Xiaojin Wei

Modeling of vapor chamber as heat spreading devices

As the heat density on the chip increases and heat dissipation is more localized, there are growing interests in developing alternative heat spreading devices. Vapor chambers have been used as heat spreading devices in heat sink bases. Before considering for integration into package heat spreaders or lids, the compatibility of the vapor chamber with the assembly processes would have to be assessed. However, before embarking down that path, this paper attempts to quantify the thermal benefit, if any, of vapor chamber heat spreaders compared to commonly used solid metal heat spreaders. A thermal model has been developed consisting of a heated chip integrated with a substrate, thermal interface material, vapor chamber lid and heat sink. The vapor chamber is represented by multiple block layers with effective thermal conductivities. It is revealed that the model can predict the temperature profile fairly well as compared with the results of a detailed numerical model. A sensitivity study shows that the thermal performance is sensitive to the effective thermal conductivity of the wick structure and insensitive to the effective thermal conductivity of the vapor space. A parametric study indicates that the vapor chamber heat spreader out-performs a copper block of the same dimension when the footprint size is larger than a certain value. It is concluded that vapor chamber is most effective in spreading heat over large areas. Consequently, it is suitable for applications where more surface area is desired due to reasons such as low heat transfer coefficients

Thermal modeling for warpage effects in organic packages

INTRODUCTION Maintaining the integrity of the heat dissipation path for high end microelectronic devices has become increasingly challenging as the industry migrates from ceramic to organic packaging. Typically, flip chip organic packages undergo significant thermal and mechanical stresses throughout the manufacturing process, including chip join, underfill, encapsulation, BGA attach and card join. As a result of the mismatch of thermal and mechanical properties between the components, significant warpage is generated in the organic substrate, the chip, and the thermal interface material (TIM1) on completion of the assembly. Warpage affects not only the substrate coplanarity but also the thermal performance of the TIM1. At room temperature, the center of an adhesive TIM1 is under compression between the lid and chip while the corners and edges are under tensile stress. The effective thermal conductivity for the portions of the TIM1 under tensile stress can be significantly lower due to the elongation of the gap and the narrowing of the heat flow area. The present paper describes an approach to model the thermal performance of an adhesive thermal interface material taking into account the warpage effects and the inherent out-of-flatness in the heat spreader. Reasonable agreement is obtained between the modeling results and thermal measurements for a representative thermal test vehicle. The present modeling approach can potentially be used to optimize the component design and the bond and assembly process to achieve optimum thermal performance.

Mechanical packaging, power, and cooling design for the IBM z13

The system-level packaging of the IBM z13™ supports the implementation of a new drawer-based Central Processor Complex (CPC). Departing from previous IBM z Systems™ designs, the introduction of distributed land-grid-array (LGA) attached single-chip modules (SCMs) required new mechanical, power, and cooling designs to address specified performance requirements and to provide enhanced reliability, availability, and serviceability (RAS) attributes. Building upon the designs created for the IBM zEnterprise® BC12 (zBC12), new CPC drawer and frame mechanical designs were created to significantly increase overall packaging density. Similar to its predecessor, the IBM zEnterprise EC12 (zEC12), the z13 utilizes water-cooling of the processors, but in contrast to the single input and return flow used to cool the multi-chip module (MCM) in the zEC12, the z13 accomplishes its processor cooling using a flexible hose internal manifold design that provides parallel input and return fluid flow to each SCM. The use of flexible hose also enabled SCM field replacement, new to high-end IBM z Systems. A new internal cooling loop unit and an updated external (building-chilled) modular water-conditioning unit were designed utilizing customized water delivery manifold systems to feed the common CPC drawer design. Revised power delivery and service control structures were also created to address the distributed nature of the z13 system design.

Development of a flexible chip infrared (IR) thermal imaging system for product qualification

A flexible and efficient chip Infrared (IR) thermal imaging system was implemented on the product manufacturing test platform by collaboration with the burn-in/wafer test, systems, process, and failure analysis teams. A liquid cooling cell was successfully designed and tested. The imaging system was applied to investigate some wafer probe power/thermal issues for server high end products. Furthermore, we applied the method of Spatially-resolved Imaging of Microprocessor Power (SIMP) [1] to translate the thermal map into a power map. Finally, we propose a new concept of product thermal qualification as a supplement and potential alternative to the traditional thermal test vehicle (TTV) qualification.

