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After the copper-clad board is cleaned, the circuit made on the film is printed on the copper board. The commonly used method is the photochemical transfer method. A layer of photosensitive film is applied on the copper surface of the copper-clad laminate, exposed to light, cleaned from the light-sensitive portion, and finally etched out of the circuit by a chemical solution. Specific steps are as follows:
1 on the photosensitive rubber centrifugal drying. The prepared photosensitive adhesive was applied on the copper surface and dried to form a photosensitive film.
Photosensitive material formula: bone glue 250～300g
Ammonium dichromate 25～30g
Water 1000ml
Preparation method:
The bone glue is soaked (12 hours or more), the bone cement is melted by heating in a water bath for 1 to 2 hours, and then filtered with 150 mesh nylon silk or multi-layer gauze.
After filtering the bone glue liquid slightly, pour the dissolved ammonium dichromate and mix well. Maintain 40 to 60°C during use.
Then sizing and drying by centrifugation to keep it from sticking.
Photoreceptor mechanism: In colloids, ammonium dichromate decomposes chromium trioxide under ultraviolet light, reacts with protein (bone gum) molecules, and forms chrome protein salts, which are difficult to dissolve in water. This is the phenomenon of photosensitizing of proteins. .
2 exposure (drying board). Generally used 1000w iodine tungsten lamp sensitive tens of seconds to a few minutes.
3 development. Place the exposed plate in warm water at 40-60°C to wash off the unexposed adhesive layer. In order to make the exposed circuit on the board clearly visible, it can be dyed with methyl violet or barium solution.
4 repair board. Repair defects in the exposed circuit. The method is to repair with a nitro spray paint of appropriate consistency after dilution with a brush, a little banana water. For excess parts of the line can be eradicated with a knife.
(4) Corrosion circuit
Corrosion circuit with ferric chloride plus water mixed in a certain proportion (about 3:5) formulated as a solution, the temperature is generally 40 ~ 50 °C is appropriate. In order to increase the etching rate, stirring methods such as spraying and compressed air entering the etching solution may be added. Immediately after the circuit is etched, remove the copper plate from the etching solution and rinse with water.
For the etching of a small number of circuits, a concentration of 31% of hydrogen peroxide (industrial use) and a concentration of 37% of hydrochloric acid (industrial use) and water can be formulated into an etching solution in a ratio of 1:3:4. First 4 points of water into the tray, then down 3 points hydrochloric acid, stirring with a glass rod and then slowly add 1 minute hydrogen peroxide, dubbed the etching solution, the corrosion of the finished circuit board. Note: When configuring the solution, please configure it in a special (corrosion resistant) container and filter the solution. At the same time, be sure to pay attention to safety, and prevent the solution of ferric chloride or hydrochloric acid from splashing on the skin and clothing. And prepare a rag to wipe off sputtered ferric chloride solution at any time to avoid contamination.
(5) Remove protective layer
After the etching is completed, the board is punched, immersed in a 5% to 20% caustic soda solution, and heated. After the protective layer is easily detached, the board is removed. The copper brush is brushed with water to remove the corrosion left on the circuit. Floor.
(6) Upper solder resist
Solder resist is a high temperature resistant coating. The purpose of the upper solder Mask is to protect the circuits and parts that do not need to be soldered with solder mask, so that the soldering can only be performed on the solder joints that need to be soldered. Solder masks should be applied to the board soldered by dip soldering and wave soldering.
The solder resist is formed according to the solder mask formation method. The solder resist can be classified into a thermosetting type, an ultraviolet curable type, and an electron diffusion type curable type.
Thermal curing solder masks have the advantages of low cost and high bonding strength. The material of the thermosetting solder resist includes phenol resin, epoxy resin, amino resin, and alkyd resin. It can be used alone or in combination. The method is to make the material into a liquid printing material, and then the solder resist is printed on the plate by screen printing, and then heat-cured to form a solder resist film. However, it needs to be baked at a temperature of 130-150° C. for several hours to cure, so the production cycle is long, the efficiency is low, and the power consumption is large, and it is not suitable for automatic production. Is gradually being replaced by photocuring agents.
(7) Pre-painted flux
Locations of solder joints that require soldering on printed circuits are pre-coated with flux. It makes the solder joint easier to solder on during welding, which improves the welding reliability and welding quality.
There are many types of flux and can be self-formulated, such as:
60% anhydrous ethanol + 10% n-butanol + 20% extra rosin + 6% triethanolamine + 4% diethylamine hydrochloride to form the salt ester diethylamine flux.
With a small number of printed boards, ready-made fluxes can be purchased in the market. The board was preheated and sprayed with flux, baked at 60°C, and then sprayed a second time until sticky and still dried. This improves solderability and finish.
(3) Photochemical Printed Circuits