Thermal impedance.doc - Report Number EV90342 Page 1 of 25...

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Report Number EV90342 Page 1 of 25 Thermal Impedance Measurements for Quality Assessment of Metallurgically Bonded Diodes. Alexander Teverovsky, Ph.D. [email protected] QSS Group/Goddard operations, Greenbelt, Maryland 1. Background A significant proportion of metallurgically bonded, glass bodied, axial diodes fail destructive physical analysis (DPA) due to excessive voiding in the attachment media. Internal examination of these diodes typically is performed by cross-sectioning the diodes in several planes parallel to the axis. However the process of cross-sectioning might change internal mechanical stresses in the diodes and release mechanical energy stored in the highly stressed diode construction. This often results in cracking of the die, glass, and/or the attachment medium between the die and slug and makes a quality assessment of the diodes difficult. Thermal impedance (TI) testing of diodes is an assay, which characterizes die attachment integrity, and could be useful during DPA screening and/or testing to evaluate quality of the diodes. This technique was applied to more than 50 DPA diode jobs in the GSFC PA Lab during 1999 to 2000. The purpose of this work was to consider some of the problems related to TI measurements, to generalize the obtained experience, and to identify the place of this technique in diode screening and DPA test flow. Content of the paper: 1. Background. 2. Parts description. 3. Experimental 3.1. What is the TI technique measuring? 3.2. Technique basics 3.3. K-factor 3.4. Effect of heating pulse amplitude and width 3.5. Effect of measurement delay time 4. Experimental results 4.1. Typical distributions 4.2. TI correlation with voiding in the die attachment 4.3. TI correlation with VF 4.4. TI variations for diodes with similar design 5. Discussion 6. Conclusion
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Report Number EV90342 Page 2 of 25 2. Parts Description The majority of parts currently used by NASA GSFC are DO-35 diodes with the Category I metallurgical bonds. The diodes are constructed using two metallurgically bonded silver-plated tungsten (or molybdenum) slugs and a thermal compression glass seal sleeve. During the metallurgical bonding silver forms a eutectic alloy with the silicon chip, dissolving part of the silicon. The glass sleeve is placed over the slug-die-slug assembly and then heated at high temperature (approximately 800 C) where the glass flows. During controlled cooling the glass fills the voids, forms intimate contact with the die, and makes a strong thermal compression bond. 3. Experimental MIL-STD-750, TM 3101, provides a detailed description of the TI technique and recommendations for test conditions and evaluation of results. Some aspects of the technique are detailed below. 3.1. What is the TI technique measuring? Electrical power in diodes is released at the P-N junction and dissipates by heat flow through the silicon die, die-to-slug interface, slug, sealing glass and lead. Using an electrical analogy of thermal behavior, with temperature (T) analogous to electrical potential and
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  • Spring '12
  • Islam
  • Impedance, thermal impedance, Report Number EV90342, Metallurgically Bonded Diodes

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