Printed Circuit Board Production Analysis (1)

For single-layer printed circuit boards where the printed circuit board pattern is relatively simple, a normal screen pattern is printed by screen printing (referred to as a screen printing method), and then the printed circuit board is etched. Photo-chemical image transfer can also be used to make printed circuit boards. For the general requirement is not high or a small number of circuit boards are produced, the printed circuit board can be produced by screen printing. Under the amateur condition, the printed circuit board is also made by the screen printing method. For demanding circuit boards, photochemical transfer methods are generally used to make printed circuit boards. In the laboratory, thermal transfer printing methods are often used to make printed circuit boards. These methods and steps for making printed circuits are described below.

1. Photochemical transfer method for making printed circuits
Photochemical transfer method and silk screen printing method to produce printed circuit, all need to design the black and white picture of the printed circuit first, make the photographic negative film from the black and white picture, and then make the circuit to the copper-clad plate through different technological process. The process of making printed circuits in two ways is shown in Figure 2-1.

(1) Designing a Printed Circuit (Black and White)

The first step, component layout

According to the characteristics of the circuit and the size of the circuit board, the layout of the circuit components on the printed circuit board layout.

The main considerations for the layout of components are to arrange components in a compact, orderly, and aesthetically pleasing manner, and to pay attention to the insulation performance and heat dissipation performance between components to maintain a proper distance. Between levels should be as close as possible to shorten the distance. Heating elements should be arranged in a position that is conducive to heat dissipation. Components with strong electromagnetic radiation and those sensitive to electromagnetic induction should be carefully arranged to avoid interference between them. Integrated circuits and transistors must maintain a certain distance from other components, which is conducive to heat dissipation. There should be no overlap or crossover between components. Heavy and large components may not be mounted on the circuit board. Due to the limitation of the size of the printed board, it is not possible to design the circuit board of the security department or the circuit of different functions. For the reason of shielding or debugging, inspection and maintenance, etc., the printed circuit board may be designed in several pieces.

The second step, printed circuit wiring and black and white drawing
The main considerations for the design of printed circuit wiring are: the width of the copper skin is determined by the strength of the transmitted signal, the size of the current flowing through the circuit, etc., for the general circuit width is not less than 0.5mm, the power line and ground may be 1.5- 2.5mm, the printed circuit board peripheral ground circuit can be wider, the outer diameter of the pad is not less than 2mm. The distance between the circuit and the circuit is generally not less than 0.5mm. The common ground wire is arranged at the most edge of the circuit board, and there should be a proper distance between the edge circuit and the edge of the printed board to facilitate the installation and mechanical processing. The circuit should pay attention to avoid long winding or parallel wiring to reduce the influence of distributed capacitance. The high-frequency circuit should pay special attention to the shielding of the circuit, and the ground line can be designed to be larger, and the ground line of each stage circuit can form a loop. The input and output lines should be far away, not close or parallel. For digital signal circuits, there are many connections, small signal currents, and the line width can be designed to be smaller. For large-area copper foil, it is best to hollow out the grid. For the individual can not be bypassed, because the printed circuit does not cross, you can use external wiring (also called jumper or jumper) method to connect.

Black and white drawing method:

1 hand draw

First, draw a sketch of a printed circuit layout.
Then use ordinary coordinate paper, drawing paper, or a dedicated photographic baseplate with a coordinate grid to draw a black and white draft of the printed circuit. The size of the black and white manuscript is preferably 2 or 3 times that of the actual printed circuit, that is, it is scaled up and drawn. In order to reduce the size of the film after the line can reduce the edge of the glitch.

When drawing a black and white draft of a printed circuit, the circuit diagram is drawn on a coordinate drawing with a pencil with a higher hardness. Then use the drawing tools (commonly used tools include compasses, small circle compasses, duckbill pens) to trace ink lines. Ink is often used to grind ink. Finally, fill in the black and repair work to make the line smooth and free of burrs along the edges.

The black-and-white printed circuit diagrams are drawn and sent to the printing factory or the makers of the plate-making photomultipliers to make glass negatives (wet sheets) or films (dry sheets).

If it is a sample or just a few circuit boards, draw pens and carbon inks directly on the polyester film that is roughened with water sandpaper.

2 Computer Drawing

Computer-aided design software (such as Protel) can now be used to design printed circuits, print black-and-white drawings with laser printers or inkjet printers, and then produce.

3 tape texture

It is also commonly used to produce different types of black tapes with different shapes and specifications. This self-adhesive tape can be used to print black and white printed circuit.

(2) Clean the surface of the copper-coated board

According to the size of the design drawings, the large copper-clad board is formed into small pieces (undercuts) of the required area size. After the material is unloaded, the surface of the copper-clad board is cleaned before the circuit is copied onto the copper-clad board due to the presence of an unclean surface, impregnated with oil stains, and an oxide layer on the surface of the original copper-clad board. The method is: Put the copper-clad board in 5% hydrochloric acid or 3% sulfuric acid for several tens of seconds. See the surface of the board is red, and remove it. Use a copper wire polishing wheel or copper wire brush to remove oxides on the copper foil surface. And oil.

If the bone glue solid film method is used, the cleaned copper sheet is immersed in a 3% dichromic acid solution for use (the purpose of the immersion solution is to prevent surface oxidation).

If spray paint is used as a protective layer, it is only necessary to polish the surface with a cloth polishing wheel.