In the production of electronic products, screen printing has become an indispensable process. Such as the product shell decoration, printed circuits, thick film integrated circuits, solar cells, electricity; Ju, capacitance, piezoelectric elements, photosensitive elements, thermal elements, liquid crystal display elements, etc., are used in varying degrees in the production process Screen printing process. Some screen printing processes have been introduced in previous chapters, such as silk screen decoration of electronic products, screen printing of liquid crystal display elements, and screen printing of contact control disks. This section will not be repeated. The following focuses on printed circuits and thick Knowledge of screen printing processes for components such as membrane integrated circuits and membrane switches.
First, the printed circuit
Conductor lines are formed on the insulation board in a printed manner and the elements are connected. This circuit connection method is called a printed circuit.
Printed circuit boards are distinguished from materials by rigid laminates (phenolic laminates, glass cloth epoxy laminates, etc.), fired boards (polyester film, polyimine film, etc.) and inorganic insulation boards (ceramics, etc.); From the composition of the layer is divided into a single panel, double panel and multi-layer board; from the manufacturing method to distinguish the denudation (etched copper foil) method (minus method), copper plating (addition method) and conductive printing method.
A multilayer printed circuit board is made by laminating a very thin single-layer printed circuit board. Multiple layers of printed conductors are interconnected with metallization holes. The application of four layers and six layers is more advantageous. The advantage is that the volume can be further reduced, and the layers can be shielded from each other to improve the stability of the circuit work.
Flexible printed circuit boards are mostly used in computers, computers, telephones, etc. The outstanding advantages are flexible, self-terminating, and three-dimensional arrangement.
1. Copper foil laminate printed circuit. In printed circuit boards, copper foil laminated circuit boards are more commonly used. The manufacturing process of this kind of circuit board will be described in sequence with the process shown in Figs. 4-20.
1 Copper foil laminate. There are many materials used to make circuit boards. This section describes the production of copper foil laminate (single-sided, double-sided) printed circuits.
The copper foil laminate is made by coating insulating material phenolic resin or epoxy resin on paper, glass fiber cloth or polyester cloth, overlaying such several layers of paper and cloth together, and hardening it under hot pressing, under hot pressing. At the same time, single-sided or double-sided stickers will be electrolytic copper foil, pressed together. This laminate is generally produced by a specialized manufacturer.
Copper foil laminates are divided into hard and soft sheets. The thickness of copper foil is mostly 18 to 70 microns.
2 cutting. Copper foil laminates are available in both hard and soft grades. Hardboards are sold in square meters. Flexible poles are sold in rolls and must be cut to a size that is easy to use. Its cutting tools can be used as saws or shears.
3 whole surface. The entire surface is the pre-treatment of the laminate before the printed circuit. The surface of the copper foil laminate is accompanied by a release agent during hot press processing and oil or swarf at the time of cutting. These stains should be removed first before printing. The method of removal is generally performed mechanically using a grinder or a brush. This is very effective for the clearing of the cross section. The debris remaining on the cut surface will cause a failure in the subsequent printing, and must be completely removed. If necessary, use degreasing and derusting scavenger to remove.
4 punch holes. When making wiring on both sides of the laminate, if you want to connect the circuits on both sides, you must make a hole in the connection and plate the silver on the wall of the hole (metallization of the hole) so that the current can be switched on. Circuit printing can be performed only after punching copper.
The punching must be fine, and the copper foil bulging cannot occur at both ends of the hole and hole. There have been various studies on the hole drilling method, and a special punching machine for a printed circuit board has also been invented. Due to the difference in laminates and drill bits, the holes are normally drilled at a high speed of 2 to 80,000 revolutions per minute. Because laminates are generally glass fibers cured with epoxy resins, the drills must be of high hardness. After punching, it must be done once more.
