PCB exposure skills and basic knowledge

Printed circuit boards, also known as printed circuit boards, are providers of electrical connections for electronic components. It 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, improve the level of automation and production labor rate. Today, the macro link circuit Xiaobian takes you to understand the circuit board exposure skills and basic knowledge.

The first is exposure

When PCB manufacturers process the board, under ultraviolet light, the photoinitiator absorbs the light energy and decomposes into free groups, which then trigger the photopolymeric monomer for polymerization crosslinking reaction, and forms a large molecular structure insoluble in dilute alkali solution after the reaction. Exposure is generally carried out in the automatic double-sided exposure machine, and now the exposure machine can be divided into air-cooled and water-cooled according to the different cooling methods of the light source.

Factors affecting the quality of exposure imaging

In addition to the performance of dry film photoresist, the selection of light source, the control of exposure time (exposure amount), and the quality of photographic substrate are all important factors affecting the quality of exposure imaging.

1) Choice of light source

Any kind of dry film has its own unique spectral absorption curve, and any kind of light source 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 match well and the exposure effect is good.

The spectral absorption curve of domestic dry film shows that the spectral absorption range is 310-440nm (nm). From the spectral energy distribution of several light sources, it can be seen that the pick lamp, high-pressure mercury lamp and iodogallium lamp have a 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 dry film exposure.

After the type of light source is selected, a light source with high power should also be considered. Because the light intensity is large, the resolution is high, and the exposure time is short, the degree of heat deformation of the photographic plate is small. In addition, the lamp design is also very important, to try to make the incident light uniformity is good, high parallelism, in order to avoid or reduce the poor effect after exposure.

2) Control of exposure time (exposure amount)

In the exposure process, the photopolymerization reaction of the dry film is not "one primer" or "one exposure is ready", but generally through three stages.

Due to the presence of oxygen or other harmful impurities in the dry film, it needs to go through an induction process, in which the free group generated by the decomposition of the initiator is consumed by oxygen and impurities, and the polymerization of monomers is very small. However, when the induction period passes, the photopolymerization reaction of the monomer takes place quickly, and the viscosity of the film increases rapidly, close to the degree of mutation, which is the stage of rapid consumption of the photosensitive monomer, and the proportion of time in the exposure process of this stage is very small. When most of the photosensitive monomer is consumed, it enters the monomer depletion zone, and this time the polymerization reaction has been completed.

Correct control of exposure time is a very important factor to obtain excellent dry film image. When the exposure is insufficient, due to the incomplete polymerization of monomers, during the development process, the film melts and becomes soft, the lines are not clear, the color is dull, and even degumming. In the pre-plating treatment or electroplating process, the film warps, permeates and even falls off. When the exposure is too much, it will cause difficulties in developing, brittle film, leaving residual glue and other ills. What is more serious is that incorrect exposure will produce a deviation of the image line width, excessive exposure will make the graphic plating line thinner, make the printed etching line thicker, on the contrary, underexposure makes the graphic plating line thicker, make the printed etching line thinner.

How to determine the correct exposure time?

Due to the different exposure machines used by various manufacturers of films, that is, the light source, the power of the lamp and the lamp distance are different, it is difficult for dry film manufacturers to recommend a fixed exposure time. Foreign companies producing dry film have their own or recommended use of some kind of optical density ruler, the dry film factory is marked with the recommended imaging level, China's dry film manufacturers do not have their own optical density ruler, usually recommend the use of iston 17 or stouffer 21 optical density ruler.

The optical density of the Rayston 17 optical density scale is 0.5, and the optical density difference AD increases by 0.05 for each subsequent stage, until the optical density of the 17 level is 1.30. The optical density of the Stuffer 2l optical density scale is 0.05, and then each stage increases with the optical density difference △D by 0.15 to the optical density of the 2l level is 3.05. When the optical density scale is exposed, the light density is small (that is, more transparent) grade, the dry film accepts more ultraviolet light energy, and the polymerization is more complete, and the light density is large (that is, the degree of transparency is poor) grade, the dry film accepts less ultraviolet light energy, and the polymerization does not occur or the polymerization is incomplete, and is displayed or only left part of it during development. In this way, different exposure times can be used to obtain different imaging levels.

