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Stencil data preparation suggestions

  • Introduction
  Is it necessary to know how to prepare ordering files properly ? 
 
   Of course, nowadays your orders can be accepted in many formats of the most popular CADs. But not always this plays a positive role. This is associated with the fact that final assembly all of stencil data is done out of your production chain, so you can control the preparation process only indirectly. But indeed, the stencil is a production tool defining more than 50% of surface mount quality level. 
   Yes, you can ask somebody to develop the stencil project by your requirements, but when you do it yourself, it can make the process faster and easier because you have the ability to see that aspects that an outsider not aware of your production features possibly doesn’t know or can’t notice. 
   It is also possible to make only a final revision of files already prepared by outside, but often this is tied with time loss many times longer than the time directly spent for stencil manufacturing. Unclear specifications set for files preparation, communication problems, absence of the required specialist on the customer side at the moment of confirmation, and at last pure and simple human factor influence while order preparation provides a great risk level of a mistake and a big delay for such a way of work. 
   The most popular recommendations summary for surface mount stencils is document IPC-7525. It describes general recommendations for preparation of stencil manufacturing files and provides explanation of terminology used for this technology. 
  Following are the key factors to be considered while data preparation for stencil manufacturing. We assume that you prefer to do this yourself in order to keep the process under your control. 
 
  • Format of files
   The most recommended format of incoming files is GERBER in all of its variations. Why? 
- It is universal. It can be read by all programs for data preparation for manufacturing. At import/export operations won't occur mistakes, related to project scale misfit or partial data loss (of course, if initially the GERBER file is prepared properly). 
- Generally, all the equipment used for stencil laser cutting recognize GERBER format as its operation format. Therefore, by making GERBER file, in a point of fact, you provide originally well-defined and clear task for the machine. 
  In short, this is a “native language” for a manufacturer. Speaking the same language with him you eliminate a case when you are not understood by anybody because all the data is represented in the file itself. 
 
What does IPC-7225 say about this?

3 STENCIL DESIGN
 
3.1 Stencil Data
 
3.1.1 Data Format Regardless of the stencil fabrication method used, Gerber data is the preferred data format. Possible alternative formats are GenCAM3, DXF, 
 
HP-GL, Barco, etc; however, they may need to be converted to Gerber® format prior to the stencil manufacturing process. Gerber® data describes the file format that provides a language for communicating with the photo plotting system to produce a tool for chemically etched stencils. It is also used to produce the laser cut or electroformed stencils. While the actual data format may vary from file to file depending on the software package or designer, the data format commonly used by photo plotter and laser equipment is known as Gerber. 

  Our many years experience in stencil manufacturing shows that this is actually an objective reality. As soon as a company ordering stencils changes to preparation orders in GERBER format, an enormous  number of questions related to source data preparation and conversion disappears. 
 
  Rather friendly and universal software for GERBER files processing is CAM350 (www.downstreamtech.com/cam350.html ) . It will allow you to import data from a variety of CAD systems, make needed modifications, see how the final stencil will appear in real scale. Data conversion from CAM350 to GERBER and back is so easy that these two formats are absolutely replaceable for a manufacturer. 
 
  •   File preparation
  Let us consider a number of key aspects of proper file preparation from the point of view of uniform information and friendliness of its presentation. Our purpose is not to teach you how to work in CAM350, but nevertheless, if you want to learn this in order to prepare stencil manufacturing files properly, we are always forward to assist you with a necessary advice. 
  If you are used to work in AutoCAD, CAM350 will provide you with ability to import DXF files and to convert them to GERBER. 
  Some aspects of files preparation are shown in small video clips, links to which you can find in this text. Viewer software for the clips you can download here.

Shortcuts used in CAM350: 
 
'R' - refresh the screen if some visual noise has appeared 
 
'Y' - show on the screen the project layers summary table
 
'A' - show on the screen the project apertures summary table
 
 'W' - change to the window selection mode. Used in cases when a group of project apertures  should be selected (for copy, move, delete, etc.) 

  CAM350 file example can be downloaded here.
 
  It is noticeable at the first sight that CAM350 format is also convenient as it can contain several layers with different data of one project at the same time while GERBER format is a set of separate files then assembled into one project when importing. 
 
  GERBER file example can be downloaded here, if you can't work with CAM350 format directly
 
 
Sample of right preparation file of stencil
 Figure 1 Appearance of prepared stencil in CAD system 
 
 
 
1. Aperture arrangement 
 
  Aperture means an opening of any shape and size formed in a stencil. A stencil aperture corresponds to a component pad on a PCB, to which solder paste is to be deposited through the stencil. 
 
  1.1 Transfer all needed components pads into GERBER file. While exporting CAD data to GERBER file you should select the layers containing such pads for export. Please note that sometimes by mistake or on purpose such pads can be arranged in several layers but not in one. 
 
 
  1.2 Mirror the data of BOTTOM PCB side. PCB has two sides which are TOP and BOTTOM (alias BOT). TOP side is always the closest side to us on computer monitor. The BOT side is seen “through” the PCB on the monitor. When BOT side data is being exported it should be mirrored to face us, otherwise the finished stencil for the opposite side will be mirrored because during assembly process we will flip PCB that side to the stencil. 
 
