PCB Blog - Light-painting technology of PCB board making

The general process of the light drawing process of the PCB board is: check the file-determine the process parameters-CAD file transfer to gerber file-CAM processing and output.

1. Check the documents
1.1 Check the user's files, the files brought by the user should first be checked as follows.
1) Check whether the disk file is intact.
2) Check whether the file contains a virus. If there is a virus, you must first kill it.
3) Check the user data format.
4) If it is a Gerber file, check whether there is a D code table or a D code (RS274-X format).
Therefore, it is necessary to correctly analyze the data format of a certain data. In particular, it is necessary to have a deep understanding of the RS274D format in Gerber, to analyze and understand the correct and standard Aperture, and to analyze the relationship between them in detail. It is very important to read the Aperture file carefully, because sometimes there are some special cases, for example, sometimes users will propose to change an Aperture from a circle to a rectangle, from a rectangle to a heat sink, etc. If you open Gerber's original file, you will find that its data only has D code and coordinates, because the graphic file consists of three parts: coordinates, size, and shape, while the Gerber file only has coordinates, so two other conditions are required. There is an Apertuer file in the file, then open it, you will find the required data in it, if you can combine them well, you will be able to read the original data of the user.

1.2 Check whether the design conforms to the technical level of our PCB board factory
1) Check whether the various spacings designed in the customer's documents conform to the factory's process, the spacing between lines, the spacing between lines and pads, and the spacing between pads and pads. The above various spacings should be greater than this The distance that the factory production process can achieve.
2) Check the width of the wire, and it is required that the width of the wire should be larger than the line width that the factory's production process can achieve.
3) Check the size of the via hole to ensure the aperture of the factory's production process.
4) Check the size of the pad and its internal aperture to ensure that the edge of the pad after drilling has a certain width.

2. Determine the process parameters
Various process parameters are determined according to user requirements. The process parameters may have the following situations.
2.1 According to the requirements of the subsequent process, determine whether the photo-painted negative is mirror image
1) The principle of the mirror image of the film, in order to reduce the error, the film surface (ie the latex surface) must be directly attached to the film surface of the photosensitive adhesive.
2) The determinant of the mirror image of the negative film, if it is a screen printing process or a dry film process, the film surface of the negative film and the copper surface of the substrate shall prevail. If it is exposed with a diazo film, since the diazo film is mirrored during copying, the mirror image should be that the film surface of the negative film is not attached to the copper surface of the substrate. Additional mirroring is required if photopainting is done as a unit negative rather than imposition on a photopainted negative.

2.2 Determine the parameters of solder mask pattern expansion
1) Determine the principle, the increase of the solder mask pattern is subject to the wire next to the pad not exposed; the reduction of the solder mask pattern is based on the principle of not covering the pad. Due to the error during operation, the solder mask pattern may deviate from the circuit. If the solder mask pattern is too small, the result of the deviation may cover the edge of the pad, so the solder mask pattern is required to be enlarged; but if the solder mask pattern is enlarged too much, the adjacent wires may be exposed due to the influence of the deviation.

2) The determinant of the expansion of the solder mask pattern, the deviation value of the solder mask process position of our factory, and the deviation value of the solder mask pattern. Due to the different deviations caused by various processes, the expansion value of the solder mask pattern corresponding to various processes is also different. The expansion value of the solder mask pattern with large deviation should be selected larger. If the wire density of the board is high, and the distance between the pad and the wire is small, the expansion value of the solder mask pattern should be smaller; if the wire density of the board is small, the expansion value of the solder mask pattern should be larger.
3) Determine whether to add process wires according to whether the plug needs gold plating (commonly known as gold finger) on the board.
4) Determine whether to increase the conductive frame for electroplating according to the requirements of the electroplating process.
5) According to the requirements of the hot air leveling (commonly known as tin spraying) process, determine whether to add conductive process lines.
6) Determine whether to add the center hole of the pad according to the drilling process.
7) Determine whether to add process positioning holes according to the subsequent process.
8) Determine whether to add contour lines according to the shape of the board.
9) When the user's high-precision board requires high line width accuracy, it is necessary to determine whether to carry out line width correction according to the production level of our factory to adjust the influence of side erosion.

