As PCBA components become smaller and more dense, the distance between parts and the stand-off height of parts, which is the gap between the part and the PCB, also become smaller. At the same time, environmental factors have a greater and greater effect on PCBA. Because of this, electronic products now need a higher level of reliability.
PCBA parts are changing from large to small, and from loose layout to dense layout. Under the trend of miniaturization and high density, failure problems caused by the environment are becoming more and more serious. For this reason, conformal coating has become a key process to protect PCBA reliability in use.
2. Effect of Different Environmental Factors on PCBA
Common environmental factors such as moisture, dust, salt spray, and mold can all cause different kinds of PCBA failure. The main effects are as follows.
2.1 Effect of Humidity
Most electronic PCB assemblies used in an open environment face a risk of corrosion. Water is the main medium that causes corrosion. Water molecules are very small, so they can pass through the molecular gaps of some polymer materials and enter the inside of the material. They can also go through pinholes in the coating and reach the metal layer below, and then cause corrosion.
When the air reaches a certain level of humidity, it can cause electrochemical migration on the PCB, leakage current, and signal distortion in high-frequency circuits.
Steam or moisture + ion pollutants such as salts and flux activators = conductive electrolyte + stress voltage = electrochemical migration
When relative humidity reaches 80%, a water film with a thickness of 5 to 20 molecules can form. At this point, many molecules can move freely. If carbon is present, electrochemical reactions may happen. When relative humidity reaches 60%, a water film with a thickness of 2 to 4 molecules can form on the surface of the device. If pollutants dissolve into it, chemical reactions may happen. When relative humidity is lower than 20%, almost all corrosion stops.
Because of this, moisture protection is an important part of product protection.
For electronic equipment, moisture exists in three forms: rainwater, condensation, and water vapor. Water is an electrolyte. It can dissolve many corrosive ions and then corrode metal. When the temperature of one area on the equipment is lower than the dew point, condensation will form on that surface. This can happen on structural parts or on the PCBA.
2.2 Effect of Dust
Dust exists in the air. Dust can absorb ion pollutants and then settle inside electronic equipment. This can cause failure. This is a common problem for outdoor electronic equipment.
Dust can be divided into two types. Coarse dust is irregular particles with a diameter of 2.5 to 15 microns. It usually does not cause failure or arcing, but it can affect the contact of connectors. Fine dust is irregular particles with a diameter smaller than 2.5 microns. Fine dust that falls on a PCBA board has some adhesion, so it can only be removed with an anti-static brush.
The harm of dust is as follows:
a. When dust settles on the surface of the PCBA, it can cause electrochemical corrosion and raise the failure rate of the device.
b. The combination of dust, heat and humidity, and salt spray causes the most damage to PCBA. Electronic devices near the coast, in desert areas or saline-alkali land, in the rainy season south of the Huai River, and near chemical plants or mining areas have the highest failure rate.
Because of this, dust protection is an important part of product protection.
2.3 Effect of Salt Spray
Salt spray is formed by wave action, tides, atmospheric circulation such as monsoon, air pressure, and sunlight. It can drift inland with the wind. The concentration becomes lower as the distance from the coast increases. Usually, at 1 km from the coast, the concentration is only 1% of the value at the shore. During typhoon seasons, salt spray can drift farther inland.
The harm of salt spray is as follows:
a. It damages the plating layer on metal structural parts.
b. It speeds up electrochemical corrosion, which can cause metal wires to break and parts to fail.
Similar corrosion sources are also common:
a. Human sweat contains salt, urea, lactic acid, and other chemical substances. These can cause the same kind of corrosion as salt spray on electronic devices. For this reason, gloves should be worn during assembly and use, and bare hands must not touch plated surfaces.
b. Flux contains halogen and acidic substances. PCBA must be cleaned, and the flux residue level must be strictly controlled.
Because of this, salt spray protection is an important part of product protection.
2.4 Effect of Mold
Mold is a common name for filamentous fungi. It means “mold-growing fungi.” They often form branched and dense mycelium, but they do not form large fruiting bodies like mushrooms. In warm and humid places, many items grow visible fuzzy, cotton-like, or web-like colonies. That is mold.
The harm of mold is as follows:
a. Mold eats and grows on organic materials. This can lower the insulation performance of the material and cause damage and failure.
b. The metabolic products of mold are organic acids. These can affect insulation and dielectric strength, and can cause arcing problems.
Because of this, mold protection is an important part of product protection.
3. Overview of Conformal Coating Process
Based on the full effect of humidity, dust, salt spray, mold, and other environmental corrosion factors, the PCBA must be isolated from the outside environment as much as possible in order to improve reliability. This is why the conformal coating process was introduced.
Conformal coating is also called surface coating or conformal coating. Its English name is coating or conformal coating. It is now the most common post-solder surface protection method for PCB. This process applies a thin insulating protective layer on the PCB surface. It separates sensitive electronic parts from harsh environments, greatly improves the safety and reliability of electronic products, and extends product life.
Conformal coating can protect circuits and parts from moisture, pollutants, corrosion, stress, shock, mechanical vibration, and thermal cycling. It can also improve mechanical strength and insulation performance. After coating, a transparent protective film forms on the PCB surface. This film can block water drops and moisture from entering, and it can prevent leakage and short-circuit failures.
