Thermal Management Of Microelectronic Packages

Published Date: 02 Nov 2017

MEE-580 Research Project

By

Muhammad Omar Memon

University of Dayton

April 2013

Introduction

Thermal management is an important design consideration for number of microelectronic components and packages. Few essential ways of thermal management of microelectronic packages are efficient cooling techniques, efficient thermal interfaces and heat dissipaters. Due to the current trend of increasing heat fluxes and smaller dimensions, thermal management becomes compulsory to achieve optimum performance and reliability of the system. In order to guarantee that, modern electronic systems are characterized by increased density of circuits which is a sole cause of rapid temperature increase within the circuit. The temperature rise caused by such intensive thermal energy dissipation may be sufficiently large to cause some damage to the system. Improved functionality and efficient performance of the products require highly efficient heat dissipation mechanisms.

Literature Review

Since thermal issues threaten to limit the thermal and electrical performance of the system, package-level thermal management has become a primary concern for any industry. There are a number of microelectronic packages that are being used in the manufacturing industries around the world. Flip chip packages are commonly used because of their improved electrical and thermal performance. These packages are primarily used in high performance microprocessors, digital signal processors, networks and data storage applications [1]. In order to achieve high performance and sustainability, flip chip packages are constructed using different types of materials, e.g. substrate either as laminate or ceramic with many metallization layers, lid, die and heat spreader [2]. These conductive materials can be used for a wide range of applications such as microelectronic packages, electrostatic discharging in electronic components, light weight coating materials for electromagnetic interference shielding, lightning strike protection of various aircraft, automobile and turbine blade structures.

It is important to research and understand how to improve the performance and reliability of the package through thermal management. This task involves considering many factors such as tradeoffs for electrical design, package layout, material selection, processing, the feasibility of manufacturing, and the cost of production [3]. The electrical design of any electronic package holds a vital importance in the efficiency of the system. A finite distance below the heat source is required for the heat flux to become uniform with the ambient. Similarly, the type and location of heat sinks also have significant effects on the overall performance of the device. There have been a number of comprehensive reviews that explored various types of heat sinks designed for efficient heat dissipation [4-9]. Lelea [4] presented a geometric optimization of the micro-heat sink with straight circular micro-channels. The inlet cross-section of rectangular shape was positioned tangentially to the tube axis with four different geometries. The results of thermal performances of heat sink were presented in terms of the temperature distribution along the bottom wall at constant heat flux and temperature of 100 W/cm2 and 293 K respectively. The author observed that the lowest temperature was obtained for case 3 where the inlet channel width covered half of the tubes cross-section with minimum temperature of 313.2 K and ΔT of 10.17 K. The author reported that the results from tangential micro-heat sink showed better thermal performance than the conventional micro-heat sink with lateral inlet/outlet cross-section.

The fin-heat sink method [10-11] has so far proved to be the most reliable and highly cost efficient method for efficient heat dissipation in similar packages. Some researchers [12-13] proposed a micro-jet array cooling system for electronic packages and they found that it has more advantages over conventional cooling system for example heat pipe and regular fin-based cooling mechanisms. Jouhara and Axcell [9] described the thermal conditions within a heat sink with rectangular fins under forced convection cooling. The authors show a numerical study on key parameters of heat transfer that change with axial distance. The heat transfer coefficient and pin efficiency rapidly changed near leading edges of cooling fins. The authors reported that despite the rapid changes in these parameters, good engineering accuracy for heat sink performance is attainable using analytical methods which incorporate average values of heat transfer coefficient and fin efficiency. Elshafei [5] performed experiments on natural convection heat transfer from circular pin fin heat sinks subject to the influence of its geometry, heat flux and orientation. The author found that the solid pin fin heat sink showed competitive performance for upward and sideward orientations. Higher heat transfer heat transfer coefficients were obtained in comparison to those of perforated/hollow pin fin ones in both arrangements. The augmentation factor was found to approximately 1.05-1.11. According to the author, the temperature difference between the base plate and surrounding air, at the same heat input value, was found to be less for hollow/perforated pin fin heat sink than that for solid pin heat sink.

