/Military pcb assembly requirements for three anti-coating materials
First, military pcb assembly generally for special-purpose products, such as high temperature, high humidity, high salinity, high corrosive conditions, will spray three anti-paint on the printed circuit board, then in the electronic processing What are the requirements for paint? let’s see it together:
1 It has the characteristics of waterproof, anti-dust, anti-acid, alkali and alcohol, anti-mildew, anti-aging and anti-friction.
2 can form a dense protective film on various printed circuit boards, component packaging materials, metal and non-metal materials, and solder joint surfaces.
3 The protective film has stable physical and chemical properties, high insulation resistance, high breakdown voltage and good high frequency characteristics.
4 color transparent, fast drying, low curing temperature.
5 easy to operate, can be sprayed, painted, dip coated.
6 non-toxic, harmless to the human body and the environment.
Types, applications and development of secondary and tertiary coating materials
The three anti-paint materials are usually of the type of acrylic, silicone, polyurethane, epoxy, and alkyd. The choice of process is generally determined by the different viscosities of the product (different concentrations have different solids content).
The three anti-coating materials can be divided into insulating protective rubber, moisture-proof adhesive and three anti-adhesive according to the application: according to whether it can be desoldered after curing, it can be divided into two types: solderable and non-weldable; Solvent-based coating materials, water-based coating materials and new solvent-free coating materials.
(1) solderable three anti-adhesive
Because some electronic products may need to be repaired or replaced after the PCB is assembled online for three-proof treatment, it is required to use solder-proof three-proof glue. The three anti-paints are required to be transparent and fast-drying, and can form a protective film on various printed circuit boards to prevent leakage and short circuit. The protective film of solderable three anti-proof can prevent the erosion of weak acid, alkali, alcohol and moisture, resists high temperature, does not drip when it is heated, and can be tinned after curing. The main component of the solderable three anti-adhesive is propylene resin or silicone resin, and the solvent is methyl propane acetate, ketone solution, acetone, xylene, and a small amount of additives.
(2) Non-explosive tin three anti-adhesive
Most of the non-weld three anti-adhesives are made of epoxy resin, one-component or two-component transparent liquid, which can be cured by heating at room temperature. After the glue is solidified, an elastic rubber layer is formed, which is dustproof and waterproof. Moisture-proof, anti-salt fog, anti-corrosion, anti-vibration, anti-surface aging, anti-leakage, etc., can work for a long time under -60 ~ 180 °C conditions, suitable for brushing, dipping and spraying.
Non-weld three anti-adhesive is mainly used for high-reliability products used in harsh environments such as high temperature, low temperature, humidity, salt spray and mold, high speed, vibration, etc., such as military weapons and equipment, aerospace, navigation, power, electricity, automobiles, And all the electronics that work in the harsh environment of the wild.
(3) Traditional solvent-based coating materials
The traditional solvent-based coating materials account for 95% of the use in the European Union, accounting for 506 in the United States. Most of the SMT patch processing plants in China currently use solvent-based coating materials. The two coating materials described above are all solvents. Type coating materials.
Since environmental legislation will limit the solvents that people are used to, the non-volatile solvent (OC-FREE) legislation has been implemented in the United States and Europe, and other solvents are also banned.
(4) Water-based coating materials
Currently, water-based coating materials are manufactured using only a small number of special prototype pcbs. The main reason is that the water-based coating material requires a longer curing time. Although the coating material can be finished in 15m (solvent-based coating material is completed in 10-12min), it needs 24h to remove internal moisture, so the PCB is charged. Performance testing cannot be performed immediately after curing. This is unacceptable for high volume production. therefore. Water-based coating materials are produced in close batches. Especially suitable for PCBA production of military products.
(5) New solvent-free coating materials
The technology means that 1009% of the material is coated on the PCB, and the process is 100% converted into a film without volatile, and the transportation requirements are not high, and the storage and operation are convenient. In the UK, silicon-based coating materials are mainly used, but the technology has not been well solved due to process problems and excessive thickness, so it has not been widely used so far.
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