Circuits Assembly - July 2008 - (Page 26)

Tech Tips A Cut Above A six-step primer on microsectioning techniques. icrosectioning is a powerful failure analysis technique when coupled with optical or scanning electron microscopy (SEM) because it permits examination of a variety of failures that could not have been seen without this destructive analysis. Some of the areas that can be investigated through microsectional analysis include component defects, thermomechanical failures, processing failures related to solder reflow, opens or shorts, voiding and raw material evaluations. The microsectioning process involves the following steps: Step 1. Identify the area of interest (Figure 1). If the sample is an assembly, some adjacent components may need to be removed, as their presence physically may hinder access to the area of interest. A common tool used to remove components and section the area is a variable speed Dremel rotary tool. A cross-section slightly larger than the area of interest is cut out to reduce damage to the adjacent areas. Step 2. The sample is mounted (potted) to provide a matrix from which a highly polished surface can be obtained without damaging critical areas. The type of potting material will depend on the sample. We use both epoxy and acrylic mounting materials. These systems both incorporate a two-part combination of resin and hardener, the ratio of which will affect the final mount characteristics. Epoxy is used whenever possible because of its lower shrinkage during curing and because the final mount is clear, which allows viewing of internal mount features during polishing. Epoxy’s drawback is that it Circuits Assembly JULY 2008 M Figure 1. Cross-sectioning of PCB. Area of interest is circled. Figure 3. A fully polished sample. Figure 2. Vacuum apparatus with molding cup inside. Figure 4. Optical image of a microsectioned via. 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. requires an overnight cure, as compared to the acrylic, which cures in 2-4 hrs. Step 3. The section or sample is held in the appropriate orientation with either a plastic or metal clip to prevent movement of the sample during curing. The supported sample is placed into a molding cup pre-sprayed with a dry release agent to facilitate removal of the mount after curing. The potting material is carefully poured into the cup, taking care to completely surround the sample and minimize entrapped air. Step 4. The sample is placed into a vacuum for approximately 5 min. to ensure the mounting material pulls into all cracks and crevices and to remove any entrapped air. (Figure 2). Step 5. The sample is progressively ground and polished with silicon carbide (SiC) paper of progressively smaller grit sizes that is fixed to a rotating platen. Polishing techniques can vary, but the objective is the same: to remove the scratches from the previously larger grit paper through a successively smaller grit paper. Final polishing is done with a rotating felt pad saturated with either alumina or diamond slurry (Figure 3). Step 6: Optical or SEM imaging is used, depending on the failure (Figure 4). Microsectioning requires good technique to section precisely the area of interest. During failure analysis, only a single sample is available for analysis. Since microsectioning is a destructive test, care must be taken not to destroy critical evidence. n 26 circuitsassembly.com http://www.aciusa.org http://circuitsassembly.com

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Circuits Assembly - July 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Tech Tips Will Electronics Follow the Sun? ‘Warm’ Manufacturing Heats Up Reputation Trumps Recession, China ‘Web Circuits’ A Novel Non-VOC Conformal Coating Krypton: Benchmarking Customer Satisfaction 2008-09 CIRCUITS ASSEMBLY Buyers Guide Wave Soldering Pb-Free Lessons Learned Process Doctor Materials World SMTA Int'l Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - July 2008

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