Kim et al. [7] suggested closed form correlations for thermal optimization of vertical plate-fin heat sinks under natural convection in a fully-developed-flow regime. Analytical solutions for velocity and temperature distributions for high channel aspect ratios, high conductivity ratios, and low Rayleigh numbers were presented using the volume averaging approach. The analytical solutions explained the explicit correlation for optimal fin thickness and optimal channel width, which minimize thermal resistance for given height, width, and length of heat sink. The authors reported that these correlations show that the optimal fin thickness is a function of height, the solid conductivity, and the fluid conductivity only and is independent of the Rayleigh number, the viscosity of the fluid, and the length of the heat sink. Huang et al [8] carried out experiments on natural convection heat transfer from square pin fin heat sinks subject to the influence of orientation. The authors tested a flat plate and seven square pin fin heat sinks with various arrangements under controlled environment. The results indicated that the upward and sideward facing orientations were of comparable magnitude and showed competitive nature. Another significant factor highlighted by the authors was the porosity of the heat sink which had secondary effect on the performance of the pin fin. The optimal porosity of the heat sink was found to be around 83% for upward arrangement and 91% for the sideward arrangement. The augmentation factor (heat transfer of a heat sink relative to the base plate) was found to be around 1.1-2.5 for the upward arrangement and between 0.8–1.8 for the sideward arrangement.

Khor et al [6] investigated the importance of the effects of thermal radiation and its view factor on the thermal performance of a straight-fin heat sink. Three different models were developed to explore these effects on the convection coefficient as well as the fin performance of the heat sink. Results show that the average convection coefficient for the case neglecting thermal radiation was largest with 30% error, followed by that with thermal radiation including view factor, and that with thermal radiation excluding view factor as the lowest with 60% error. The fin effectiveness was overrated for the cases when thermal radiation and view factor were excluded with more than 40% error. The authors concluded that it is reasonable to exclude thermal radiation under certain circumstances but to consider thermal radiation without incorporating the view factor could be resulting in more critical and detrimental errors.

An essential type of electronic packages is made of ceramic. Ceramic packages show better thermal expansion and dissipation than other packages such as those made of plastic. Kandasamya and Mujamdar [2] considered major contributing factors to the thermal performance such as bond line thickness, bulk thermal conductivity of the material and effect of void parameters during the assembly. The authors carried out a numerical study on three different ceramic ball grid array (FC-CBGA) packages (35 mm, 22 mm and 18 mm) using cup lid, flat lid and no-lid configurations to simulate power dissipation of the package. They found that either eliminating the thermal interface material TIM or using one with least interfacial resistance was very effective in cooling of high power thermal applications with bare die packages. The die size plays a significant role in efficient thermal power dissipation of the package. Higher Theta-JC performance was observed for the large die in comparison to the smaller die. In comparison to that, Mujumdar et al. [14] compared the thermal performance of a 35 x 35 mm FC-CBGA package with different die sizes that included 5x 5 mm, 15 x 15 mm, and 20 x 20 mm. The lid fitted heat sink performance was investigated in application specific integrated circuit (ASIC) chip, in correspondence with the JEDEC criteria. An exceptional agreement and consistency was found between the numerical results and the data that was measured. The thermal performance was observed with a package that was lidded as opposed to the un-lidded package. However, there was no noticeable improvement observed between lidded and un-lidded packages when they were fitted with a heat sink subjected to forced convection. The package thermal budget estimate variations with and without heat sinks was also discussed in the paper. Circuit board that was printed and package top surface patterns of temperature were analyzed and measured using an infrared thermal camera. Parametric studies were also carried out in order to understand the effect of radiation effect, die size, and gird size variations and airflow rate on die junction temperature and the thermal resistance on the package. The study incorporates the effect of substrate, lid, PCB temperatures, and die for different die sizes in natural and forced convection environments.

Light-emitting diode (LED) is a type of solid-state semiconductor device that directly convert electrical energy into light. Due to their extraordinary color saturation and excellent reliability over time, these are one of the technologies with superior potential. These do not only produce great light, but also guarantee energy savings when compared to the conventional sources of light. High power LEDs are very popular in the solid illumination industry and is considered as next generation device for wide variety of applications such as LCD displays, visual indicators in computers and microelectronic devices, automotive lighting including interior and exterior displays, headlights, signals and luminaries [15-18]. Majority of researchers have carried out extensive research on the thermal management of the LED packages. In those studies, these LED packages were studied for many different applications such as LED package design and analysis [19,20], high thermal conductivity materials for thermal packaging [21,22] and thermal interface materials [23] and efficient cooling mechanisms [24,25].

