Circuits Assembly - November 2008 - (Page 38)

Tech Tips The Benefits of Good 'PR' Teaching the wirebonder pattern recognition. lighting from a vertical source is useful for polished die ire bonders have pattern recognition capabilities surfaces. essential to modern electronics manufacturing. Figure 1 shows several different PR modes, including Pattern recognition refers to computer identiSingle Point, Dual Point, Edge, Chain and Line Find. The fication of substrate or die features important for precise differences between these modes depend on the type of position of bond wires. A PR system will include a camera feature to be recognized. It is advised to try Single Point head, a camera control unit and a bonder controller that recognition first, as this setting will work most of the time. includes a microcomputer, a control circuit and a servo An edge finder will examine the motor drive unit. Some wirebondcontrast due to difference in the finers also come with proprietary grayish of the surface. scale pattern recognition to improve Image enhancements can aid the yield and throughput rates. PR process (Figure 2). Contrast setTo summarize the process, a feating between gray scales level can be ture of interest is identified, the area set either automatically or manually. that it resides in specified, and the Typically, if the image appears good visual properties of the feature are to the human eye, the computer can defined; i.e., the machine is taught recognize the feature. to recognize a pattern. Here are Figure 3 shows the definition some steps that can be followed to of the search area of interest. This ensure proper PR recognition. Figure 1. Pattern recognition modes. region is defined by the user and First, a reference system referred is indicated by the outer dashed to as a surface must be defined. Part box. It represents the area that will of this requires simply setting the be searched. Next, multiple modsurface Z-height. This is performed els can be created (inner box) for by focusing the system camera on the pattern. In this example, two the surface, using a motor drive models have been created for the to move vertically on the Z-axis. same feature. The number of pixels The system has a relative focal scale also can be set. After being defined, that typically ranges from zero to the model then can be tested, and 200 units. This Z-height is different the best matches are displayed and from the common Z-height proranked with a quality score. The grammable home value. This axis Figure 2. Image enhancements can be set process also determines the match is independent of the bond head automatically or manually and aid in PR. robustness. If the robustness is too Z-axis. The bonder also will ask for low, it can be improved by other a range of locations for the bonds. techniques such as the Dual Point This is defined by three points on method. the corners of the bonding surface. Because yield and throughput Next, assist points are added. are important criteria, wire bondAssist points are features, such as ers have high pattern recognition global and local fiducials used to system search speeds. reference positions. Pattern recogniPR enables a wire bonder to eastion is an automatic feature once a ily identify fiducial marks and other program is running; however, the alignment features. In this way, areas user is required to teach a pattern to the machine. In this example, Figure 3. The search area of interest is user- where a wire bond will be placed the pattern to be taught is a “plus defined, and multiple models can be created. easily can be referenced and determined. It is even possible to perform sign.” Once a feature of interest is a pattern recognition step between each bond wire. This is identified, lighting is used to increase contrast between used when each location is critical, and a dependency on the background and feature. Usually for gold surfaces, alignment fiducials may not be possible. PR also permits oblique lighting levels are increased. Lighting from an the wirebonder to find the position of each die in MCMs oblique angle is useful on the diffuse surfaces such as gold to maximize throughput. thick film surfaces and other gold-plated surfaces, whereas n Circuits Assembly NOVEMBER 2008 circuitsassembly.com W 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. 38 http://www.aciusa.org http://www.circuitsassembly.com

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Circuits Assembly - November 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing Better Manufacturing RoHS Conversion for Medical Devices Supporting Full-Service Customer Requirements at the Regional EMS Level Speed Thrills Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned The Defects Database Getting Lean Materials World Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - November 2008

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