| On Your Marks |
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| Written by American Competitiveness Institute | |||
| Wednesday, 31 December 2008 19:00 | |||
Sometimes a placement fiducials program needs to be tricked. Pick-and-place machines are often programmed using CAD data. These data increase the accuracy, precision and repeatability of the machine’s component placement objectives. CAD data make fine-pitch and small component assemblies repeatable, but cannot adjust to a particular board unless they are exactly the size and shape of the original board used for programming. The process by which printed circuit boards are made only permits certain minor changes in board size and shape, but these small differences are enough for parts to be misplaced. For this reason, fiducial marks are used to increase the chances of precise component alignment.Fiducial marks are standardized symbols (typically circles, squares, triangles and crosses) used for circuit pattern recognition that provide common measurable points for an automated assembly process. The circle is often selected for automated pick-and-place and is most commonly used for panel, circuit and local fiducials. Panel fiducials are located outside the body of the board where components are placed. Usually only three are necessary for programming, but when possible, four are placed on the PCB. If the board orientation needs to be changed for any reason, the extra fiducial permits the programmer flexibility in the machine setup. Panel fiducials are the most often recommended method because they are far from the components. This permits simpler camera resolution and contrast for automated machine inspection and less chance a via or component might be confused as a fiducial. Circuit fiducials are located inside the component placement area near the corners or as far apart as possible (Figure 1). When three (or four) circuit fiducials are used properly, they can correct theta distortions and compensate for scaling, stretching, and twisting or warpage of the PCB. These fiducials are a secondary method for increasing placement reliability, and are used in place of, or in addition to, panel fiducials. If panel fiducial inspection does not provide a precise placement for a panelized matrix PCB, then the fiducials must be used on each board in the matrix. Vision recognition is more difficult to program for circuit and local marks because vias, surface mount and through-hole components can be mistaken for fiducials.  Local fiducials are used to find the precise placement of an individual land location. They are usually used for fine-pitch components like QFPs or BGAs. Local fiducials increase fine-pitch placement precision, but it is vital to program the mark using the proper light level and contrast because of the close proximity of parts and vias to the fiducial. Troubleshooting. Fiducials are 1 to 3 mm in size and should not vary on the same PCB. Marks should be at least 2 mm from any similar feature that could cause an error. In addition, the mark should be more than 4.75 mm from the board edge to prevent obstruction by the board-locking mechanism. The search window size varies from machine to machine. A window three times the size of the mark in x and y direction is an acceptable search size. Search windows typically are set so the mark is in the center, but it may be necessary to move the search window slightly off-center to avoid mistaking the desired fiducial with a similar-looking, unwanted mark. Nevertheless, some machines have automatic fiducial search window correction that will automatically set the window back to the center and occasionally find the wrong fiducial mark. When assigning a light level during programming, you must be able to clearly see the mark, but not create a shiny reflection. When an automatic fiducial inspection is not working properly, additional contrast can often help. If the image is blurry, change the contrast level until the shapes are solid and easily identified. If fiducial marks are intermittently recognized, check to see if oxidation, solder mask, or some sort of coating is covering the mark. Rubbing a pencil eraser over the unrecognized mark sometimes can alleviate a very annoying problem. If this still does not work, and there is no manual fiducial alignment, washing and reprogramming the PCB may be necessary. The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.
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