B. P. Sood, M. H. Azarian, M. Pecht, CALCE, University of Maryland, College Park, MD; J. Miker, T. Wanek, Emerson Network Power, Lorain, OH

Summary: Schottky diodes are semiconductor switching devices with low forward voltage drops and very fast switching speeds. Schottky diodes respond more quickly than conventional diodes to changes in bias conditions. Their low forward voltage drop causes a lower loss of power when compared to ordinary p-n junction diodes. Schottky diodes are widely used as discrete components in applications such as high frequency, high voltage, high current switching circuits, industrial power supplies and rectifiers, where a low voltage drop improves efficiency. Schottky diodes typically consist of a deposited metal layer on a doped p-type or n-type semiconductor junction which is in contact with an n-type or p-type semiconductor substrate. Commercially available Schottky diodes are available in single monolithic chip packages, in dual chip packages with common cathode, or in diode packs known as ring quads containing four Schottky diodes. The packages can take a variety of different forms depending on design requirements, including small outline transistor plastic packages (SOT), diode outline type (DO) or diode-67 (D-67) type half packs. This paper introduces the Failure Modes, Mechanisms and Effects Analysis (FMMEA) methodology which enhances the value of standard Failure Modes and Effects Analysis (FMEA) and Failure Modes, Effects and Criticality Analysis (FMECA) procedures by identifying high priority failure mechanisms and failure models. It provides an overview of the common failure modes in Schottky diodes and corresponding failure mechanisms associated with each failure mode. Results of material level evaluation on diodes and packages as well as manufacturing and assembly processes are analyzed to identify a set of possible failure sites with associated failure modes, mechanisms and causes. A case study is then described which illustrates the application of a systematic FMMEA methodology to the analysis of a specific failure in a Schottky diode package. The FMMEA process helped in the identification of potential failure sites, enabling a detailed materials evaluation to be conducted and interpreted in conjunction with the life cycle profile before it was included or eliminated as a possible failure cause in the case study. The most likely failure site and associated mechanism were then carefully investigated. The D-67 Schottky diode package was exhibiting an extremely high failure rate in the field. The diode was used in a 24 Volt circuit where it was subjected to a reverse voltage of 139 Volts, and an average current of 93 Amperes at a junction temperature of 127 degrees Celsius. This package design had been in use for over 20 years, although it is now obsolete and has been replaced by an equivalent package which complies to requirements of the Restriction of Hazardous Substances (RoHS) legislations. The package base was a copper heat-sink that also acted as a mounting flange for the device. On top of the heat sink was a build-up of various materials consisting of solder and molybdenum discs, diode die, some washers and a threaded insert. During the failure analysis, materials identification and characterization steps we found numerous instances of manufacturing defects and poor materials selection choices in the D-67 package analyzed in this case study. Silver, in the form of fiducials, was found on the surface of the diode dice. Delamination and separation between die surfaces or heat-sinks and plastic molding compound constituted the path creation step for silver migration between the die top surface (anode) and the cathode. Moisture absorption experiments also confirmed the propensity of the molding materials to contribute to the silver migration. The case study illustrates the effectiveness of the FMMEA methodology and its use in designing and conducting a failure analysis. The analysis of these results led to implementation of several design and assembly process changes, which resulted in dramatic improvements in reliability and performance.