Thermal and thermo-mechanical performance of voided lead-free solder thermal interface materials for chip-scale packaged power device

The need to maximise thermal performance of electronic devices coupled with the continuing trends on miniaturization of electronic packages require innovative package designs for power devices and modules such as Electronic Control Unit (ECU). Chip scale packaging (CSP) technology offer promising solution for packaging power electronics. This is as a result of the technology’s relatively improved thermal performance and inherent size advantage. In CSP technology, heat removal from the device could be enhanced through the backside of the chip. Heat dissipating units such as heat spreader and/or heat sink can be attached to the backside (reverse side) of the heat generating silicon die (via TIM) in an effort to improve the surface area available for heat dissipation. TIMs are used to mechanically couple the heat generating chip to a heat sinking device and more crucially to enhance thermal transfer across the interface. Extensive review shows that solder thermal interface materials (STIMs) apparently offer better thermal performance than comparable state-of-the-art commercial polymer-based TIMs and thus a preferable choice in packaging power devices. Nonetheless, voiding remains a major reliability concern of STIMs. This is coupled with the fact that solder joints are generally prone to fatigue failures under thermal cyclic loading. Unfortunately, the occurrence of solder voids is almost unavoidable during manufacturing process and is even predominant in lead (Pb)-free solder joints. The impacts of these voids on the thermal and mechanical performance of solder joints are not clearly understood and scarcely available in literature especially with regards to STIMs (large area solder joints). Hence, this work aims to investigate STIM and the influence of voids on the thermo-mechanical and thermal performance of STIM. As previous results suggest that factors such as the location, configuration (spatial arrangement) and size of voids play vital roles on the exact effect of voids, extensive three dimensional (3D) finite element modelling is employed to elucidate the precise effect of these void features on a Pb-free STIM selected after thermo-mechanical fatigue test of standard Pb-free solder alloys. Finite element analysis (FEA) results show that solder voids configuration, size and location are all vital parameters in evaluating the mechanical and thermal impacts of voids. Depending on the location, configuration and size of voids; solder voids can either influence the initiation or propagation of damage in the STIM layer or the location of hot spot on the heat generating chip. Experimental techniques are further employed to compare and correlate levels of voiding and shear strength for representative Pb-free solders. Experimental results also suggest that void size, location and configuration may have an influence on the mechanical durability of solder joints. The findings of this research work would be of interest to electronic packaging engineers especially in the automotive sector and have been disseminated through publications in peer reviewed journals and presentations in international conferences

A review of photovoltaic module technologies for increased performance in tropical climate

The global adoption and use of photovoltaic modules (PVMs) as the main source of energy is the key to realising the UN Millennium Development Goals on Green Energy. The technology – projected to contribute about 20% of world energy supply by 2050, over 60% by 2100 and leading to 50% reduction in global CO2 emissions – is threatened by its poor performance in tropical climate. Such performance discourages its regional acceptance. The magnitude of crucial module performance influencing factors (cell temperature, wind speed and relative humidity) reach critical values of 90 °C, 0.2 m/s and 85%, respectively in tropical climates which negatively impact module performance indices which include power output (PO), power conversion efficiency (PCE) and energy payback time (EPBT). This investigation reviews PVM technologies which include cell, contact and interconnection technologies. It identifies critical technology route(s) with potential to increase operational reliability of PVMs in the tropics when adopted. The cell performance is measured by PO, PCE and EPBT while contacts and interconnections performance is measured by the degree of recombination, shading losses and also the rate of thermo-mechanical degradation. It is found that the mono-crystalline cell has the best PCE of 25% while the Cadmium Telluride (CdTe) cell has the lowest EPBT of 8-months. Results show that the poly-crystalline cell has the largest market share amounting to 54%. The CdTe cell exhibits 0% drop in PCE at high-temperatures and low irradiance operations – demonstrating least affected PO by the conditions. Further results establish that back contacts and back-to-back interconnection technologies produce the least recombination losses and demonstrate absence of shading in addition to possessing longest interconnection fatigue life. Based on these findings, the authors propose a PVM comprising CdTe cell, back contacts and back-to-back interconnection technologies as the technology with latent capacity to produce improved performance in tropical climates

Numerical assessment of the effect of void morphology on thermo-mechanical performance of solder thermal interface material

The presence of voids in solder thermal interface material (STIM) layers affects the reliability and mechanical performance of formed solder joint. Previous studies suggest that the level of void effect depends not only on the size of the voids but also on the distribution and location of voids. In this work, a void morphology generating algorithm was used to generate representative volume elements (RVEs) depicting the seeming randomness of voids in a given STIM layer. The generated 2D RVEs were converted to 3D RVEs within a finite element modelling (FEM) environment and subsequently incorporated into the chip and heat spreader to complete the geometric model. The maximum damage site in the Sn-3Ag-0.5Cu (SAC305) solder joint as predicted by finite element analysis (FEA) of the geometric model showed a good qualitative agreement with experimental observations elsewhere. Further numerical assessment of the thermo-mechanical performance of SAC305 alloy as STIM layer due to the different generated void morphology was carried out. Results showed that solder voids can either influence the initiation or propagation of damage in the STIM layer, depending on the configuration, size and location of voids. While the small voids around the critical region of the solder joints appeared to enhance stress and strain localisation around the maximum damage site thus facilitating damage initiation; small voids also showed potentials of arresting damage propagation. In addition, results from this study indicated that void located in the surface of the solder joint, particularly voids at the solder/silicon die interface are more detrimental compared to void embedded in the middle of the solder layer. The innovative technique employed in this study to numerically generate realistic solder void morphologies would be beneficial to the solder voids modelling research community