Christensen and Graham [26] showed in their report that the package temperature distributions that were of a high power light emitting diode array had been investigated using quantitative heat flow models. A thermal resistor and network model was combined with a 3D finite element sub-model of an LED structure for the analysis. There were varying degrees of function between the different groups. In order to better understand the various roles of thermal resistance in cooling, the resistance network was analyzed to draw up estimates. This study overall, also had an aspect to give methods that would help reduce the packaging resistance for high power LEDs. Tsai et al. [27] conducted experiments that showed in the results that the thermal resistance of the low-cost high power LED package was similar and comparable of commercial packages. Also, it was found that the Tj and thermal resistances of the package, calculated from the 3D TRC, 2D ANSYS, and 3D CF design are formed and consistent with those from the experimentation. There are differentiations between the different designs for thermal analysis and the equation based convection coefficients. The variations between the LED module and the others showed the results in the forced convection and the design parameters.

Weng [28] demonstrated the cooing of LED package by 20%-30% decrease in the thermal resistance over the package geometry. The author found that by reducing thermal resistance at the interface, the cooling of the LED package is very efficient compared to that with higher interfacial resistance. This interfacial resistance plays a vital role in the heat transfer from the heat source to the heat sink. This could be complimented with the use of high thermal conductivity materials for microelectronic packages such as aluminum oxide (Al2O3, ~20 W/mK), silicon nitride (Si3 N4,~70 W/mK), and aluminum nitride (AlN, > 170 W/mK) ceramics instead of conventional flame retardant (FR-4) epoxy (0.2 W/mK), (Pb,La), (Zr,Ti)O3 films for electro-optics, Pb(Zr,Ti)O3 films for piezoelectrics, yttria films for plasma damage-free coatings, and the phase transformation of cubic AlN during the aerosol deposition process [29].

Cho and Kim [30] investigated, by the aerosol deposition mechanism, the heat dissipation properties of metal-core printed circuit boards (MCPCBs) with an alumina (Al2O3) layer. The authors showed that the total thermal resistance of the MCPCBs with an alumina dielectric layer in a packaged LED form was obtained to be approximately 34.5 K/W as compared to the conventional MCPCBs with thermal resistance of approximately 38.5 K/W. Heo et al. [29] applied high thermal conductivity aluminum nitride (AlN) films (directly deposited on the aluminum plates) to replace low thermal conductivity epoxy resin and various alumina substrates. This caused the removal of the thermal adhesives sheets which are traditionally used as thermal interface materials in metal printed circuit boards. The authors calculated the thermal resistance of the LED package and found that the package mounted on the AlN thick film was 28.5 K/W, while an LED package mounted on a conventional epoxy-based metal PCB and a PCB with thermal vias were 47.2 K/W and 36.5 K/W, respectively. This shows significant enhancement in the heat transfer across the interface in the aerosol-deposited AlN-based LED package.

Lu et al. [31] performed a thermal analysis of high power LED package using a flat heat pipe (FHP). The authors experimentally investigated the thermal characteristics including startup performance, temperature uniformity and thermal resistance of high power LED package with FHP heat sink. They showed that the junction temperature of LED is about 52 °C for the input power of 3 W, and correspondingly the total thermal resistance of LED system is 8.8 K/W. In addition to that, they indicated that different filling rates and inclination angles of the heat pipe to the heat transfer performance of the heat pipe should be taken into account since these parameters effect the cooling system [31]. Wang [32] investigated the thermal performance of heat sinks with one and two pairs of embedded heat pipes. The embedded heat pipes transfer the total heat capacity from the heat source to the base plate and disperse heat into the ambient. The author found that that two and four heat pipes embedded in the base plate carry 36% and 48% of the total dissipated heat respectively. It was also reported that when the total heating power of the heat sink with two embedded heat pipes was 140 W, the total thermal resistance reached the minimum value of 0.27 °C/W, while for the heat sink with four embedded heat pipes, when the total heating power was between 40 W and 240 W, the total thermal resistance was calculated to be 0.24 °C/W. Chen et al. [33] derived various boundary conditions based on the heat flow and temperature to design a compact thermal model for LED packages. The authors performed finite element method modeling for simulating the LED package with different heat slug, PCB, cooling condition and chip sizes. They found surprising correlations in the thermal design with varying boundary conditions. These correlations direct to the system level thermal management for these applications.

