PCB plate making technology: CAM and photolithography
PCB plate making technology: CAM and photolithography
PCB manufacturing technology includes computer-aided manufacturing processing technology, that is, CAD/CAM, and photolithography technology. The general process of photolithography is: check the file – determine the process parameters – convert CAD file to Gerber file – CAM processing and output.
1.Computer-aided manufacturing processing technology
Computer-aided manufacturing (CAM) is to carry out various process processing according to the specified process. All the process requirements mentioned above must be prepared before photolithography. For example, mirroring, solder mask expansion, process line, process frame, line width adjustment, center hole, outline line and other issues must be completed in the CAM process. Special attention should be paid to the places where the spacing in the user’s file is too small, and corresponding processing must be made.
Because the process flow and technical level of each factory are different, in order to meet the user’s final requirements, necessary adjustments must be made in the production process to meet the user’s requirements for accuracy and other aspects. Therefore, CAM processing is an indispensable process in modern printed circuit manufacturing.
1.Work completed by CAM
① Correction of pad size and combination of D code
② Correction of line width, merging D codes.
③ Check the minimum spacing between pads, pads and lines, and lines.
④ Check the aperture size and merge.
⑤ Check the minimum line width.
⑥ Determine the solder mask expansion parameters.
⑦ Mirror.
⑧ Add various process lines and process frames.
⑨ Correct line width to correct side erosion.
⑩ Form a center hole.
⑩ Add an outer corner line.
⑥ Add positioning holes.
⑩ Make a plate, rotate, and mirror.
⑩ Make a piece.
⑩ Overlay processing of graphics, corner cutting and tangent processing.
⑩ Add user trademarks.
2.Organization of CAM processes
Since there are dozens of popular CAD software on the market, the management of CAD processes must first start from the organization. A good organization will achieve twice the result with half the effort. Since the Gerber data format has become the standard of the photolithography industry, the Gerber data should be used as the processing object in the entire photolithography process. If CAD data is used as the object, the following problems will arise.
(1) There are too many types of CAD software.
If all process requirements are to be completed in CAD software, each operator must be proficient in the operation of each CAD software. This will require a long training period for the operator to become a skilled worker and meet the actual production requirements. This is not cost-effective from a time and economic perspective.
(2) Due to the numerous process requirements, some requirements cannot be achieved for certain CAD software.
Because CAD software is used for design, it does not take into account the special requirements in process processing and cannot meet all requirements. CAM software is specifically used for process processing and is the best at doing these jobs.
(3) CAM software is powerful, but all operations are performed on Getber files, not CAD files.
(4) If CAD is used for process processing, each operator is required to be equipped with all CAD software, and each CAD software has different process requirements. This will cause unnecessary confusion in management.
In summary, the CAM organization should be the following structure (especially for large and medium-sized enterprises).
① All process processing is unified to Gerber data as the processing object.
② Each operator must master the skills of converting CAD data into Gerber data.
③ Each operator must master the operation methods of one or several CAM software.
④ Establish a unified process specification for Gerber data files.
⑤ CAM processes can be relatively concentrated and processed by several operators for easy management.
A reasonable organization will greatly improve management efficiency and production efficiency, and effectively reduce the error rate, thereby achieving the effect of improving product quality.
2.Photolithography process
The general process of photolithography process is: check the file – determine the process parameters – convert CAD file to Gerber file – CAM processing and output.
- Check the file
(1) Checkhe user’s file The file brought by the user must first be checked as follows.
① Check whether the disk file is intact.
② Check whether the file contains viruses. If there are viruses, it must be disinfected first.
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) inside.
The original data provided by the user is usually in the following format.
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).
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, analyze and understand the correct and standard Aperture, and make a detailed analysis of the relationship between them. It is very important to read the Aperture file carefully, because sometimes there are some special situations. For example, sometimes users will ask 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 graphic file consists of three parts: coordinates, size, and shape, and the Gerber file only has coordinates, so two other conditions are required. If there is an Apertuer file in the received file, then open it and you will find that it contains the required data. If you can combine them well, you will be able to read the user’s original data.
(2) Check whether the design meets the factory’s process level
① Check whether the various spacings designed in the customer file meet the factory’s process. The spacing between lines, the spacing between lines and pads, and the spacing between pads should be greater than the minimum spacing that can be achieved by the factory’s production process.
② Check the width of the wire. The wire width should be greater than the minimum line width that can be achieved by the factory’s production process.
③ Check the size of the via to ensure the minimum aperture of the factory’s production process.
④ Check the pad size and its internal aperture to ensure that the pad edge has a certain width after drilling.
2.Determine the process parameters
Determine various process parameters according to user requirements. The process parameters may be as follows.
