What's the conformal coating?

Conformal coating is named the following names: PCB protective oil, coating oil, damp-proof paint, three-proof coating, water-proof glue, insulating paint, anti-corrosion paint, salt fog-proof paint, dust-proof paint, protective paint, coating paint and three-proof glue. The PCB with the conformal coating is of three-proof as damp proof, dust proof and anti-chemical corrosion. It is also with the performances as below: thermal shock resistant, anti-aging, radiation resistant, anti salt fog, ozone corrosion resistant, vibration resistant, good flexibility and strong adhesive force.  

Conformal Coating Types

By its chemical composition, the conformal coating can be defined the following 3 types: the acrylate conformal coating, the Polyurethane (pu) conformal coating and the silicone conformal coating. By its curing mode, the conformal coating can be classified as the solvent curing, the indoor temperature curing, the thermal curing and the UV-light curing. 

1.Features of conformal coating with acrylate (the most popular and widely used one): surface drying and curing fast, good three-proof, cheap price, transparent color, flexible quality, and be easy to recover. 

2.Features of conformal coating without acrylate: UV curing, the surface drying within a few seconds to more then 10 seconds, transparent color, hard quality, good corrosion resistance and abrasion resistance

3.Features of Polyurethane (pu) conformal coating: brittle quality, perfect solvent resistance performance. Apart from the perfect damp-proof, it has the stable performance in low temperature environment.  

4.Features of silicone conformal coating: Soft elastic coating material releasing the pressure very well, high temperature resistance up to 200℃, be easy to recover.  

Furthermore, comprehensively considering the price and performance, the above three kinds of conformal coating sometimes will be present overlapping, for example the silicone modified conformal coating.  

Roles of conformal coating

Moisture is the most popular and most destructive main factor to the PCB. Too much moisture will reduce the insulating resistance between the conductors and increase the high-speed decomposition. It will also reduce the Q value and corrode the conductor. The verdigris we often meet on the metal part of PCB is caused by the chemical reaction happens when the copper without conformal coating meets water, steam and oxygen. Hundreds kinds of pollutants we can find on the PCB have the same damages. They will cause the same results as the moisture corrosion such as electric deterioration, corrosion of the conductor or even the inevitable short circuit. The most popular pollutant found in the electronic system may be the chemical substance left in the production process. Those pollutants can be listed as flux, solvent releasing agent, metal particles and marking in for example. The main pollutant could also be produced because of the careless handling, such as the body fat, the fingerprint, the cosmetics and the food residual dirty. There are many pollutants in the working environment, such as salt spray, dust, fuel, acid, the corrosive steam and mould. The conformal coating on the PCB or its parts and components will reduce or remove the deterioration of the electric operation performance when they are affected by the negative factors in the working environment. If the coating can keep its effect for a satisfied period, longer than the products service life for example, it can be regarded as getting the coating purpose. Even the coating is very thin, it will stand to some extent the mechanical vibration and swing, thermal shock, as well as the operation in high temperature. The concept that the film makes the individual part inserted in the PCB be of mechanical strength or enough insulation is certainly wrong. The parts and components should be inserted strong in mechanical way. They also need their suitable joint mixture. Therefore, the double insurance avoids the accident. 

Where to Use Conformal Coating

At the beginning, the conformal coating was only used on the PCB in high-tech fields. As the more and more usage of the electric devices in our daily life, the consumers care more and more about the quality and reliability of the products. The conformal coating helps the manufacturer improve the products quality effectively and reduce the expensive failure cost in warranty period. 

Typical Application Includes the Followings:

Home appliances、Industrial control、Automotive、Electrical source、Military industry、Aerospace、Navigation、Medical industry  ction on-line and suitable for the large or middle scale of mass production.

Three Proof Process

There four processes for conformal coating. They are brush coating, dipping coating, sprinkling and spraying.

The spraying is the most popular coating method in the industry. There are two kinds of spraying. One is machine automatic spraying, the other is manual spraying. 

The machine automatic spraying is applied in the mass production and the customers with quantity or long-term production. The advantages of the machine automatic spraying are as below: selective spraying available, high accuracy, uniform coating and efficient. The disadvantages comes as expensive cost and the strictly requirements to the operation accuracy.

The manual spraying normally are the combination of the handheld spray gun, the air pipe and the air compressor (and the exhaust fan). Its advantages are cheap cost and suitable for the small customers. But it needs the air pressure more than 5kg to form the fog. Thus the requirements of the pressure adjustment is much high. In case the air pressure is not high, the conformal coating would not be atomization. Then the cobwebbing happens. The manual spraying requires the operator excellent quantity.

In conclusion, the manual spraying is low investment and easy to operate. The machine automatic spraying has the advantages of consistent coating and the highest production efficiency. It is easy to realize the automatic production on-line and suitable for the large or middle scale of mass production.

Requirements of Conformal Coating Process

There are 4 steps in the conformal coating process as below: 1.Board cleaning and drying 2. Coating work, 3. Curing and drying, 4. Inspection and partial repair

Board cleaning and drying: Clean and dry the board to get rid of the moisture and water. First, should remove all the dust, moisture and oil dirt from the product surface to be coated so that to make the conformal coating play its full protective role. Thoroughly cleaning insures the removal of the corrosive residue clearly and the conformal coating will well adhere in the PCB surface. Board drying 

condition: 60℃, 10-20minutes. It gets the better coating effects if be coated before cooling after taking from the dryer. 

Coating operation: we have made detailed introduction of the coating method above.

Curing and drying: Curing and drying is the step we take after the PCB conformal coating finished to cure it. At present, there are mainly 3 kinds of curing and drying. They are UV curing (to the UV kind), hot wind curing and infrared curing. For the normal curing of conformal coating, if it is cured on-line, we usually take the infrared and hot wind convection oven. Sometimes, we also use the infrared heating or hot wind heating alone. If it is off-line, the drying oven is more popular to be used. If it is organic silicon, humidifying is better when heating. If it is UV, the UV oven is needed. Some UV conformal coating also needs humidifying curing.  

Inspection and Partial Repair Method: To repair the part being coated, only need to make the welding electric soldering iron contact with the coating layer directly, you can remove the part and replace it with a new one. Clean the area by brush or solvent and dry it, then re-coat it.

Cautions in the Conformal Coating Process

1.If you want to get a thicker coating, you'd better to get it by coating two layers. And you should paint the second layer only after the first layer is totally dry.

2.When you are doing the PCB coating, the coating material is not allowed to exist in the following areas: the connectors, the software socket, the switches, the radiator, the cooling area and the plate inserting area. We suggest use the solder mask of tearability to cover.     

3.The layer thickness: The layer thickness depends on the application method. If the volume of the thinner is large and the glue viscosity will be low, then the coating thickness is thin. Contrariwise, the glue viscosity will be high, and the coating thickness is thick.    

4.All the coating work should be made in the condition of being no lower then 16℃ and relative humidity less than 75%. As the composite material, the PCB will absorb the moisture. Without moisture removal, the conformal coating can not play its protective role thoroughly. Most moisture can be removed by predrying and vacuum drying. 

Advantages of Ouli Coating Process Solutions

The selective conformal coating system of Ouli Automation Technology Co., Ltd provides the users a clean and high efficient automatic selective conformal coating process which avoids the coating beyond the selected area. Thus the processes of coating, coating removal and repair are taken out. The spray nozzle controlled precisely guarantees the uniform conformal coating thickness of 50u-200u. Comparing with the manual spraying, dipping and brushing method, it is revolutionary improvement which improves the production efficiency and product quality greatly.