Jeff Punch

From Chip to Cooling Tower Data Center Modeling: Influence of Server Inlet Temperature and Temperature Rise Across Cabinet

To achieve reductions in the power consumption of the data center cooling infrastructure, the current strategy in data center design is to increase the inlet temperature to the rack, while the current strategy for energy-efficient system thermal design is to allow increased temperature rise across the rack. Either strategy, or a combination of both, intuitively provides enhancements in the coefficient of performance of the data center in terms of computing energy usage relative to cooling energy consumption. However, this strategy is currently more of an empirically based approach from practical experience, rather than a result of a good understanding of how the impact of varying temperatures and flow rates at rack level influences each component in the chain from the chip level to the cooling tower. The aim of this paper is to provide a model to represent the physics of this strategy by developing a modeling tool that represents the heat flow from the rack level to the cooling tower for an air cooled data center with chillers. This model presents the performance of a complete data center cooling system infrastructure. After detailing the model, two parametric studies are presented that illustrate the influence of increasing rack inlet air temperature, and temperature rise across the rack, on different components in the data center cooling architecture. By considering the total data center, and each components influence on the greater infrastructure, it is possible to identify the components that contribute most to the resulting inefficiencies in the heat flow from chip to cooling tower and thereby identify the components in need of possible redesign. For the data center model considered here it is shown that the strategy of increasing temperature rise across the rack may be a better strategy than increasing inlet temperature to the rack.

Effect of Ag content on the microstructure of Sn‐Ag‐Cu based solder alloys

Purpose – The purpose of this paper is to examine the microstructure and evaluate the intermetallic compounds in the following lead‐free solder alloys: Sn98.5Ag1.0Cu0.5 (SAC105) Sn97.5Ag2.0Cu0.5 (SAC205) Sn96.5Ag3.0Cu0.5 (SAC305) and Sn95.5Ag4.0Cu0.5 (SAC405).Design/methodology/approach – X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to identify the main intermetallics formed during solidification. Differential scanning calorimetry (DSC) was used to investigate the undercooling properties of each of the alloys.Findings – By using XRD analysis in addition to energy dispersive spectroscopy (EDS) it was found that the main intermetallics were Cu6Sn5 and Ag3Sn in a Sn matrix. Plate‐like e‐Ag3Sn intermetallics were observed for all four alloys. Solder alloys SAC105, SAC205 and SAC305 showed a similar microstructure, while SAC405 displayed a fine microstructure with intermetallic phases dense within the Sn matrix.Originality/value – Currently, low‐silver content SAC alloys are bei...

From chip to cooling tower data center modeling: Part I Influence of server inlet temperature and temperature rise across cabinet

To achieve reductions in the power consumption of the data center cooling infrastructure, the current strategy in data center design is to increase the inlet temperature to the rack, while the current strategy for energy-efficient system thermal design is to allow increased temperature rise across the rack. Either strategy, or a combination of both, intuitively provides enhancements in the coefficient of performance (COP) of the data center in terms of computing energy usage relative to cooling energy consumption. However, this strategy is currently more of an empirically based approach from practical experience, rather than a result of a good understanding of how the impact of varying temperatures and flow rates at rack level influences each component in the chain from the chip level to the cooling tower. The aim of this paper is to provide a model to represent the physics of this strategy by developing a modeling tool that represents the heat flow from the rack level to the cooling tower for an air cooled data center with chillers. This model presents the performance of a complete data center cooling system infrastructure. After detailing the model, two parametric studies are presented that illustrate the influence of increasing rack inlet air temperature, and temperature rise across the rack, on different components in the data center cooling architecture. By considering the total data center, and each components influence on the greater infrastructure, it is possible to identify the components that contribute most to the resulting inefficiencies in the heat flow from chip to cooling tower and thereby identify the components in need of possible redesign. For the data center model considered here it is shown that the strategy of increasing temperature rise across the rack may be a better strategy than increasing inlet temperature to the rack. Part II of this work expands on this paper with further parametric studies to evaluate the robustness of this data center cooling strategy, with conditions for optimal strategy deployment.