5 hole copper plating. The method of copper plating on the hole wall after drilling is not electrolytic but chemical. In this method, the perforated laminate is first immersed in a salt solution, and then the reductant mixed with the copper ion solution and the formaldehyde solution is shaken to reduce the steel ions to metal due to the adsorption of hydrogen. Copper adheres to the wall of the hole.
The thickness of the copper plating is generally about 1 micron. Sometimes, in order to strengthen this layer of steel, copper pyrophosphate and copper sulfate are used for electroplating. The plating thickness after electroplating can be increased to 20-30 micrometers.
After the copper plating of the hole wall, the entire surface treatment is performed once more.
6 circuit board screen printing.
a. Printing of resist film. The printed circuit pattern is printed on the laminate to form anti-corrosion strands, which are done by screen printing.
Screen printing plate production, optional nylon mesh, polyester mesh, stainless steel mesh in any one. Stretching the tension of the net should be appropriate, and the stretched net should be used for a reward for a couple of days to make the tension even and stable. Plate production can use direct photolithography or direct photosensitization. When using direct method plate, the thickness of the coated photosensitive film should be well mastered. Generally, the printed solder mask is preferably 25-30 μm, and the printed conductors and characters are preferably 15-20 μm. With the direct legal plate, it is generally possible to print conductors of 0.3 mm or more and meet the technical requirements of printed circuit boards.
There are two cases of screen printing of a resist film. One is to use anti-corrosion ink to print the part of the circuit, that is, the resist ink is printed on the part where the copper foil needs to be retained, so that during the etching, the circuit part is retained to form a conductive circuit. The other is that the printed circuit is not printed with a resist ink, but the part other than the printed circuit is printed on the contrary, that is, the part other than the printed circuit is provided with corrosion resistance. Soldering solder is used when mounting components, and double-sided, multi-layered circuit boards are almost printed using this method.
When plating, solder or gold should be used. Plating solder should use fluorinated copper plating solution. The ratio of tin to lead in the solder is 6O:4O. Gold plating uses a weak acid solution.
The electroplated resist ink film must be able to withstand the etch of the electroplating pretreatment liquid and the electroplating liquid, and finally must be easily stripped.
When electroplating lines, they should be plated with copper to a certain thickness and then plated with solder. The electroplated thickness is 30 micrometers for copper, 10 micrometers for solder, 7 micrometers for nickel underlayer plating, and 25 microliters for gold electroplating. After the plating is completed, the resist film is peeled off, and the method is to remove the resist with an ink scavenger and a fur brush.
Anti-corrosion ink commonly known as alkali-soluble ink, screen printing printed circuit conductive patterns can be resistant to ferric chloride, acid chloride and other acidic etching liquid etching.
The single-sided copper foil laminates must be put into screen printing anti-corrosion ink immediately after drying by cutting, brushing and cleaning. Before screen printing, the anti-corrosion ink should be mixed evenly, and one do not use thinner. The screen printing scraper uses polyurethane. Rubber type, thickness is 8mm, its Shore hardness is about 70°, the shape of the scraper is right angle, the angle between the plate and the screen is 50～60°, and the static distance between the screen and the substrate is 3～5mm. Whether using screen printing machine or manual operation, the film thickness of the resist ink should be properly controlled, if the film thickness is too thick, it will cause the conductive pattern to expand, affect the precision, and if it is too thin, it will form pinholes and increase the workload of revision. After silk screen printing, it can be dried in far-infrared drying channel for 2 minutes, but when it is dried at 25-30°C in natural drying mode, it takes 4 hours or more, and the hardness of the film forming pencil reaches 2H.The dried circuit board can enter. Etching process, after the etching is completed, the film is sprayed with 1-2% NaOH dilute alkaline solution.
