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PCB Tech

PCB Tech

PCB plate making technology: CAM and light painting process
2021-08-19
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Author:IPCB

PCB manufacturing technology, including computer-aided manufacturing processing technology, that is, CAD/CAM, as well as light painting technology, the general process of light painting technology is: check the file-determine the process parameters-CAD file to Gerber file-CAM processing and output .


1. Computer Aided Manufacturing Processing Technology


Computer Aided Manufacturing (CAM) is to perform various process treatments according to a predetermined process. The various process requirements mentioned above must make necessary preparations before light painting. Such as mirroring, solder mask enlargement, process line, process frame, line width adjustment, center hole, outline line and other issues must be completed in this process of CAM. Special attention should be paid to the small spaces in the user files, which must be dealt with accordingly.


Because the process flow and technical level of each factory are different, to meet the final requirements of the user, necessary adjustments must be made in the production process to meet the user's requirements in various aspects such as accuracy. Therefore, CAM processing is an indispensable process in modern printed circuit manufacturing.


1. Work done by CAM


① The size of the pad is revised, and the D code is combined.

②Correction of line width, combined with D code.

③Check the minimum spacing, between the pad and the pad, between the pad and the line, and between the line and the line.

④Check the aperture size and combine them together.

⑤Check the minimum line width.

⑥Determine the expansion parameters of the solder mask.

⑦ Perform mirroring. ,

⑧Add various craft lines and craft frames.

⑨ Carry out line width correction in order to correct the undercut.

⑩Form the center hole.

⑩Add the shape angle line.

⑥Add positioning holes.

⑩ Imposition, rotation, mirroring.

⑩ Piece together.

⑩Graph overlay processing, corner cutting and tangent processing.

⑩Add user trademark.


2. CAM process organization


As there are dozens of popular CAD software on the market now, the management of CAD procedures must first be organized from the perspective of organization, and a good organization will achieve the effect of getting twice the result with half the effort. Since the Gerber data format has become a standard in the light painting industry, Gerber data should be used as the processing object in the entire light painting process. If CAD data is used as the object, the following problems will arise.


(1) There are too many types of CAD software. If various technological requirements must be completed in the CAD software, every operator is required to be proficient in the operation of each kind of CAD software. This will require a long training period to enable operators to become skilled workers and to meet actual production requirements. This is uneconomical from a time and economic point of view.


(2) Due to the numerous process requirements, some requirements cannot be achieved with certain CAD software. Because the CAD software is used for design, it does not take into account the special requirements in the process, so it cannot meet all the requirements. The CAM software is specially used for process processing, and it is the best to do these tasks.


(3) The CAM software has powerful functions, but all of them operate on Getber files and cannot operate on CAD files.


(4) If CAD is used for process processing, each operator is required to be equipped with all CAD software, and there are different process requirements for each kind of CAD software. This will cause unnecessary confusion to management.


In summary, the CAM organization should be the following structure (especially large and medium-sized enterprises).


① All process processing is unified with Gerber data as the processing object.

②Each operator must master the skills of converting CAD data into Gerber data.

③Each operator must master one or several CAM software operation methods.

④ Develop a unified process specification for Gerber data files.

⑤The CAM process can be handled by several operators in a relatively centralized manner for management.


A reasonable organization will greatly improve management efficiency, production efficiency, and effectively reduce the error rate, so as to achieve the effect of improving product quality.


2. Light painting process


The general process of the light painting process is: check the file, determine the process parameters, convert the CAD file to the Gerber file, and process and output the CAM.


1. Check file


(1) Check the user's file The file that the user brings, first of all check the following.


① Check whether the disk file is intact.

②Check whether the file contains viruses. If there is a virus, it must be disinfected first.

③Check the user data format.

④ If it is a Gerber file, check whether there is a D code table or contains D code (RS274-X format).


The original data provided by the user, the usual format is as follows.


Gerber (RS274D&RS274X);

HPGL1/2 (HP Graphic Layer);

Dxf&Dwg (Autocad for Windows);

Protel format(DDB\pcb\sch\prj);

Oi5000 (Orbotech output format);

Excellon1/2 (drill\rot);

IPC-D350 (netlist);

Pads2000 (job).