4. Key Points of Conformal Coating Process Based on IPC-A-610E
4.1 Coating Area Classification
4.1.1 Areas Where Coating Is Not Allowed
These include areas that need electrical connection, such as gold pads, gold fingers, plated through holes, and test holes; batteries and battery holders; connectors; fuses and housings; heat sinks; jumpers; camera lenses in optical devices; potentiometers; sensors; switches without sealed structures; and all areas where coating will affect performance or normal operation.
4.1.2 Areas That Must Be Coated
All solder joints, pins, and conductive parts of components must be coated.
4.1.3 Optional Coating Areas
These areas have no clear mandatory coating requirement. They can be coated based on product needs and customer standards.
4.2 Coating Thickness Standard and Test Method
Coating thickness should be measured on a flat, uncovered, cured surface of the printed circuit assembly. It can also be measured on a companion board made during the same process as the assembly. The companion board can be made from the same material as the PCB, or it can be a non-porous material such as metal or glass. Wet film thickness measurement can also be used as an optional way to check coating thickness. In this case, the dry-to-wet film conversion ratio must be known in advance, and the records must be kept.
Thickness test methods:
- Dry film thickness measurement: a micrometer can be used, following the IPC-CC-830B standard. A ferrous dry film thickness gauge can also be used.
- Wet film thickness measurement: use a wet film thickness gauge to measure the wet film thickness, then convert it into the actual dry film thickness based on the solid content ratio of the coating material.
4.3 Edge Resolution Requirement
Definition: the edge of a line sprayed by a normal spray valve cannot be perfectly straight. Small burrs will exist. The width of these burrs is called edge resolution.
Process requirement: the smaller the edge resolution value, the higher the coating precision. There is no single fixed standard. It only needs to meet the custom requirement of the customer.
4.4 Uniformity Requirement
The conformal coating must form a complete film on the product surface. The film must be even in thickness, smooth, and clear. The coating coverage must be uniform across the whole product. After coating, the product surface must not show cracks, layer separation, orange peel, contamination, capillary effect, bubbles, or other process defects.
5. Full Conformal Coating Process and Main Methods
5.1 Early Preparation
Prepare the PCBA product, conformal coating material, and other helper materials. Identify the areas that need local masking protection. Confirm and lock down all key process details.
5.2 Cleaning Process
Cleaning should be done as soon as possible after PCBA soldering is finished. This helps avoid solder residue becoming hard to remove after it cures. First, judge whether the contaminant is polar or non-polar, and then match it with the right cleaning agent.
If an alcohol-based cleaning agent is used, the work area must have good ventilation. After cleaning, drying must be done strictly, so that leftover solvent does not cause an oven explosion when it evaporates. If a water-wash process is used, use a slightly alkaline emulsified cleaning liquid to remove flux residue, then rinse fully with pure water. Make sure the cleaning quality standard is met.
5.3 Masking Protection Process
When selective coating equipment is not used, the no-coating areas must be masked before coating. Use adhesive tape that will not leave residue or transfer glue. For IC parts, anti-static paper tape should be used first. All parts must be masked and protected strictly according to the drawing.
5.4 Dehumidification Process
After cleaning and masking, the PCBA must be pre-baked and dehumidified before coating. Based on the maximum temperature that the PCBA can tolerate, choose the right pre-bake temperature and hold time. This removes moisture fully from the board and from inside the components.
5.5 Main Coating Methods
The conformal coating process can be chosen flexibly based on the protection level of the PCBA, the production equipment, and the technical setup. The main methods are the following four types.
5.5.1 Manual Brush Coating
Brush coating has a wide use range. It is mainly suitable for small batch production, products with complex and dense PCBA structures, and products with strict masking protection needs. It gives strong manual control and can avoid no-coating areas very precisely, so it can prevent contamination.
This process uses very little coating material, so it is suitable for high-cost two-component coatings. It requires a high level of skill from the operator. Before work starts, the operator must understand the coating requirements in the drawing, identify different components, know the no-coating areas clearly, and make the marks easy to see. During the whole work process, bare hands must not touch the printed plug-in parts, so that secondary contamination can be avoided.
5.5.2 Manual Dip Coating
Dip coating can form a uniform, continuous, and complete coating layer. The overall coating effect is very good. However, it is not suitable for PCBA products that have adjustable capacitors, adjustable magnetic cores, potentiometers, cup-shaped magnetic cores, or parts with poor sealing.
The key process parameters for dip coating are: adjust the viscosity of the conformal coating accurately; control the PCBA lifting speed strictly; the lifting speed must not be more than 1 meter per second, so that bubbles do not form in the coating.
5.5.3 Manual Spray Coating
This method fits products with complex structures, many types, and small batch sizes, where mass production by automatic equipment is not possible. It can cover special corners and gap areas very well.
Work notes: spray mist can easily contaminate no-coating areas such as PCB plug-in parts, IC sockets, precision contacts, and grounding areas. Therefore, masking protection must be done before spraying. Operators must not touch printed plugs with bare hands, so that the contact surface will not become dirty.
5.5.4 Automatic Spray Coating
This method usually uses selective automatic coating equipment. It is suitable for large-scale standardized production. It has the advantages of good coating consistency, high precision, and less environmental pollution. As the industry upgrades, labor costs rise, and environmental rules become stricter, automatic spray equipment is slowly replacing traditional manual coating. It is becoming the main method in the industry.
