Why PCB Need Conformal Coating
Conformal coating is a specially formulated coating used to protect the circuit boards from environmental erosion. Conformal coating has good resistance in high temperature and low temperature. It is a transparent protective film after solidification. It has excellent insulation, moisture resistance, anti leakage, shock proof, dustproof, anti-corrosion, anti-aging and corona resistance.
We often see the metal parts are patina of circuit boards, , that was no coating of conformal coating. The copper metal is combined with water vapor and oxygen to cause the chemical reaction.
The composition of conformal coating
Acrylic acrylic conformal coating is flexible and provides comprehensive protection. Because it is a single component system with good adhesion and simple operation, the requirements and conditions of the equipment is not high, convenient construction, high transparency, high brightness, the characteristics of the short operation cycle, so they are easy to use and easy to clean. Some acrylic products meet military standards, they are dry quickly and not anhydration, organic solvent available supporting its elimination, so this kind of circuit board conformal coating is one of the most widely use on the market,also the most effective product.
There are four types for the use of conformal coating
- Brushing – commonly used, can produce smooth coating on the surface .
- Spray – spray can be conveniently applied to maintenance and small scale production, gun production suit for large scale.but the two sprayings require high accuracy of operation, and may produce a shadow (place where not covered with conformal coating).
- Automatic immersed coating – it can ensure full film coverage without causing material waste due to excessive spraying.
- Selective coating – coating accurate and not waste materials, suitable for a large number of film coating, but the coating equipment requirements is high.Using a set of XYZ programs reduces coverage. When boards are painted, there are many connectors which don’t need be painted. Stick tape too slowly and there is too much residual glue when tear, can consider do a fixture according to the shape, size and position with mounting hole location. Covered area where no conformal coating.
Operation requirements of conformal coating
- Clean and dry the board to remove moisture and vapor,should remove the dust,moisture and oil stain on the surface of object in order to give full play to its protective effectiveness.Thorough cleaning ensures that the corrosive residue is completely removed and that conformal coating is well adhered to the surface of the circuit board.Drying conditions: 60°C, 10-20 minutes,the effect is better hot coating when boards removal from oven.
- With brush coating method, brush surface area should be larger than components to ensure that all covered of components and pads.
- The circuit board should be flat and no dew after be brushed, brush should be smooth and no exposed part, 0.1-0.3mm is appropriate.
- Before brushing and spraying, make sure that the diluted product is fully stirred and placed for 2 hours before brushing or spraying. Use a high quality natural fiber brush and brush gently at room temperature. If use machine,should measure the viscosity of the coating(using a viscosity agent or a flow cup), and the viscosity can be adjusted with a diluent.
- Circuit board assembly shall be immersed vertically in the paint lees. Don’t immerse the connector,unless it is covered carefully, the circuit board should be immersed for 1 minutes until the bubble disappears and then slowly take out. A uniform film is formed on the surface of the circuit board. Most coating residues should flow back from the circuit board to the film dipping machine. TFCF has different coating requirements. Circuit boards or components should not be immersed in too fast, so as not to produce excessive bubbles.
- The second using after the end of the dip, if the surface has crust phenomenon, remove the epidermis and can continue to use.
- After brushing, flat on the stent, ready to cure, need to use heating method is coating accelerated curing. If the surface of the coating is uneven or contains bubbles, it should be put in a high temperature furnace for more time at room temperature to allow the solvent to flash out.
Note of conformal coating：
- If you wish to get a thicker coating, preferably by coating two layers of thinner coating – and requiring that the first coat be completely dried, second layers are allowed to be applied.
- In the coating on the circuit board, the general connector, software socket, switch, radiator, heat sink area, plug plate region is not allowed to have coated materials, it is recommended to use tear resistant solder covering.
- Film thickness: the thickness of the film depends on the application method. The addition of diluent is large, the viscosity of the gum is low, the thickness of the coating is thin; on the contrary, the viscosity of the glue is high, and the thickness of the coating is thick.
- All coating operations shall be performed at a temperature of not less than 16 DEG C and relative humidity below 75%. The circuit board will absorb moisture as a composite material. If not to tide, three paint can not fully protect the role. Pre drying and vacuum drying can remove most of the moisture.