ATL

Therefore, it is necessary to correctly analyze the data format of a certain data. Especially the format of RS274D in Gerber should be deeply understood, the correct and standard Aperture should be analyzed and understood, and the relationship between them should be analyzed in detail. It is very important to read the Ap eture file carefully, because sometimes there are some special situations. For example, sometimes the user will propose to change an Aperture from a circle to a rectangle, from a rectangle to a heat sink, etc. If you open the original Gerber file, you will find that its data only has D code and coordinates, because the graphics file consists of three parts: coordinates, size, and shape, while the Gerber file only has coordinates, so two other conditions are required. If it is received If there is an Apertuer file in the file, open it and you will find that there is the required data in it. If you can combine them well, you will be able to read the original data of the user.


(2) Check whether the design meets the technical level of the factory


①Check 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, the spacing between pads and pads, the above various spacings should be greater than this The smallest distance that can be achieved by the factory's production process.

②Check the width of the wire. The width of the wire should be greater than the minimum line width that can be achieved by the factory's production process.

③Check the size of the via hole to ensure the smallest diameter of the factory's production process.

④Check 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 process parameters


Various process parameters are determined according to user requirements. The process parameters may have the following situations.


(1) Determine whether the light-painted film is a mirror image according to the requirements of the subsequent process


①The principle of negative film mirroring In order to reduce the error, the film surface (that is, the latex surface) must be directly attached to the film surface of the photosensitive glue.


②The decisive factor for the mirror image of the film. If it is a screen printing process or a dry film process, the copper surface of the substrate on the film side of the film shall prevail. If the diazo film is used for exposure, since the diazo film is a mirror image when copied, its mirror image should be the film surface of the negative film without the copper surface of the substrate. If the light-painting is a unit film, instead of imposition on the light-painting film, you need to add another mirror image.


(2) Determine the parameters for the expansion of the solder mask pattern


① Principle of determination The increase of the solder mask pattern is based on not exposing the wire next to the pad; the reduction of the solder mask pattern is based on the principle of not covering the pad. Due to errors 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 wires next to it may be exposed due to the influence of the deviation.


② The determinants of the expansion of the solder mask pattern The deviation value of the solder mask process position and the deviation value of the solder mask pattern of our factory. Due to the different deviations caused by various processes, the enlargement values of the solder mask patterns corresponding to various processes are also different. The enlargement value of the solder mask pattern with large deviation should be selected larger. The board wire density is large, the distance between the pad and the wire is small, and the solder resist pattern expansion value should be selected smaller; the board wire density is small, and the solder mask pattern expansion value can be selected larger.


(3) Determine whether to add process wires according to whether the plug is gold-plated (commonly known as gold fingers) on the board.

(4) Determine whether to add a conductive frame for electroplating according to the requirements of the electroplating process.

(5) Determine whether to add a conductive process line according to the requirements of the hot air leveling (commonly known as tin spraying) process.

(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 an outline corner line according to the board shape.

(9) When the user's high-precision board requires high line width accuracy, it is necessary to determine whether to perform line width correction according to the factory's production level to adjust the influence of side erosion.


3. Light-drawing of the base plate as an output


Since many printed board manufacturers do not directly use the negatives drawn by the light-painting machine for imaging production, but use it to remake the working negatives, here we call the light-painting negatives the master. Before starting the light drawing, the parameters of the light drawing machine must be adjusted to make it in a suitable working state.


(1) Setting the parameters of the light plotter


① Setting the intensity of the light source In the light drawing process, if the intensity of the light source is too high, the drawn graphics will appear halo; if the intensity of the light source is too low, the drawn graphics will be underexposed, so whether it is a vector light plotter or a laser All light plotters have a light intensity adjustment problem. A light intensity detection circuit is installed in the high-end light plotter. When the light intensity is insufficient, the light plotter will refuse to work or the shutter will not open, and an error will be displayed on the screen. Sometimes the film drawn by the laser light plotter shows no signs of exposure at all, which is caused by insufficient light intensity. Generally, 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 piece is used to check whether the light intensity is appropriate.


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


③ Due to the changes of various external factors, the placement of the negative film during light painting, the light painting negative film will be slightly stretched and deformed. Under normal circumstances, it will not have much impact on the processing of the printed circuit board, but sometimes it will also make the negative film unable to use. 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 a negative film, try to ensure that the X and y directions of different layers of the same printed circuit diagram (such as component surface and soldering surface) to be drawn are consistent with the X and y directions of a negative film. Identity. For some low-precision plotters, start from the origin of the drawing table as much as possible when drawing the film. When drawing graphics of the same circuit at different levels, try to be on the same coordinate range of the table as possible, and pay attention when placing the film. In addition, when placing the film, keep the film surface of the film facing up to the light source to reduce the diffraction effect of the film medium on the light.