The use of Ruston 17 optical density ruler is described as follows:

a. When exposure is made, the film is downward;

b. Put the film on the copper-clad plate for 15 minutes and then expose it.

c. After exposure, leave for 30 minutes to develop. Any exposure time is selected as the reference exposure time, expressed by Tn, and the series left after development is called the reference series. The recommended use series is compared with the reference series, and calculated according to the coefficient table of [sensitive word].

Series difference

Coefficient K

Series difference

Coefficient K

one

1.122

6

2.000

2

1.259

7

2.239

3

1.413

8

2.512

4

1.585

9

2.818

5

1.778

10

3.162

When the use series needs to be increased compared to the reference series, the exposure time of the use series T = KTR. When the use series needs to be reduced compared to the reference series, the exposure time of the use series T = TR/K. In this way, the exposure time can be determined by only one test.

In the case of no light density scale can also be observed by experience, using the method of gradually increasing the exposure time, according to the brightness of the dry film after development, whether the image is clear, whether the image line width is consistent with the original negative to determine the appropriate exposure time. Strictly speaking, it is unscientific to measure exposure by time, because the intensity of the light source often changes with fluctuations in external voltage and the aging of the lamp. The light energy is defined by the formula E = IT, where E represents the total exposure, in millijoules per square centimeter; I represents the intensity of light in milliwatts per square centimeter; T is the exposure time, in seconds. As can be seen from the above formula, 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 amount also changes, so although the exposure time is strictly controlled, the total exposure amount accepted by the dry film at each exposure is not necessarily the same, and the degree of polymerization is different. To make each exposure the same energy, a light energy integrator is used to measure the 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 substrate

The quality of the photographic substrate is mainly manifested in two aspects: optical density and dimensional stability.

For optical density, the optical density Dmax is greater than 4, and the minimum optical density Dmin is less than 0.2. Optical density refers to the lower limit of the surface light blocking film in the left ultraviolet light of the base plate, that is, when the optical blocking density of the opaque area of the base plate exceeds 4, a good light blocking purpose can be achieved. The minimum optical density refers to the upper limit of light blocking presented by the transparent film outside the backplate in ultraviolet light, that is, when the optical density Dmin of the transparent area of the backplate is less than 0.2, good light transmission can be achieved. The dimensional stability of the photographic substrate (referring to changes in temperature, humidity and storage time) will directly affect the dimensional accuracy and image overlap of the printed board, and the serious expansion or reduction of the photographic substrate size will cause the photographic substrate image to deviate from the drilling of the printed board. The original domestic SO hard film is affected by temperature and humidity, the size changes greatly, the temperature coefficient and humidity coefficient are about (50-60)×10-6 / ℃ and (50-60)×10-6 / %, for a length of about 400mm S0 base version, the size change in winter and summer can reach 0.5-1mm, The distance from half a hole to a hole may be skewed when imaging on a printed board. Therefore, the production, use and storage of photographic plates are in a constant temperature and humidity environment.

The use of thick polyester based silver salt sheets (e.g. 0.18mm) and diazo sheets can improve the dimensional stability of photographic substrates. 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.

Exposure positioning

1) Visual positioning

Visual positioning is usually suitable for the use of diazo plates, diazo plates are brown or orange translucent; However, it is not transparent to ultraviolet light, through the diazo image, the welding pad of the bottom plate is aligned with the hole of the printed board, and the exposure can be fixed with tape.

2) Out of stock positioning system positioning

The out-of-stock positioning system includes a photographic film punch and a double round hole out. The positioning method is as follows: first, align the front and back plates of the drug film under the microscope; Use a film punch to punch two positioning holes outside the effective image of the base plate. Take one of the base plate with the positioning holes and program the drilling process to obtain the data tape with the component holes and positioning holes drilled at the same time. After drilling the component holes and positioning holes at one time, the metalizing holes of the printed board and the pre-copper plating, the double round holes can be used for positioning exposure.

3) Fix pin positioning

The fixed pin is divided into two sets of systems, one set of fixed photographic plate, the other set of fixed printed board, by adjusting the position of the two pins, to achieve the coincidence and alignment of the photographic plate and printed board. After exposure, the polymerization reaction will continue for a period of time, in order to ensure the stability of the process, do not immediately remove the polyester film after exposure, so that the polymerization reaction can continue. Remove the polyester film before development.