(1) Determine whether the photolithography film is mirrored according to the requirements of the subsequent process
① Principles of film mirroring In order to reduce errors, the film surface (i.e., the latex surface) must be directly attached to the film surface of the photosensitive resin.
② Determinants of film mirroring If it is a screen printing process or a dry film process, the film surface is attached to the copper surface of the substrate. If it is exposed with a diazo film, since the diazo film is mirrored when copied, its mirror image should be the film surface not attached to the copper surface of the substrate. If the photolithography is a unit film, rather than a plate on the photolithography film, an additional mirror image is required.
(2) Determine the parameters for enlarging the solder mask pattern
① Determination principle The enlargement of the solder mask pattern is based on not exposing the wires next to the pad;
the reduction of the solder mask pattern is based on not covering the pad. Due to errors during operation, the solder mask pattern may cause deviations in the circuit. If the solder mask pattern is too small, the deviation may cause the edge of the pad to be covered, 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 Determinants of solder mask pattern enlargement The deviation value of the solder mask process position of our factory and the deviation value of the solder mask pattern. Since the deviations caused by various processes are different, the enlargement values of the solder mask pattern corresponding to various processes are also different. The enlargement value of the solder mask pattern with large deviation should be larger. The board has a large wire density and a small distance between the pad and the wire. The enlargement value of the solder mask pattern should be smaller; the board has a small wire density, and the enlargement value of the solder mask pattern can be larger.
(3) Determine whether to add process wires based on whether the plug needs to be gold-plated (commonly known as gold fingers) on the board.
(4) Determine whether to add a conductive frame for electroplating based on the requirements of the electroplating process.
(5) Determine whether to add conductive process wires based on the requirements of the hot air leveling (commonly known as tin spraying) process.
(6) Determine whether to add a pad center hole based on the drilling process.
(7) Determine whether to add a process positioning hole based on the subsequent process.
(8) Determine whether to add an outer corner line based on the board shape.
(9) When the user’s high-precision board requires very high line width accuracy, it is necessary to determine whether to perform line width correction based on the factory’s production level to adjust the impact of side etching.
3.Output of photoplotting of the base
Since many printed circuit board manufacturers do not use the negatives drawn by the photoplotter directly for imaging production, but use it to copy the working negatives, here we call the photoplotting negative the base. Before starting photoplotting, you must first adjust the parameters of the photoplotter to put it in a suitable working state.
(1) Setting of photoplotter parameters
① Setting of light source intensity During the photoplotting process, if the light source intensity is too high, the drawn figure will have a halo; if the light source intensity is too low, the drawn figure will be underexposed. Therefore, both vector photoplotters and laser photoplotters have a light intensity adjustment problem. A light intensity detection circuit is set in high-end photoplotters. When the light intensity is insufficient, the photoplotter will refuse to work or the shutter will not open, and an error message will be displayed on the screen. Sometimes the film drawn by the laser photoplotter shows no signs of exposure at all, which is due to insufficient light intensity. Usually, the intensity of the light source can be controlled by adjusting the voltage of the light-emitting device. Every time the light-emitting device or the developer is replaced, a photoplot test piece should be used to check whether the light intensity is appropriate.
② Adjustment of photoplotting speed The drawing speed of a photoplotter, especially a vector photoplotter, is also an important factor affecting the quality of the plotted film.
When the vector photoplotter is marking, if the drawing speed is too fast, that is, the light beam stays on the film for too short a time, underexposure will occur; if the drawing speed is too slow, that is, the light beam stays on the film for too long, it will be overexposed and halo will appear. Not only will the photoplotting speed affect the plotting effect, but also the acceleration during photoplotting and the delay time of shutter opening and closing during exposure will affect the result. These parameters also need to be carefully adjusted.
③ Placing of negative film during photolithography Due to changes in various external factors, photolithography negative film will undergo slight expansion and contraction deformation. Generally, it will not have much impact on the processing of printed circuit boards, but sometimes it will make the negative film unusable. Therefore, in addition to trying to eliminate the influence of external environmental factors, attention should also be paid during photolithography operations. When placing the negative film, try to ensure that the X and Y directions of different layers of the same printed circuit diagram to be drawn (such as the component surface and the welding surface) are consistent with the X and Y directions of a negative film, so that the deformation is more or less the same. For some photolithography machines with low precision, try to start from the origin of the drawing table when drawing the film. When drawing graphics of different levels of the same circuit, try to be on the same coordinate range of the table. Pay attention to the placement of the film. In addition, when placing the film, keep the film’s film film facing up to the light source to reduce the diffraction effect of the film medium on the light.
④ Maintenance of the film table The cleanliness and flatness of the drawing table (or curved surface) is an important guarantee for the drawing quality. There should be no other objects on the film table (curved surface) except the film to be drawn, and the working surface should not be scratched. The small holes for vacuum adsorption of the film should be kept unobstructed, so that high-precision films can be drawn.
