Due to the different exposure machines used by different manufacturers of film, that is, the light source, lamp power and lamp distance are different, so it is difficult for dry film manufacturers to recommend a fixed exposure time. Foreign companies that produce dry film have their own or recommended optical density scales. The dry film is marked with the recommended imaging series when it leaves the factory. my country's dry film manufacturers do not have their own optical density scales. It is usually recommended to use Reese. 17-level or Stouffer (stouffer) 21-level densitometer.

　　The optical density of the Reston 17-level optical density ruler is 0.5, and the optical density difference AD is incremented by 0.05 for each subsequent level, and the optical density of the 17th level is 1.30. The optical density of the Stuffier 21-level optical density ruler is 0.05, and the optical density difference ΔD of each subsequent level is increased by 0.15, and the optical density of the 21-level optical density is 3.05. When exposing with an optical density ruler, the grade with low optical density (ie more transparent), the dry film receives more ultraviolet light energy, and the polymerization is more complete, while the grade with high optical density (ie, poor transparency), dry film The film receives less ultraviolet light energy, does not polymerize or is incompletely polymerized, and is revealed or only partially left during development. In this way, different exposure times can be selected to obtain different imaging levels.

　　The method of using the Reston 17-level optical density ruler is now introduced as follows:

　　a. During exposure, the film is down;

　　b. Put the film on the copper clad laminate for 15 minutes and then expose it;

　　c. After exposure, it was left for 30 minutes to develop. Choose one exposure time as the reference exposure time, denoted by Tn, the series left after development is called the reference series, compare the recommended series with the reference series, and calculate according to the following coefficient table.

　　When the number of levels used needs to be increased compared with the reference level, the exposure time T=KTR of the number of levels used. When the number of levels used needs to be reduced compared to the reference level, the exposure time T=TR/K of the number of levels used. This allows only one experiment to determine the exposure time.

　　In the absence of an optical density ruler, it can also be observed empirically. The method of gradually increasing the exposure time is used to determine the appropriate exposure according to the brightness of the dry film after development, whether the image is clear, and whether the line width of the image is consistent with the original film. time. Strictly speaking, it is unscientific to measure exposure in time, because the intensity of the light source often changes with the fluctuation of the external voltage and the aging of the lamp. The formula E=IT for the definition of light energy, where E represents the total exposure, in millijoules/square centimeter; I represents the light intensity, in milliwatts/square centimeter; T is the exposure time, in seconds. It can be seen from the above formula that the total exposure E varies with the light intensity I and the exposure time T. When the exposure time T is constant, the light intensity I changes, and the total exposure also changes, so although the exposure time is strictly controlled, the total exposure received by the dry film in each exposure is not necessarily the same, so The degree of aggregation is also different. Exposures were metered using a light energy integrator so that the energy was the same for each exposure. The principle is that when the light intensity I changes, the exposure time T can be automatically adjusted to keep the total exposure E unchanged.

　　3) The quality of the photographic plate

　　The quality of photographic negatives is mainly reflected in optical density and dimensional stability.

　　Regarding the optical density, the optical density Dmax is required to be greater than 4, and the minimum optical density Dmin is less than 0.2. Optical density refers to the lower limit of the light blocking film on the surface of the left ultraviolet light of the bottom plate, that is to say, when the light blocking density Dmax of the opaque area of the bottom plate exceeds 4, the good light blocking purpose can be achieved. The minimum optical density refers to the upper limit of light blocking exhibited by the transparent film base other than the light blocking film of the bottom plate in the ultraviolet light, that is to say, when the optical density Dmin of the transparent area of the bottom plate is less than 0.2, good light transmission can be achieved. The dimensional stability of the photographic plate (referring to changes with temperature, humidity and storage time) will directly affect the dimensional accuracy and image coincidence of the printed board. The serious expansion or reduction of the size of the photographic plate will make the image of the photographic plate and the printed plate. The drill hole is deviated. The original domestic SO rigid film is affected by temperature and humidity, and its size changes greatly. Its temperature coefficient and humidity coefficient are about (50-60)×10-6/℃ and (50-60)×10-6/%. For an S0 plate with a length of about 400mm, the size change in winter and summer can reach 0.5-1mm, and the distance from half a hole to a hole may be offset when imaging on a printed board. Therefore, the production, use and storage of photographic plates are all in a constant temperature and humidity environment.

　　The use of thick polyester base silver salt flakes (eg 0.18mm) and diazonium flakes can improve the dimensional stability of the photographic negative. In addition to the above three main factors, the vacuum system of the exposure machine and the choice of vacuum frame materials will also affect the quality of exposure imaging.

Read recommendations:

Display board (six layers)

Six-layer Immersion Gold Board (BGA)

Six-layer Immersion Gold Board (BGA)

Mobile phone board

Tips for Electromagnetic Problems in PCB Design