Since maximum power of the LEDs is converted to heat, it is really important to dissipate that heat. The life of any electronic package decreases exponentially as the junction temperature increases. Narendran and Gu [34] demonstrated the use of a low-operation temperature for LEDs since these have dense packaging and require high output power which contradicts between the power density and operation temperature, especially when applications require LEDs to operate at full power to produce the desired brightness. Faranda et al. [35] proposed and investigated a prototype based refrigerating liquid for heat dissipation of power LEDs. The authors carried out optimization investigation of the proposed solution to find an optimum thermal performance to be established and set as a primary boundary condition for similar systems. They performed experiments with different heights of liquid levels since the liquid determines better heat dissipation and diminution of the junction temperature. Larger operating current was supplied to the components until the junction temperature of the traditional solution was reached. By obtaining an increment of light radiation, the authors revealed that the refrigerating liquid cooling is a powerful way for heat dissipation of high power LEDs, and the fabrication of prototype was feasible and useful.

Yung et al. [36] presented experimental thermal analysis of natural convective air cooling of a high brightness 3×3 LED array package on a printed circuit board (PCB) during operation from 0 to 180° inclinations. The authors used IR camera and thermocouples to conduct thermal profile measurement for temperature distribution and heat flow analysis of the LED package. They found that the effect of position and inclination plays an important role in the heat dissipation of the LED package. The authors were able to establish a criteria for not only setting up a LED array system, but also to adopt design features that would be beneficial to achieve efficiency in thermal management. Hu et al. [37] used high power light emitting diodes (LEDs) with ceramic packages to carry out thermal and mechanical analysis. The authors found that there was high level of thermal and mechanical stress even though there were less mismatching coefficients of thermal expansion compared to those in plastic packages. Usually, the performance of the LEDs is degraded as such high power operations are undertaken. In order to reduce that degradation, the application of silicon based thermoelectric cooler integrated with high power LED can be used. This LED has lower thermal resistance and the thermoelectric cooler package helps reduce the thermal resistance even further to almost zero [38]. The mismatching in the coefficient of thermal expansion can create distortions, cracks and even catastrophic damages to the microelectronic device. Hence, a good match in these coefficients of new package materials to the semiconductor device is essential. The thermal design of GaN electronic packages using new materials not only provide good match for coefficients of thermal expansion, but also have higher thermal conductivity than the current conventional materials.

A number of diamond-reinforced composites are used in the development of GaN based electronics such as GaN based high electron mobility transistors (HEMTs) and LEDs. The popularity of these materials is growing because of the high demand of power densities. Using the diamond materials, the cost constraints should also be taken into account. The reduced cost of synthetic diamond grains and latest diamond fabrication technologies have given an edge to the use of diamond composite for thermal management. In addition to that, the greater bonding between the diamond particles within the metal matrix gives it superiority over other composites to be used as an efficient material for thermal management applications. Faqir et al. [39] presented a novel packaging solution for GaN power electronics for efficient heat extraction in high power devices. The authors carried out Micro-Raman thermography measurements to probe the device temperature for devices mounted on different base plates, silver diamond composites and standard Cu platted material at a range of operating power levels. Since the base-plate roughness of less than 1 lm is desirable for many applications, the bare silver diamond coating was used. The authors found a significant reduction in GaN power electronics temperature by up to a factor of two, was obtained when using silver diamond composites base plates. They reported a significant improvement in the thermal management of GaN devices with respect to the existing packaging technologies.