A reliability model for SAC solder covering isothermal mechanical cycling and thermal cycling conditions

This paper presents an alternative to the use of energy-based methodologies for life cycle predictions of solder interconnects. Isothermal mechanical cycling testing has been conducted using joint-scale solder samples on a novel testing apparatus. The test data shows that work as a single parameter is insufficient in predicting failure; nor does the inclusion of cyclic frequency and mean temperature improve work-based methodologies. Here, a novel semi-empirical approach is presented in which stress, strain, strain rate and temperature are individually treated to create a model capable of predicting material behaviour under arbitrary cyclic loading conditions. The model constants are fitted to the results of the isothermal mechanical cycling tests, using load drop as a measure of damage. The calibrated model is then employed to predict the failure of a BGA device under thermal cycling. The modelling results show state-of-the-art agreement with the test data and superiority over Morrow model constants from literature that have been applied to this data set.

Design and Fabrication of a Hybrid Superhydrophobic–Hydrophilic Surface That Exhibits Stable Dropwise Condensation

Condensation of water vapor is an essential process in power generation, water collection, and thermal management. Dropwise condensation, where condensed droplets are removed from the surface before coalescing into a film, has been shown to increase the heat transfer efficiency and water collection ability of many surfaces. Numerous efforts have been made to create surfaces which can promote dropwise condensation, including superhydrophobic surfaces on which water droplets are highly mobile. However, the challenge with using such surfaces in condensing environments is that hydrophobic coatings can degrade and/or water droplets on superhydrophobic surfaces transition from the mobile Cassie to the wetted Wenzel state over time and condensation shifts to a less-effective filmwise mechanism. To meet the need for a heat-transfer surface that can maintain stable dropwise condensation, we designed and fabricated a hybrid superhydrophobic-hydrophilic surface. An array of hydrophilic needles, thermally connected to a heat sink, was forced through a robust superhydrophobic polymer film. Condensation occurs preferentially on the needle surface due to differences in wettability and temperature. As the droplet grows, the liquid drop on the needle remains in the Cassie state and does not wet the underlying superhydrophobic surface. The water collection rate on this surface was studied using different surface tilt angles, needle array pitch values, and needle heights. Water condensation rates on the hybrid surface were shown to be 4 times greater than for a planar copper surface and twice as large for silanized silicon or superhydrophobic surfaces without hydrophilic features. A convection-conduction heat transfer model was developed; predicted water condensation rates were in good agreement with experimental observations. This type of hybrid superhydrophobic-hydrophilic surface with a larger array of needles is low-cost, robust, and scalable and so could be used for heat transfer and water collection applications.

Determination of the Anand Viscoplasticity Model Constants for SnAgCu

The major focus of this work was the determination of the nine constants required for Anand’s viscoplastic constitutive model for a lead-free solder alloy, 95.5Sn3.8Ag0.7Cu and to compare them with those for SnPb. The test specimen was a cast dog bone shape based on ASTM E 8M-01, with a diameter of 4mm and a gauge length of 20mm. A series of tensile experiments were carried out: constant displacement tests ranging from 6.5 × 10−5 /s to 1.0 × 10−3 /s at temperatures of 20°C, 75°C, and 125°C; constant load tests at a range of loads from 10MPa to 65MPa, also at temperatures of 20°C, 75°C, and 125°C. A series of non-linear fitting processes was used to determine the model constants. Comparisons were then made with experimental measurements of the stress-plastic strain curves from constant displacement rate tests: it was found that the model matched the experimental data at low strain rates but did not capture the strain hardening effect, especially at high strain rates. A finite element model of the test was also constructed using ANSYS software. This software includes the Anand model as an option for its range of viscoplastic elements, requiring that the nine constants be input. In this case, an 8-noded axisymmetric element (VISCO108) was used to model the test specimen under constant displacement rate loading. The model was then used to predict the stress-plastic strain curve and this was compared to both the experimental measurements and the fitted Anand model. Reasonable agreement was found between the Anand model and the FE predictions at small strain rates. Finally, a BGA device was simulated under accelerated temperature cycling conditions using ANSYS with the fitted Anand for the SnAgCu solder joints. A Morrow-type fatigue life model was applied using empirical constants from two published sources and good agreement was found between experiment and predicted fatigue life.Copyright