(2) Drawing of the graphic base When the photoplotter is in normal working state, it inputs the photoplotting data through the disk, RS232 port or tape (the tape method is rarely used at present), and then draws the graphics described by these data on the film. In fact, in addition to simple operations on the photoplotter, there is no need to do more work. A lot of work on photoplotting graphics is in the generation and processing of photoplotting files.
① Drawing of circuit chips Generally, it is only necessary to directly generate photoplotting data for the design graphics that have passed the review, and input the photoplotting data into the photoplotter. Usually the circuit sheet should be 1:1. For some more complex circuits, attention should be paid to whether the error between the size of the graphic element on the photolithography film and the design value will affect the production. If there is an impact, the size of the designed graphic element should be modified to compensate for the deviation of the photolithography value.
② Drawing of solder mask The requirements for solder mask are lower than those for circuit sheet, but according to different process requirements, the pads of solder mask should be enlarged compared with circuit sheet, and attention should be paid when generating photolithography data for solder mask.
③ Drawing of character sheet The requirements for character sheet are slightly lower
but because the characters of the device are often called out from the library 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 silk screen effect is not good. This requires careful inspection of the characters before generating the photolithography file of the characters. When generating the photolithography file of the characters, try to merge the line width of the characters into one or several types to meet the process requirements.
④ Drawing of drilling film Generally, it is not necessary to draw drilling film, but sometimes,
in order to better check the drilling situation or clearly distinguish the aperture, you can also draw a drilling film. For vector photoplotters, when drawing drilling holes to distinguish the aperture, you should consider saving photoplotting time, that is, when generating photoplotting data, you should pay attention to using simple symbols to identify the aperture.
⑤ Drawing of power supply and ground layer with large area copper coating For power supply and ground layer with standard design
the negative film drawn according to the design is opposite to the graphics on the printed circuit board, that is, the unexposed part on the negative film is the copper foil, and the part with graphics on the negative film is the isolation part on the printed circuit board without copper layer. Due to the needs of the process, when drawing power supply and ground layer, the isolation plate should be larger than the pad of the circuit layer. For the holes connected to the power supply or ground layer, it is best not to draw anything, but to draw a special flower pad. This is not only a problem of ensuring solderability, but more importantly, it is conducive to the inspection of the negative film. It is clear at a glance which position has a hole, which position has no hole, and which hole is connected to the power supply or ground.
⑥ Mirror image drawing:
In the process of printed circuit board imaging, the film surface (graphic surface) of the film needs to be pasted on the dry film attached to the copper foil of the printed circuit board. Therefore, the phase of the graphic (i.e. the front and back of the graphic surface) should be considered when drawing the film. It is not recommended to adjust the phase of the graphic by turning the film surface upside down. Moreover, when several small graphics are drawn on a large film, this method cannot make their phases different. This should be paid attention to when generating the photolithography data file. In general, since the film needs to be flipped before imaging, the photolithography data generated for the single-layer (1, 3, 5, …, layer) graphics of the printed circuit board should be in positive phase, and for the double-layer graphics, the graphics described by the generated photolithography data should be mirror images. If the photolithography film is used directly for printed circuit board imaging, the phase mentioned above should be reversed..
⑦ Graphic level identification It is very important to identify the PCB level corresponding to the negative graphic.
For example, if the surface (layer) of the graphic is not identified for a simple single-sided board, the welding surface may be made into the component surface, which will eventually make the device difficult to install, especially for double-sided and multi-layer boards. Some PCB auxiliary design software can automatically add the level of the graphic when generating the photo-drawing file, which undoubtedly brings a lot of convenience. However, two points should be noted when applying it. First, whether the level of the printed circuit wiring is the level arranged by the processing; secondly, the zero point of the graphic is often far away from the origin of the coordinates during design, and the automatically added level mark is near the origin of the coordinates, so there will be a large gap between the level mark and the graphic, which will not only affect the effect of the identification, but also cause waste of negatives.
⑧ Aperture matching Whether it is a vector photo-plotter or a laser photo-plotter, there is an aperture matching problem.
If a 40mil pad is used in the design graphics, and a 50mil aperture is used for photoplotting, the graphics drawn will obviously be different. However, since the size of the primitives (line segments, pads) can be set at will during graphic design, it is impossible for the vector photoplotter to completely match the aperture, and it is also troublesome for the laser photoplotter. Moreover, from the processing point of view, it is not necessary. Even if the aperture is completely matched, the size of the primitives drawn on the film will still be slightly different from the design value due to the influence of factors such as focusing and development. 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 used. In many cases, it is allowed to use a 50mil aperture to correspond to a design value of 46mil or 55mil, or even a 60mil aperture to correspond to a design value of 40mil.
