A printed circuit board, also known as a printed circuit board, is a provider of electrical connections for electronic components. Its development has a history of more than 100 years; its design is mainly layout design; the main advantage of using circuit boards is to greatly reduce wiring and assembly errors, and improve the automation level and production labor rate. Today, the editor of Honglian Circuit will take you to understand the circuit board exposure skills and basic knowledge.

　　The first is exposure

　　Exposure means that when the PCB manufacturer is processing the board, under the irradiation of ultraviolet light, the photoinitiator absorbs the light energy and decomposes into free radicals, and the free radicals trigger the photopolymerization monomer to carry out the polymerization and crosslinking reaction. Body size macromolecular structure. Exposure is generally carried out in an automatic double-sided exposure machine. Now the exposure machine can be divided into two types: air-cooled and water-cooled according to the cooling method of the light source.

　　Factors Affecting Exposure Image Quality

　　In addition to the performance of the dry film photoresist, the factors that affect the quality of exposure imaging are the selection of light sources, the control of exposure time (exposure amount), and the quality of photographic plates.

　　1) The choice of light source

　　Any kind of dry film has its own unique spectral absorption curve, and any kind of light source also has its own emission spectral curve. If the main peak of spectral absorption of a certain dry film can overlap or mostly overlap with the main peak of spectral emission of a certain light source, the two are well matched and the exposure effect is achieved.

　　The spectral absorption curve of the domestic dry film shows that the spectral absorption region is 310-440 nm (nanometer). From the spectral energy distribution of several light sources, it can be seen that pick lamp, high pressure mercury lamp and iodine gallium lamp have relatively large relative radiation intensity in the wavelength range of 310-440nm, which is an ideal light source for dry film exposure. Xenon lamps are not suitable for exposure of dry films.

　　After the light source type is selected, the light source with high power should also be considered. Because the light intensity is high, the resolution is high, and the exposure time is short, the degree of thermal deformation of the photographic plate is also small. In addition, the design of lamps is also very important. It is necessary to try to make the incident light uniform and parallel, so as to avoid or reduce the poor effect after exposure.

　　2) Control of exposure time (exposure amount)

　　During the exposure process, the photopolymerization reaction of the dry film is not "one-shot" or "one-exposure", but generally goes through three stages.

　　Due to the obstruction of oxygen or other harmful impurities in the dry film, an induction process is required, in which the free radicals generated by the decomposition of the initiator are consumed by oxygen and impurities, and the polymerization of the monomer is minimal. However, when the induction period is over, the photopolymerization of the monomer proceeds rapidly, and the viscosity of the film increases rapidly, approaching the level of sudden change. This is the stage of rapid consumption of the photosensitive monomer, and the time occupied by this stage during the exposure process The ratio is small. When most of the photosensitive monomer is consumed, it enters the monomer depletion zone, and the photopolymerization reaction has been completed at this time.

　　Correct control of exposure time is a very important factor in obtaining good dry film resist images. When the exposure is insufficient, due to the incomplete polymerization of the monomers, during the development process, the film swells and becomes soft, the lines are not clear, the color is dim, and even degumming. , or even fall off. When the exposure is too high, it will cause problems such as difficulty in development, brittle film, and residual glue. What is more serious is that incorrect exposure will cause deviation of image line width. Excessive exposure will make the lines of pattern plating thinner and the lines of printing and etching thicker. On the contrary, insufficient exposure will make the lines of pattern plating thicker. Makes the printed etched lines thinner.

Read recommendations:

Six-layer Immersion Gold Board (BGA)

High frequency PCB

Six-layer Immersion Gold Board (BGA)

Six-layer Immersion Gold Board (BGA)

What is the relationship between PCB design and microcontroller