Finite element modelling of a BGA package subjected to thermal and power cycling

The finite element techniques of substructuring and submodelling have been applied to a 9/spl times/9 ball grid array in order to estimate the fatigue life of the solder joints under various thermal loading conditions. Darveauxs method, which relates the accumulated viscoplastic strain energy density and crack growth data to fatigue life, has been used in all cases to predict the life of the solder joint. Three types of cycle were considered: (i) isothermal temperature cycling, (ii) isothermal temperature cycling with constant heat generation in the die, and (iii) power cycling (transient heat generation in the die). Results indicate that for the first two cases, the solder joint closest to the centre will fail first and that the superimposed constant heat generation in the die has little effect on fatigue life. In the case of power cycling, the outermost diagonal joint is predicted to fail first. The two finite element techniques examined are shown to produce similar results, however, substructuring is not applied to the power cycling case due to the transient nature of the problem.

The influence of the Pb-free solder alloy composition and processing parameters on thermal fatigue performance of a ceramic chip resistor

This paper presents the results of a thermal fatigue study of a 2512 ceramic chip resistor assembled with various Pb free solders including SnCu, SAC105, SAC205, SAC305, and SAC405. The test matrix also includes some limited evaluations with other variables such as cooling rate (solidification rate), thermal preconditioning and nitrogen (inert) reflow atmosphere. The matrix also includes a SnPb eutectic control cells. The resistor test vehicle provides an expedient and self-consistent method for evaluating the relative fatigue performance of the various alloys. A study of the as-assembled solder joints was conducted to characterize the microstructure of the solder joints with varying silver content. Thermal fatigue was evaluated using an accelerated temperature cycle of 0/100 °C with dwell times of 10 and 60 minutes. The test results show a direct relationship between characteristic fatigue life and Ag content, with the higher Ag content alloys outperforming those with the lowest Ag content. As might be anticipated, there also was a consistent inverse relationship between fatigue life and dwell time for the Pb free solders. The failure analysis and microstructural evolution is characterized with optical metallography and scanning electron microscopy and the fatigue reliability of the Pb free solders is discussed in terms of the microstructures.

Phenomenological Study of the Effect of Microstructural Evolution on the Thermal Fatigue Resistance of Pb-Free Solder Joints

Unlike SnPb solders, the thermal fatigue reliability of the Sn-Ag-Cu (SAC) solders is believed to be influenced significantly by both the initial and evolving microstructures. This paper presents a phenomenological study of the relationship between the initial SAC solder joint microstructure, the evolving microstructure, and the thermal fatigue performance measured by accelerated temperature cycling (ATC). To reflect the board assemblies that are in field use, commercial surface mount components with multiple geometries and materials and from different package assemblers were joined to the board with different lead free SAC alloys. The initial microstructures of the board level solder joints were altered in a variety of ways including: 1) varying the solder joint cooling rate; 2) varying the number of solder reflow exposures; and 3) exposure to different isothermal temperature exposures. In all cases the solder joint microstructure was exposed to one or more of these treatments prior to exposure to temperature cycling. In addition, some of the test boards were exposed to different cycling dwell times to determine if the microstructural evolution that occurred during ATC testing effected the respective characteristic lifetimes of the joints. The microstructural evolution was tracked and characterized with optical metallography and scanning electron microscopy. These results could have practical implications in terms of limiting the ability to develop acceleration factors and effective strain-based models for predicting Pb-free solder joint life.

Enhanced vibrational energy harvester based on velocity amplification

This article presents a fundamental investigation in which velocity amplification is employed in non-resonant structures to enhance the power harvested from ambient vibrations. Velocity amplification is achieved utilising sequential collisions between free-moving masses, and the final velocity is proportional to the number of masses and the mass ratios selected. The governing theory is discussed, particularly how the final velocity scales with the number of masses. This article examines n-mass velocity-amplified vibration energy harvesters and examines their performance relative to single-mass harvesters. Electromagnetic energy conversion is chosen as it is fundamental in allowing the free movement of the masses. Experimental results from two- and three-mass prototypes are presented that demonstrate a wider frequency response and a gain in power of 33 times compared to single-mass configurations under wideband random excitation. The volume of the devices was constrained, which resulted in the two-mass system outperforming the triple-mass system counter to expectations. This was caused by the triple-mass device experiencing an increased number of impact due to the volume constraint, leading to high losses in the system. It is recommended that in order to realise the full benefits of the triple-mass system, additional volume for mass actuation is required.