④Maintenance of the negative film table The clean and flat surface of the drawing table (or curved surface) is an important guarantee for the quality of drawing. There should be no other items on the table surface (or curved surface) of the negative film except the film to be drawn, and do not scratch the working surface. The small holes of the vacuum suction film should be kept unblocked, so that the high-precision film can be drawn.


(2) The drawing of the graphic base plate. When the light drawing is in the normal working state, it inputs light drawing data through disk, RS232 port or tape (the tape method is rarely used at present), and then draws the graphics described by these data on the negative. . In fact, there is no need to do more work in addition to simple operations on the gerber plotter. A large amount of work on the gerber plot is in the generation and processing of the gerber file.


①The drawing of the circuit piece generally only needs to directly generate the light drawing data for the approved design graphics, and input the light drawing data into the light drawing machine. Usually the circuit slice should be 1:1, for some ratios


For more complex circuits, you should pay attention to whether the error of the graphic element size on the light painting film and the design value will affect the production. If there is an impact, the designed graphic element size should be modified to compensate for the deviation of the light painting value.


②Drawing of solder mask The requirement for solder mask is lower than that of circuit chip. However, according to different process requirements, the pad of the solder mask should be larger than the circuit chip. Pay attention to the light drawing data of the solder mask.


③Drawing of character slices The requirements for character slices are slightly lower, but because the characters of the device are often called out from the library with the device during the 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 silk screen effect is not good. This requires careful inspection of the characters before generating the light drawing file of the characters, and the light of the characters is generated. When drawing files, try to combine the line widths of characters into one or more types to make them meet the process requirements.


④Drawing of drilling film. Under normal circumstances, it is not necessary to draw a drilling film, but sometimes in order to better inspect the drilling situation or clearly distinguish the hole diameter, you can also draw a drilling film. For vector light plotters, when drawing holes with different apertures, you should consider saving light drawing time, that is, you should pay attention to using simple symbols to identify the apertures when generating light drawing data.


⑤ Drawing of large-area copper-clad power supply and ground layer. For the power supply and ground layer of standard design, the negative film drawn according to the design is opposite to the graphics on the printed board. That is to say, the unexposed part of the negative film is copper foil, and the negative film The patterned part is an isolated part on the printed board without a copper layer. Due to the needs of the process, when drawing the power supply and ground layer, the isolation disk should be larger than the pad of the circuit layer. For the hole connected to the power supply or ground layer, it is best not to draw nothing, but to draw a special soldering. Disk, this is not only a problem of ensuring solderability, but more importantly, it is beneficial to the inspection of the film. It is clear at a glance which position has a hole, which position has no hole, and which hole is connected to power or ground.


⑥ Mirror image drawing The film surface (graphic surface) of the negative film needs to be pasted on 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 a film, the problem of the phase of the graphics (that is, the front and back of the graphic surface) should be taken into consideration. When filming large negatives, this method can not make their phases different, and care should be taken when generating the gerber data file. Under normal circumstances, because the film needs to be turned once before imaging with the negative, the light drawing data generated for the single-layer (1, 3, 5,..., layer) graphics of the printed circuit board should be positive phase, facing the double Several layers of graphics, the graphics described by the generated light drawing data should be mirrored graphics. If directlyThe graphics described should be mirrored graphics. If you directly use the light-painted film for printed board imaging processing, the phases mentioned above should be reversed.


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


⑧Aperture matching Whether it is a vector light plotter or a laser light plotter, 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 because the size of the primitives (line segments, pads) can be set freely during the graphic design, Therefore, if the aperture of the light plotter is required to completely match it, it is not possible for the vector light plotter, and it is also troublesome for the laser light plotter, and from the processing point of view, this is mostly unnecessary, just do When the aperture is completely matched, due to factors such as focus and development, the size of the picture element drawn on the film will still be a little different from the design value. Therefore, in the actual processing process, as long as the processing technology allows, the existing aperture (for vector light plotters) or a set aperture (for laser light plotters) can be used. In many cases, it is allowed to use a 50mil aperture to correspond to the design value of 46mil or 55mil, and even a 60mil aperture to correspond to the design value of 40mil.