Prabjit Singh

Tracer study of diffusion and electromigration in thin tin films

A comprehensive study of lateral self‐diffusion and electromigration in evaporated thin Sn films over the temperature range −50 to 198 °C is reported. Sn119m atoms were deposited over a central portion of thin Sn film stripes and the subsequent diffusional spreading and migration of the tracer distribution were evaluated by a high‐resolution autoradiographic technique. Activation energies suggestive of grain boundary transport processes were obtained. At low temperatures the ionic drift velocity in grain boundaries Vb is measured to be VbT/j=3.4×10−2 exp[−(8710±360 cal/mol)/RT] cm3 K/A s, while the low‐temperature grain boundary diffusivity Db is given by Db=4.9 exp[−(11 700±840 cal/mol)/RT] cm2/s. At higher temperatures (i.e., >80 °C) transport measurements were conducted employing Sn films of two different grain sizes, and apparent drift velocities and diffusivities were obtained. Application of the Hart model enabled the extraction of grain‐size independent values of the grain boundary ionic drift velo...

A power, packaging, and cooling overview of the IBM eServer z900

This paper provides an overview of the power, packaging, and cooling aspects of the IBM eServer z900 design. The semiconductor processor chips must be supported and protected in a mechanical structure that has to provide electrical interconnects while maintaining the chip junction temperature within specified limits. The mechanical structure should be able to withstand shock and vibrations during transportation or events such as earthquakes. The processor chips require electrical power at well-regulated voltages, unaffected by the ac-line voltage and load current fluctuations. The acoustical and electromagnetic noise produced by the hardware must be within the limits set by national regulatory agencies, and the electronic operations must be adequately protected from disruption caused by electromagnetic radiation. For high availability, the power, packaging, and cooling hardware must have redundancy and the ability to be maintained while the system is operating. This paper first overviews the packaging hardware, followed by a description of the first- and second-level packaging, which includes the mother board and the multichip module. Thermal management is discussed from the point of view of both the multichip module and the overall system. Power conversion, management, and distribution are presented next. Finally, the design aspects involved with meeting the requirements of electromagnetic compatibility, acoustics, and immunity to shock, vibration, and earthquakes are discussed.

Packaging the IBM eServer z990 central electronic complex

The z990 eServerTM central electronic complex (CEC) houses four multichip-module-based processor units instead of one, as in the previous-generation z900 eServer. The multichip module (MCM) input/output pin density in z990 processor units is more than twice that of the MCMs in z900 processor units. This increase in packaging density and the consequent tripling of the current drawn by the processor units were accommodated by the first-time use of land grid array (LGA) MCM-to-board interconnections in an IBM zSeries® eServer. This was done by using innovative refrigeration cooling of the MCM with air cooling as backup, and by a new mechanical packaging and power distribution scheme. This paper describes the mechanical engineering of the CEC cage, the LGA MCM-to-board interconnection scheme, and the mechanical isolation of the MCM evaporator-heat-sink mass from the LGA contacts. The paper also describes the electrical power and the cooling solutions implemented to meet the more demanding requirements of the denser CEC package.

Creep corrosion failure analysis on ENIG printed circuit boards

The proliferation of the Internet of Things (IOT) and the increasing reliance on cloud computing are critically dependent on the reliable operation of Information Technology (IT) equipment. The associated increase in the power consumption by IT equipment is putting pressure on the data center administrators to expand the temperature-humidity operating envelope and in some cases resorting to airside economizers, thereby, exposing their mission critical IT equipment to gaseous and particulate contamination, especially in regions with high pollution levels. It is therefore incumbent on the IT equipment manufacturers to not only improve the performance of their products but to also make them more robust against the harsher environments in which their products are expected to operate. It is necessary to develop more accurate failure models and as well as more reliability qualification test methods for the known failure modes such as the creep corrosion on printed circuit boards (PCB). Creep corrosion is the corrosion of copper and sometimes silver metallization on PCBs and the creep of the associated corrosion products to the extent that the corrosion products electrically short circuits closely spaced metallized features on PCBs. An iNEMI task force has been successful in developing a flowers of sulfur (FOS) based method for the qualification testing of PCBs for robustness against creep corrosion. The test has proven successful in correctly predicting the high propensity to creep corrosion on PCBs from lots that suffered creep corrosion in the field. The test has also consistently shown very extensive creep corrosion on PCBs from an iNEMI experimental test lot with electroless nickel on immersion gold (ENIG) finish. Since the ENIG finished PCBs are very popular in the consumer electronics industry due to their excellent solderability, there is an urgent need to determine the root cause of their susceptibility to creep corrosion and to provide a solution that will make them more robust against creep corrosion. The morphology of the ENIG finish and the creep corrosion products were analyzed using a range of analytical tools including optical microscopy (OM), focused ion beam (FIB), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Preliminary cross-section analysis has shown that there is a gap formed between two nickel layers at the edge of the solder mask. The gap provides an easy path for copper ion migration and for copper sulfide creep. In an effort to improve the robustness against creep corrosion, ENIG finishes with various phosphorus content and post-treatments were studied. The paper will present the root cause of the severe creep corrosion on the ENIG finished PCBs and ways to improve their robustness against creep corrosion.