An imperative way to improve the heat dissipation problem is to improve the thermal contacts using efficient material. A vital property of any material is its thermal conductivity. All modern materials such as aluminum oxide, aluminum nitride, boron nitride, graphite and diamond powder, silver that increase thermal conductivity up to the 2 W/m-K. A number of researchers [40-42] used diamond with several other materials to develop a unique thermal package. Aluminum diamond was developed with a CTE of 7.5 ppm/K and thermal conductivity of 500 W/(m K) [40]. The copper diamond composite material made with 55 vol.% of good quality diamond was developed with a thermal conductivity of 420 W/(m K) [41]. The copper diamond composites with diamond volume fraction of 62% can lead to a better performance, with a thermal conductivity of 530 W/(m K), and a CTE of 5.5 ppm/K [42]. Silver diamond composites (consisting of diamond particles) in a matrix of silver alloy were developed with the ultra-high thermal conductivity of 700 W/(m K), significantly larger than CuW, at room temperature and a CTE close to that of the semiconductor materials [43]. Because of ultra-high thermal conductivity and excellent bonding between these materials, they are capable of showing optimized thermal management solutions when used in microelectronic packages for high power requirements.

Unfortunately, high thermal conductivity alone is not enough to guarantee optimal system performance. Thermal resistance obtained from a one-dimensional heat flow calculation should also be minimized. For solids of high thermal conductivity, the contact resistance may be reduced by increasing the area of contact spots, accomplished either by increasing contact pressure which will flatten the peaks of the micro-roughness, and deflecting the mating surfaces to reduce any non-flatness, or reducing the roughness of the surfaces before the interface is formed by grinding the surfaces to remove non-flatness and buffing the surface to reduce micro-roughness [44].Other properties taken into account are interfacial thickness and applied forces. In addition to thermal performance, materials for thermal packaging are selected based on their inherent thermal properties and long-term stability (reliability). Researchers are constantly seeking "better" thermal interface products. This is usually expressed as a request for higher thermal conductivity and lower thermal resistance. Phase change materials (PCMs) have been indicated by the researchers as high storage capacity materials and significantly efficient for heat dissipation at the thermal contact. Liu and Chung [45] carried out a comparative study on phase change materials for electronic packages with various melting temperatures close to room temperature. The authors used the Parafin wax as well as the microcrystalline in their study. They found that paraffin wax was potentially good thermal interface material because of the negative super-cooling of −7°C, large heat of fusion (up to 142 J/g) and excellent thermal cycling stability. Paraffin also exhibited clear endothermic and exothermic DSC peaks which were not clear for microcrystalline. Incongruent melting and decomposition behavior was also seen in microcrystalline with high supercooling of 8°C or more and instable thermal cycling. Most PCMs with high energy storage density have an unacceptably low thermal conductivity. Therefore the performance of the PCMs can be enhanced by mixing the PCM with polymers and with particulate fillers of high thermal conductivity.

Kandasamy et al. [46] had included in their studies the application of a novel PCM package for thermal management of devices that were portable. It was experimentally investigated for the effects of different parameters. There was also a two-dimensional numerical study that was made and compared to the experiment results. The outcome showed that the increase power inputs increased the rate of melting, while gravity had a negligible effect (on the package) on thermal performance of the PCM package. The thermal resistance of the device and the power level applied to the package were of vital importance for the design of a passive thermal control system. The association with numerical results confirmed the PCM-based design was an exceptional candidate design for electronic cooling applications. Krishnan and Garimella [47] carried out a transient analysis of the phase change process inside a rectangular enclosure for microelectronics cooling applications with pulsed power dissipation. The experimental investigation included the melting of a pure PCM, n-eicosane, inside a rectangular aluminum container with multiple discrete heat sources mounted on one side. The authors examined the influence of frequency changes of the pulses, heat source location and aspect ratio of the containment volume on the thermal performance of the PCM unit. The performance of the PCM system was quantified by studying maximum temperature observed in the container, cumulative heat energy absorbed heat loss through the container walls and instantaneous heat absorbed. The authors reported that the heat source location and container aspect ratio played a very significant role in the performance of the PCM system with pulsed heat input. Krishnan and Garimella [48] also performed a transient thermal analysis to investigate thermal control of power semiconductors using phase change materials, and to compare the performance of this approach to that of copper heat sinks. The authors concluded that a significant suppression of junction temperatures was achieved by the use of PCMs when compared with copper heat sinks. The researches indicate that, although the thermal conductivity of a sold material is important for microelectronic packages, the efficiency and conductance of the plating material also plays an important role in consenting efficient thermal management.