3. The light drawing of the bottom plate is output as an output
Since many printed board manufacturers do not directly use the negatives drawn by the light plotter for imaging production, but use it to remake the working negatives, here we call the light-painted negatives negatives. Before starting a new light painting, you must first adjust the parameters of the light plotter to make it in a suitable working state.

3.1 Setting of Light Plotter Parameters
1) The setting of the light source intensity, in the light drawing process, if the light source intensity is too high, the drawn graph will appear halo; if the light source intensity is too low, the drawn graph will be underexposed, so whether it is a vector light plotter There is also a problem of light intensity adjustment in laser photoplotters. There is a light intensity detection circuit in the photoplotter. When the light intensity is insufficient, the photoplotter will refuse to work or the shutter will not be opened, and an error will be displayed on the screen. Sometimes the negatives drawn by the laser photoplotter have no sign of exposure at all, which is caused by insufficient light intensity. Usually, the intensity of the light source can be controlled by adjusting the voltage of the light-emitting device. Whenever the light-emitting device is replaced or the developer is replaced, a light-drawing test strip is used to check whether the light intensity is appropriate.

2) Adjustment of the light drawing speed, the drawing speed of the light drawing machine, especially the vector light drawing machine, is also an important factor affecting the quality of the drawing. When the vector light plotter draws lines, if the drawing speed is too fast, that is, the beam stays on the film for too short time, underexposure will occur; if the drawing speed is too slow, that is, the beam stays on the film for too long, it will Overexposure appears as a halo phenomenon. Not only the speed of light painting will affect the effect of painting, but also the acceleration of light painting and the delay time of shutter opening and closing during exposure will affect the results, and these parameters also need to be carefully adjusted.

3) The placement of the negative during light painting, due to the changes of various external factors, the light-painted negative will undergo slight expansion and deformation. Generally, it will not have much impact on the processing of the printed circuit board, but sometimes it will cause Film cannot be used. Therefore, in addition to eliminating the influence of external environmental factors as much as possible, attention should also be paid to the light painting operation. When placing the film, try to ensure that the X and y directions of different layers (such as the component surface and the soldering surface) of the same printed circuit diagram to be drawn are consistent with the X and y directions of a negative film, so that the shape is somewhat deformed. identity. For some photoplotters with low precision, start from the origin of the drawing table as far as possible when drawing. When drawing graphics of the same circuit at different levels, try to be on the same coordinate range of the table, and pay attention when placing negatives. In addition, when placing the film, the film side of the film should be kept facing the light source to reduce the diffraction effect of the film medium on the light.

4) The maintenance of the negative table top, the cleaning and smoothness of the drawing table (or arc surface) is an important guarantee for the quality of the drawing. There should be no other items on the negative table (arc surface) except the negative to be drawn, and do not scratch the work. On the surface, the small holes of the vacuum adsorption film should be kept unobstructed, so that the high-precision negative can be drawn.

3.2 The drawing of the graphic base plate, when the light drawing is in normal working state, it inputs the light drawing data through the disk, RS232 port or tape (currently the tape method is rarely used), and then draws the graphics described by these data on the negative film. In fact, in addition to simple operations on the light plotter, there is no need to do more work, and a large amount of work for light plotting graphics is in the generation and processing of light plotting files.
1) The drawing of circuit chips generally only needs to generate light drawing data directly for the design graphics that have passed the review, and input the light drawing data into the light drawing machine. Usually the circuit sheet should be 1:1. For some complex circuits, attention should be paid to whether the error between the size of the primitives on the light-drawn negative and the design value will affect the production. If so, the designed primitives should be modified. size to compensate for deviations in lightpaint values.

2) The drawing of the solder mask requires lower requirements for the solder mask than the circuit, but according to different process requirements, the pad of the solder mask should be larger than that of the circuit, and should be added when generating the light drawing data of the solder mask. Notice.