Effect of Relative Humidity, Temperature and Gaseous and Particulate Contaminations on Information Technology Equipment Reliability

The energy used by information technology (IT) equipment and the supporting data center equipment keeps rising as data center proliferation continues unabated. In order to contain the rising computing costs, data center administrators are resorting to cost cutting measures such as not tightly controlling the temperature and humidity levels and in many cases installing air side economizers with the associated risk of introducing particulate and gaseous contaminations into their data centers. The ASHRAE TC9.9 subcommittee, on Mission Critical Facilities, Data Centers, Technology Spaces, and Electronic Equipment, has accommodated the data center administrators by allowing short period excursions outside the recommended temperature-humidity range, into allowable classes A1-A3. Under worst case conditions, the ASHRAE A3 envelope allows electronic equipment to operate at temperature and humidity as high as 24°C and 85% relative humidity for short, but undefined periods of time. This paper addresses the IT equipment reliability issues arising from operation in high humidity and high temperature conditions, with particular attention paid to the question of whether it is possible to determine the all-encompassing x-factors that can capture the effects of temperature and relative humidity on equipment reliability. The role of particulate and gaseous contamination and the aggravating effects of high temperature and high relative humidity will be presented and discussed. A method to determine the temperature and humidity x-factors, based on testing in experimental data centers located in polluted geographies, will be proposed.Copyright

Creep corrosion test in flowers of sulfur chamber

Creep corrosion is the corrosion of the copper (and sometimes silver) metallization on PCBs and the creeping of the corrosion product (mostly sulfides of copper and sometimes silver) on the board surfaces which may lead to the electrically shorting of neighboring features on PCBs. The problem of creep corrosion has been largely brought under control by selecting finishes, by trial and error, that have less propensity to creep corrosion. But the challenge of a reliable qualification test for creep corrosion remains, though some progress has been made developing the mixed flowing gas (MFG), the Chavant clay and the flowers of sulfur tests. The iNEMI (International Electronics Manufacturing Initiative) technical project team on creep corrosion is developing a flowers-of-sulfur (FOS) based qualification test for creep corrosion on printed-circuit boards (PCBs). The test setup consists of a 300-mm cube chamber with two means of mounting the test specimens and flowing air over them to expose them to constant, predefined humidity and temperature conditions and sulfur and other contaminants. The FOS chamber performance has been evaluated using copper and silver foils and preliminary test runs have been conducted on PCBs from a manufacturing lot known to have failed in service. The effect of air velocity on the copper and silver corrosion rates was quite linear. The effect of humidity on copper and silver corrosion rates in the low air velocity range of less than 0.1 m/s showed a strong dependence on relative humidity. In the high velocity range of 1 m/s, there was no clear dependence of humidity on copper and silver corrosion rates. A means has been developed for applying controlled concentration of ionic contamination on selected local areas of test PCBs. Preliminary test runs have shown that ionic contamination found in fine dust may be a necessary condition for copper creep corrosion. The focus of this paper is the design of a FOS chamber and its performance evaluation using copper and silver foils and the preliminary testing of the procedure using PCBs from a manufacturing lot known to have failed in service.

Predicting arcing in power supplies

Two novel techniques are described to predict arcing in switch mode power supplies used in large computer systems. One technique, called the partial vacuum test, can be used to determine locations susceptible to arcing. The second technique, called the zinc spray test, can help determine the minimum spacing between features, subjected to high voltages with high frequency harmonics, that will avoid arcing in the field.

Energy Efficiency and Air Quality Considerations in Airside Economized Data Centers

Many data center operators are considering the option to convert from mechanical to free air cooling to improve energy efficiency. The main advantage of free air cooling is the elimination of chiller and Air Conditioning Unit operation when outdoor temperature falls below the data center temperature setpoint. Accidental introduction of gaseous pollutants in the data center along the fresh air and potential latency in response of control infrastructure to extreme events are some of the main concerns for adopting outside air cooling in data centers. Recent developments of ultra-high sensitivity corrosion sensors enable the real time monitoring of air quality and thus allow a better understanding of how airflow, relative humidity, and temperature fluctuations affect corrosion rates. Both the sensitivity of sensors and wireless networks ability to detect and react rapidly to any contamination event make them reliable tools to prevent corrosion related failures. A feasibility study is presented for eight legacy data centers that are evaluated to implement free air cooling.Copyright

Preventing fire and smoke in power supplies

Leakage current through a printed circuit board epoxy has a strong Arrhenius dependence on temperature. Fire and smoke occurrences can be prevented by monitoring the leakage current between a ground plane and an adjacent test plane and shutting off power if the leakage current rises above a predetermined critical value.

Power supply arcing

The trend to pack more power in smaller spaces is leading to higher rates of computer power supply arcing in the field. Power density increase is being achieved by decreasing the spacing between features such as the power train MOSFET leads and by increasing the switching frequency. Both of these changes make power supplies more prone to field arcing. This paper discloses a technique called the partial vacuum test to predict the arcing propensity in power supplies. The partial vacuum test also helps determine the corrective actions needed to avoid field arcing by indicating the locations susceptible to arcing. The paper also describes a test called the zinc spray test that can help determine the minimum spacing between features, subjected to high voltages with high frequency harmonics, that will not arc in the field.