  It is most intelligent to make the mirroring by yourself. For this purpose it is enough as a rule to mark the associated checkbox in CAD system while exporting. This allows to avoid misunderstanding by file processing engineer whether such a mirroring has been done or not, as when importing your data he see only the final result on the monitor. Consequently, in this case the project is sent so as for TOP side, because the mirroring has been already done by you. 
 
 
  1.3 Arrange apertures inside stencil outline so as you want. To avoid questions about how to arrange apertures on the stencil it is better to do it yourself by arranging them within stencil outline so as you need. Thus, you will extremely clear define your requirements to the manufacturer not losing time for various drawings and descriptions of what you need to have as a result. In present-day world the single drawing for production is properly prepared computer-aided project file. It is the data source for operation of all the equipment. 
 
  Apertures arranging on the stencil can be a critical factor for the production equipment as the squeegee of most printer models often travels only within some stencil area, thus shifting of aperture pattern can take them out of paste printing area. 
 
  PCB orientation also can be important. As a rule, long side of PCB is orientated along the stencil’s long side.
 
  1.4 Arrange stencil apertures of various PCBs so as it is needed. If the stencil is rather big and the PCB is not panelized (multiplied) and its outline is small, you can combine apertures designated for different sides of PCB or several different PCBs on one stencil. Obviously, in this case solder paste is deposited only on one PCB and all other apertures simply sealed for this time. The best solution for this purpose is a common paper masking tape. It doesn’t leave sticky residues on the stencil surface after removal. 
 
  In case of such a combined arrangement of PCBs on the stencil it is better if you allocate PCBs aperture patterns within the stencil yourself so as it will be acceptable for you production. 
 
 
2. Stencil outline and perforation holes
 
  2.1 Draw stencil outline and perforation holes. There is a large variety of different stencil types designed for various mounting systems. All of them have their own outlines and perforation holes arrangements. The better practice is to define stencil outline and perforation holes explicitly. This allows to avoid misunderstanding of aperture arranging at file preparation.

  Advice:
  If it is difficult for you to prepare the stencil outline data by yourself, please, send its drawing to the manufacturer and request for files confirmation. Received file containing outline drawing can be used later for self-made preparation of new orders. 

  Stencil outline and perforation (mounting) holes arrangement for industry printers are determined by stretching system used by them. For this purpose it is necessary to create such a file once. Then, for a new stencil only aperture pattern should be change in it. 
 
   If the stencil is designed for manual solder paste printing with a simple printer without mounting holes (for instance, fixing by metal jaws), such a stencil may have no mounting holes and can be of any size. 
 
  The general recommendation in this case is to make edge clearance from aperture pattern not less than 150 mm at each side in the direction of squeegee travel and not less than 50 mm at lateral sides of the stencil. These clearances are needed to allow a formation of solder paste roll moved by the squeegee which then moves over stencil apertures. 
 
  In addition, the larger size of the stencil and larger distance between apertures and stencil mounting point, the less is the impact of twisting force caused by dual side stencil mounting. 
 
 
3. Stencil apertures modifications
 
  It is better when needed changes of aperture sizes are made by the customer initially in the file sent by it. There are no standard rules for modification; the need of changes is determined only by production engineer on the assembly facility and none outside him. All this is done depending on specific equipment and expendable materials used. The simple modifications like aperture size change and rounding can be easily made with CAM350 software. 
 
   Certainly, we will do our best to help you in complicated aperture modification (for instance, in aperture separation with material spaces, etc.), but it’s preferable to make simple modifications by yourself because in this way you can have full control of preparation process and implement it to meet your assembly facility requirements. 
 
  IPC-7525 recommendations provide several methods that help to change the stencil apertures shape and size. Not all of them should be implemented by default. It is more useful to determine from an experiment for yourself what should be done and what doesn’t make sense in your manufacturing environment. 
 
   Following are some methods of practically proven vitality: 
 
  3.1 Stencil aperture size change (as a rule, this means reduction against PCB pad size). If you are not sure you need it, start with 1:1 aperture to pad size ratio. But if you feel that too much solder paste is deposited, you can begin to reduce aperture size. Typically, reduction within 10% of initial size is sufficient. 
 
   It is important not to overdo, because too small aperture will cause the solder paste not to pass through it. Empirical limit beyond which there is a risk of paste deposition problems is 240 microns. 
 
   Apertures size change can be made either all over the stencil in one go or for individual components. This can be done easily in a single step with the help of CAM350. 
 
   It is recommended to become familiar with this operation yourself as this provides you with precise control of source data change. You know, the problems with paste deposition should be then solved by you! 
 
  If you have trouble doing this yourself, it will be needed to specify while stencil ordering how much reduction of apertures should be. In the case of selective reduction, coordinates of components with apertures to be changed should be additionally specified.  After modification is complete it will be also needed to perform an extra confirmation and approve the file received from manufacturer. 
 
 
Please note that the modified apertures will be moved to another project layer. To move apertures between project layers please refer to link below. 
 
