CircuiTree - January 2009 - (Page 22) New Capability for HDI Microvia Defect Detection portion of production that contains HDI or microvias of concern. provide 100 percent coverage and does not replace final electrical test. The nature of the AEI system’s contact array, product surface contamination, and product geometry will present some combinations that are not evaluated. But on a typical high-technology cell phone panel, the AEI scanner can measure more than 98 percent of all HDI networks using a 20 ohm threshold. Note that this coverage includes the critically important internal routes and connections discussed equipment, the abnormal defect population will be reported resulting in appropriate process control and disposition of the product lot as early as possible. Detect It Better. Detect It Earlier. Improve Process Control The AEI scanner applies only 3 grams of force per contact. Note that this is between 1/4 and 1/40 of the force applied by a typical universal grid spring probe. Not only is the per-contact force reduced, but the AEI system applies its limited force in a single Reduce Cost and Risk, Improve Customer Satisfaction Many common HDI applications involve relatively harsh mechanical, thermal, or electrical environments that exacerbate the risk of latent failure. HDI is especially valuable where product size and weight are to be minimized. Portable electronics such as mobile phones, PDAs, cameras, portable media devices, and laptop computers are subject to frequent vibration and flexure, they are dropped, and they may see large temperature variations when left in hot automobiles or packed in baggage. High-performance industrial and automotive applications see constant extremes of vibration and temperature throughout the product lifetime—with high-reliability expectations. Semiconductor carrier and motherboard applications to notebook/ desktop computers and game consoles carry high electrical currents and thermal loads that will greatly stress weakened interconnections. Brands offering these products are highly sensitive to the customer’s experience and perception of product reliability. No tool is perfect, and the AEI process Where AOI is purely an optical surface inspection, the AEI process measures the actual HDI network connectivity for all layers of the board, and does so with real resistance thresholds. row of contacts across the breadth of the panel rather than in the area array formations typically present in traditional electrical test fixtures. This further reduces the accumulated force in any one region of the panel. This extremely low force is essential in not compressing the HDI interconnect and causing a false test pass. Placing AEI inspection early in the manufacturing process (at the bare copper level) provides the opportunity for true process control feedback while there is still time to correct the problem (Figure 7). This is especially true when addressing a problem capable of producing intermittent or latent failures. Early identification of a defect population rise allows for faster response and correction of the root cause. A full complement of real-time reports enables managers to monitor and react. Alarm conditions can be set to detect failure trends that exceed acceptable levels. As with AOI, the AEI method does not previously, where AOI does not. As such, the coverage of connectivity within a laminated panel is vastly improved over AOI’s surfaceonly scan. Enhancing AEI Latent Defect Detection Using Thermal Cycles Expansion of the substrate due to heating during soldering or end use increases the stress on improperly formed vias and may result in field failure. The higher temperatures associated with lead-free soldering may elevate the substrate above its glass transition temperature (Tg) where many materials may demonstrate substantially higher temperature coefficient of expansion. By adding one or more heating/cooling cycles prior to AEI inspection, the PCB manufacturer can simulate the stress of the solder reflow (Figure 8). Within defective lots, some of the otherwise intermittent defects will become steadily open. During subsequent scanning by AEI Figure 8 Thermal Cycle Prior to Test Simulates the Effect of Reflow and Increases the Probability of Detecting Latent Defects Using AEI Figure 7 AEI Placement Earlier in the Process Accelerates Process Control Feedback 22 January 2009 • circuitree.com Figure 9 Rule of 10s: Cost Is Minimized When Defects Are Detected Early 26 http://www.circuitree.com
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