3) The drawing of character slices has slightly lower requirements for character slices. However, since the characters of the device are often transferred from the library along with the device during layout, the size of the characters and the line width of the characters are often uneven. Some characters are too small and will be blurred when printed with ink; some lines are too thin and the screen printing effect is not good. When drawing files, try to combine the line widths of characters into one or more types to make them meet the technological requirements.

4) For the drawing of the drilling slices, it is generally not necessary to draw the drilling film, but sometimes in order to better check the drilling conditions or clearly distinguish the apertures, a drilling slice can also be drawn. For vector photoplotters, when drawing boreholes that distinguish apertures, saving photoplotting time should be taken into account, that is, when generating photoplot data, attention should be paid to using simple symbols to identify apertures.

5) Drawing of large-area copper-clad power supply and stratum. For standard designed power supply and stratum, the negative drawn according to the design is opposite to the pattern on the printed board, that is to say, the unexposed part of the negative is the copper foil. The part with graphics on the negative is an isolated part on the printed board, and there is no copper layer. Due to the needs of the process, when drawing the power supply and the ground layer, the isolation pad should be larger than the pad of the circuit layer. For the holes connected to the power supply or the ground layer, do not draw nothing, but draw special flower pads. This is not only a matter of ensuring the solderability, but more importantly, it is beneficial to the inspection of the negative film. It is clear at a glance which position has holes, which positions have no holes, and which holes are connected to power or ground.

6) Mirror drawing, because the film surface (graphic surface) of the negative film needs to be attached to the dry film attached to the copper foil of the printed circuit board during the imaging process of the printed circuit board. Therefore, when drawing the film, the phase of the pattern (that is, the front and back of the pattern surface) should be considered, and the method of adjusting the phase of the pattern by reversing the film surface of the negative is not recommended, and when several small patterns are drawn on one This method also does not make them out of phase when on large negatives, and should be taken care of when generating the photoplot data file. Under normal circumstances, since it is necessary to turn the film before imaging with the negative, the generated light drawing data should be positive phase for the single-numbered layer (1, 3, 5, ..., layer) pattern of the printed circuit board, facing the double-numbered layer. Layer graphics, the graphics described by the generated light drawing data should be mirror graphics. If the printing plate imaging processing is performed directly with the light-painted film, the phase mentioned above should be reversed.

7) The identification of the graphic level is very important to identify the level of the printed board corresponding to the graphic of the negative. For example, for a simple single panel, if the surface (layer) where the graphic is located is not identified, the welding surface may be made into the component surface. It will cause the device to be difficult to install, especially for double-sided and multi-layer boards. Some printed board aided design software can automatically add the level of the graphic when generating the light drawing file, which undoubtedly brings a lot of convenience. However, two points should be paid attention to in application. First, whether the level of printed circuit wiring is the level arranged by processing; second, the zero point of the graphic is often far away from the coordinate origin during design, and the automatically added level mark is near the coordinate origin. , so that there will be a large interval between the level mark and the graphics, which will not only affect the effect of the logo, but also cause a waste of negatives.

8) Aperture matching, whether it is a vector photoplotter or a laser photoplotter, there is a problem of aperture matching. If a 40mil pad is used in the design graphics, and a 50mil aperture is used for light drawing, obviously the drawn graphics will be different, but since the size of the primitives (line segments, pads) can be arbitrarily set during graphic design, Therefore, if the aperture of the photoplotter is required to be completely consistent with it, it is impossible for a vector photoplotter, and it is also troublesome for a laser photoplotter. When the aperture is completely matched, due to factors such as focusing and development, the size of the primitives drawn on the negative will still be slightly different from the design value. Therefore, in the actual processing process, as long as the processing technology allows, the existing aperture (for vector photoplotters) or the set aperture (for laser photoplotters) can be selected. In many cases it is permissible to use a 50mil aperture to correspond to a design value of 46mil or 55mil, and even a 60mil aperture to correspond to a design value of 40mil on PCB board.