  3.2 Aperture shape changes. As a rule, the most popular method is aperture corners rounding. It makes sense only for small apertures in order to eliminate the risk of solder paste trapping in the corners.
 
   If you have trouble doing this yourself, it will be needed to specify while stencil ordering where apertures rounding should be done. After modification is complete it will be also needed to perform an extra confirmation and approve the file received from manufacturer.
 
 
  It is sensible to divide very large apertures (with size in order of 10 mm) into several parts with metal spaces in order to prevent solder paste scooping while squeegee travel. This operation should be better put into manufacturer’s hands, because specialized software user by it makes this faster. 
 
3.3 Apertures deletion, multiplication, copying, moving. It is easier to become familiar with these operations yourself as they are not so complex, and explanation of what should be in result and further verification will take much time and will not eliminate a risk of mistake at all. And an incorrect stencil means equipment downtime till its remaking! 
 
 
 
 
 
4. Fiducials and text engraving 
 
   Everything related to laser engraving process should be exported to an individual GERBER layer which should be associated with any bright color (if CAM350 is used). This is concerned with need to clearly separate the things on the stencil that should be made with engraving and that should be cut through. Typically, there is much more cut data, and that is engraved should be clearly noticeable against it. For instance, just try to find a couple of fiducials against a background of 10,000 apertures of the same color even if you know that they should be somewhere. Take pity on who will prepare your stencil to manufacturing! 
 
  4.1 Fiducials (refer to item 1 of figure 1) 
 
  Fiducials are made on stencils for its use by printers equipped with vision systems. As a rule, they are formed on stencil surface by engraving, but it is just possible to use through-cut fiducials. 
 
  The most optimal option is a circle with diameter of 1, 1.5 or 2 mm. Smaller ones will have lower contrast due to features of laser engraving technology. Too large ones are undesirable because of increased risk that weakened metal of engraved area will simply drop out while operation, plus contrast of such a fiducial degrades faster under the impact of a printer squeegee. 
 
  The stencil fiducial is aligned against a PCB fiducial by means of printer vision system, thereby stencil-to-PCB registration is performed. 
 
  It is enough to use two fiducials for this purpose. Typically, three fiducials are made, one of which plays a role of reserved one in case of one of operational fiducials is out of order. 
 
  It is not needed to make tens of fiducials on the stencil! A panelized PCB has many of them because they are used by pick-and-place machine to place most precise components. They are not related to paste printing. Thereby you will only make the stencil manufacturing process more complicated which can affect its cost increasing actually no one of these fiducials will be used. Engraving of one fiducial takes more time than cutting of several tens of apertures!
                       
  The best practice is to locate a fiducial outside of apertures pattern. This will make its recognition by vision system easier. For this purpose they should be included in the design at the edge of PCB itself or on a PCB's panel, but not between component pads. 
 
   In laser cutting technology fiducials as well as text engraving are always made on one side which be applied to PCB. Engraving on the other side is another technical problem, therefore you will be likely asked several times if you need this. Due to this, it is very easy to determine how the stencil should be positioned while operation. If it cannot be situated properly, it means that a mistake was made at file mirroring step during file preparation. 
 
  4.2 Text engraving (refer to item 2 of figure 1) 
 
  Text engraving is optional but rather advisable stencil feature. Ease your life, don’t make an anonymous stencils! 
 
  As a rule, it is enough to plot the stencil manufacturing file name with engraving.  Any additional details like date, material thickness, etc. are unnecessary information. Actually, it is seldom needed by anybody, but all this costs money and takes manufacturing time. In addition, massy engraving on a stencil can lead to metal bend in this area affecting the deposition quality of solder paste. 
 
   One of the easiest procedures in CAM350 is text insertion. It would be better if you do it yourself in correct place and in correct style. This will save time of production startup! This is absolutely uncomplicated! 
 
  For text plotting it is better to use outline fonts without inside letter filling. The font height should not be less than 4 mm to make it better readable. Letter filling is undesirable as it will take much machine time and can affect the stencil operation performance. 
 
   One of the features of stencil engraving is that in the file it appears mirrored. That just related to the fact that actually you see the engraving in the file from “that” but not “this” stencil side; therefore it should be mirrored. The text engraving location on a stencil, as well as fiducials, determines the stencil side to be applied to PCB. 
 
 
 
 
  Now let us summarize the advantages of correctly prepared stencil manufacturing file in accordance to international standards: 
  • Source data is understandable for manufacturer and will not cause any questions. 
  • The file doesn’t need any additional approving. Huge amount of time for communication and negotiation of various technical questions is saved. 
  • The order goes to production immediately. The production time of such order is minimal as there are no question arising regarding it. 
  • Influence of human factor at the preparation step is minimized, because no need to make changes (or these quantity is minimal) in initial files of customer, sent for order manufacturing. 
  • The order manufacturing cost is also lower than for orders difficult in preparation because no additional data preparation is needed. Many manufacturers charge an extra fee, if source data needs additional work.
 
  We are ready to help you in questions of file preparation. We will be happy if our advices are helpful for you and help to significantly decrease the number of arising questions.