Anti-corrosion inks can also be formulated as follows:
Rosin (industrial grade) 0.8-1kg
Lide powder (32 mesh) 3kg
Stearic acid (industrial grade) 32g
Dark green or dark blue offset ink 200g
Turpentine oil (industrial grade) 1000～1200ml
When preparing, firstly heat and melt rosin and stearyl awakening. After melting, you must leave the heater and slowly add turpentine. Stir and do not make the rosin resolidify. Then add the ink to dissolve it. Finally, add the livid powder. Stir or roll evenly. When preparing, the number of heads of lithopone must be strictly selected; do not use water bath method to prevent water vapor from entering the printing material to make it clump.
b. Solder mask printing. Printed circuit boards that have been cleaned and etched should be screen printed with solder resist immediately. The solder resist inks are classified into two types: light-fixing and thermosetting, and each type can be divided into two types: single-component and weighted components.
Photo-curable solder mask ink. Photo-curable solder resist ink, also known as UV curable ink, has the same silk screen process conditions as resist ink. The thickness of the solder mask should be appropriate, pencil hardness should reach 6 ~ 7H. If the film surface is sticky, indicating that the curing time is insufficient or the strand is too thick, the curing time can be appropriately extended.
Thermosetting solder mask ink. The thermosetting solder mask inks are single-component in foreign countries, and the materials used in China are generally two-component. When used, the solder mask ink and the curing agent are mixed at a weight ratio of 100:5 and fully stirred, and can be used after being deaerated for 30 to 45 minutes. The solder resist after mixing has a life of 7 days and the curing condition is 120±5°C for 10 minutes. Such as the use of far-infrared Hong Road 3 to 5 minutes to cure the film.
Compared with the two types of solder mask inks, especially single-component photocurable solder mask inks have many advantages, such as no solvent, low volatility; low toxicity, can improve the production operating environment; fast drying speed, adapt to automatic or semi-automatic assembly lines. Work; The biggest advantage is that the curing temperature is low, there is no thermal reaction to the laminate, the substrate will not change and cause warping phenomenon. The adhesion and heat resistance of the thermosetting solder mask ink to the substrate is better than that of the photo-curing type. The requirements for the pre-press cleaning treatment of the printed circuit board by the radiation-solid solder mask ink are relatively strict.
c. Marking ink printing. The classification of marking inks is the same as that of solder masking inks, and they are divided into two major categories, namely light-shaping and thermosetting, and each category is divided into two types: single-component and two-component.
The printed circuit board requires two sides, namely, the insert surface and the solder surface are to be screen printed ink, and the production operation process is the same as the solder resist ink.
The film thickness of various inks has a certain influence on the quality of the printed circuit board. The thickness of the ink film is related to many process factors. Therefore, various factors must be integrated to control the thickness of the film in accordance with actual operations. Generally, the thickness of the resist ink film should reach about 20 μm, and the thickness of the solder resist film should reach 20-30 μn.
d. Circuit board silk screen positioning method. The correctness and reasonableness of the positioning method in the circuit board silk screen will directly affect the quality of the circuit board. Single-sided printed circuit boards are generally screen printed four times (anti-corrosion ink, solder mask ink, positive and negative surface marking ink). In order to ensure the accuracy of each screen printing, a strict positioning method must be adopted. At present, the inner positioning method is widely used.
The internal positioning method is to select two points on the diagonally upper line at the left bottom of the inside of the outer shape of the machining circuit of the printed circuit board. It is required that the spacing between the two points be as large as possible without affecting the electrical performance and appearance of the circuit. The aperture of the two holes is 0.8 to 1+0.5 mm.
The inner positioning method is applicable to both mechanical operation and manual screen printing. When printing silk resist for the first time, the two process holes are printed together with the circuit pattern. After the etching is completed, a double-headed bench drill is used to drill. If a single-head drilling machine is used, the operating hole position must be accurate so as to avoid an excessive distance between the two holes, resulting in inaccurate positioning. In the screen printing ink and positive and negative surface marking ink, these two process holes are located. When the printed circuit board is machined, such as punching and blanking, the two process holes should be positioned.
e. Circuit board silk screen notes. The silk screen precautions of the circuit board are listed in Table 4-29.
Table 4-29 Notices for Circuit Board Screen Printing
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