Thermal-mechanical Co-design of Cold Plate, Second Level Thermal Interface Material (TIM2) and Heat Spreaders for Optimal Thermal Performance for High-end Processor Cooling

Cooling high-end system processors has become increasingly more challenging due to the increase in both total power and peak power density in processor cores. Junction peak temperature at worst case corner conditions often establish the limits on the maximum supportable circuit speed as well as processor chip yield. While significant progress has been made in cooling technology (e.g., cold plate design and thermal interface materials at first and the second level package), a systematic approach is needed to optimize the entire thermal and mechanical stack to achieve the overall (optimal) thermal performance objectives. The necessity and importance of this is due to the thermal and mechanical design interdependencies contained with the overall stack. This paper reports an in-depth study of the thermal-mechanical interactions associated with the cold plate, second level thermal interface material (TIM2) and heat spreaders. Thermal test results are reported for different cold plate designs and TIM2 pad sizes. Thermal and mechanical modeling results are provided to quantify the TIM2 thermal performance as a function of the TIM2 mechanical stress, the TIM2 dimensions and cold plate design. As described via both modeling and testing results, an optimal TIM2 pad size results as a trade-off between heat transfer area for conduction and TIM2 compressive pressure. In addition, pressure sensitive film study results are also provided revealing that heat spreader design affects the module level and TIM2 thermal performance. Results from this set of work clearly demonstrate the close interactions between cooling hardware in the stack hence the importance of thermal-mechanical co-design to achieve optimal thermal performance for the high-end processors.

Modeling blower flow characteristics and comparing to measurements

Thermal management for high performance electronic systems such as servers and I/O boxes has become increasingly challenging due to the ever growing demand for higher computing performance and packaging density. Proper modeling, design and characterization of the cooling system have become critical to the overall system performance, reliability and energy consumption. Air moving devices such as blowers (centrifugal fans) are key components of the air-cooled electronic systems. This paper focuses on the flow characteristics of blowers and the impact on the system air flow distribution. To capture the flow characteristics, different levels of numerical modeling methodology are considered using a commercially available tool. It is demonstrated that a simple compact model, typically used in system level models, is not sufficient to resolve the air flow distribution near the exhaust. A more detailed model which includes the actual geometry of the blower blades resolves the body forces using MRF and predicts the flow distribution with better agreement with measurement data. Comparing the different modeling methodologies for systems of different impedance characteristics, a general guideline is subsequently proposed.

Air-water hybrid cooling for computer servers: A case study for optimum cooling energy allocation

Air-water hybrid cooling offers flexible design choices for computer systems with components of different thermal management needs. On one hand, water cooling enables the continuous growth of CPU performance and increasing packaging density. High performance cold plates such as microchannels have been successfully implemented for water cooling in previous high-end systems. When coupled with an air-water heat exchanger or radiator, the water loop becomes a closed one with no need for facility chilled water. This significantly reduces the complexity to deploy the server in the data center. On the other hand, for components with less thermal demand, traditional air-cooling technology is adequate with low cost, high availability and better serviceability. For the computer system as a whole, an air-water hybrid cooling system may be optimized. Such a hybrid system typically requires pumps to drive the water loops, air-movers to drive air through the radiator and blowers or fans to drive the air flow for component cooling. It is the focus of this paper to study the optimum allocation of energy between the pumps and air-movers for a given total cooling energy budget and overall load. The goals are to achieve better overall thermal performance and to reduce the cooling energy consumption. To this end models for each cooling block are established based on test data. These include the air-water heat exchanger, pumps, blowers, and cold plates. These models are linked together to predict the overall thermal system operating points for different application scenarios. A parametric study is then conducted to define the near optimum allocation of cooling energy for these scenarios that meets the thermal design objectives. Additionally, sub-threshold leakage for the CPU is taken into account to enhance the model since temperature provides positive feedback. It is shown through modeling that additional performance enhancement is possible with judicious allocation of cooling energy for a given overall energy budget. It is argued in this paper that overall energy efficiency can be improved significantly through intelligent data driven energy allocation.

In-situ cure assembly and thermal performance verification of electronic packages using chip heat

An innovative in-situ electronic package assembly process is presented using the on-chip power to cure the package thermal interface and seal materials and to verify the thermal performance of the electronic package. The in-situ curing process was thermally modeled to demonstrate that the thermal interface and seal materials would reach their required curing temperatures by controlling the chip power and heat sink air flow. Experimental validation of the in-situ cure method was conducted using thermal test vehicles. Thermal resistance characterization of the in-situ cured package in comparison to a batch oven cured package showed better thermal performance stability with increasing temperature.