Thermal management materials for electronic control unit: trends, processing technology and R&D challenges

The development of advanced thermal management materials for Electronic Control Unit (ECU) is the key to achieving high reliability and thus safety critical operations in areas of ECU applications such as automotives and power systems. Thermal management issues associated with the operation of ECU at elevated temperature have accounted for some of the recent reliability concerns which have culminated in current systems failures in some automobiles. As the functions of ECU in systems have increased in recent times, the number of components per unit area on its board has also risen. High board density boosts internal heat generated per unit time in ECU ambient. The generated heat induces stress and strain at the chip interconnects due to variation in the Coefficient of Thermal Expansion (CTE) and thermal conductivity of different bonded materials in the assembly. Thermal degradation impacts device’s efficiency and could become critical. The life expectancy of electronic components reduces exponentially as the operating temperature rises, thus making thermal management pivotal in electronic system reliability. Since materials’ properties vary with operating condition, material performance has become a major consideration in the design of heat dissipation mechanism in ECU. The development of advanced thermal management materials and hence improving the performance of ECU require an in-depth understanding of the complex relationship between materials’ properties and their behaviours at elevated temperatures. The paper presents an overview of thermal management materials for ECU in terms of material properties and processing technology. In addition, the paper outlines the crucial challenges in materials selection, including cost and manufacturing and other outstanding R & D issues

Comparative study of the effects of coalesced and distributed solder die attach voids on thermal resistance of packaged semiconductor device

Solder thermal interface materials are often used in power semiconductors to enhance heat dissipation from silicon die to the heat spreader. Nonetheless, the presence of voids in the die bond layer impedes heat flow and thus increases the chip junction temperature. Such voids which form easily in the solder joint during solder reflow process at manufacturing stage are primarily occasioned by out-gassing phenomenon. Three-dimensional finite element analysis is employed to investigate the thermal effects of lead-free solder void percentages and configurations on packaged semiconductor device. The thermal resistance for each voiding case is calculated to evaluate the thermal response of the resultant electronic package. The results show that for equivalent void percentage, thermal resistance increases more for large coalesced type voids in comparison to the small distributed void configurations. The results would assist packaging and design engineers in setting criteria for assessments of the thermal impacts of different solder void patterns

Emerging nanotechnology-based thermal interface materials for automotive electronic control unit application

The under-hood automotive ambient is harsh and its impact on electronics used in electronic control unit (ECU) assembly is a concern. The introduction of Euro 6 standard (Latest European Union Legislation) leading to increase in power density of power electronics in ECU has even amplified the device thermal challenge. Heat generated within the unit coupled with ambient temperature makes the system reliability susceptible to thermal degradation which may result in catastrophic failure if not efficiently dissipated. Previous investigations show that the technology of thermal interface materials (TIMs) is a key to achieving good heat conductions within a package and from a package to heat sinking device. With studies suggesting that conventional TIMs contribute about 60% interfacial thermal resistance, innovative technology is required to improve the thermal performance of TIMs. A review of emerging nanotechnology in TIMs shows that carbon nanotubes (CNTs) and carbon nanofibres (CNFs) when used as the structure of TIM or TIM filler could improve the overall thermal and mechanical properties of TIMs. Hence, CNTs/CNFs have the potentials to advance thermal management issues in ECU. This search identifies chemical vapour deposition (CVD) as a low cost process for the commercial production of CNTs. In addition, this review further highlights the capability of CVD to grow nanotubes directly on a desired substrate. Other low temperature techniques of growing CNT on sensitive substrates are also presented in this paper

Finite element analysis of the effect of silver content for Sn–Ag–Cu alloy compositions on thermal cycling reliability of solder die attach

Thermal performance of a chip-scale packaged power device can be improved by attaching a heat spreader to the backside of the heat generating silicon die via solder thermal interface materials [STIMs]/solder die-attach. Driven by government legislation, electronics industry has advanced to lead (Pb)-free solders due to environmental and health concerns emanating from the use of Pb-based solders. Though solder compositions in the form of Sn–Ag–Cu (SAC) ternary system have been widely accepted and preferred by the electronic industry as replacements for the traditional Pb-based solder alloys, debate continues over the optimal silver content in the Sn–Ag–Cu (SAC) solder alloys. Apparently, the effect of silver (Ag) content on thermo-mechanical reliability of SAC alloy compositions as small area solder joints (flip chip solder bump or ball grid array (BGA)) has been extensively studied but not enough information exist on the effect of Ag percentage in SAC solder alloys when employed as large area solder joint (die-attach application). In this study, non-linear finite element method (FEM) is used for a comparative analysis of the effect of Ag content on the thermal fatigue performance of Sn–3Ag–0.5Cu (SAC305) and Sn–4Ag–0.5Cu (SAC405) when used in die-attach applications under three different thermal cyclic loading cases. The results show that Von-Mises stresses and strain energy in each of the two different SAC solder joints were strong function of the thermal cycle profiles, increasing in the order −55–80 °C < −55–125 °C < −65–150 °C. In addition, this study suggests that the range of stress was relatively greater for the SAC alloy with higher Ag content (SAC405) while the lower Ag content SAC solder (SAC305) experiences a comparatively larger accumulated plastic work under the same thermal cycling condition. Further failure analysis via visual inspection reveals that for all cases of thermal cyclic loading employed in this study, maximum values of strain energy were all located in the corner regions (critical regions) of the solder joints at the side next to the silicon die independent of the Ag content of the solder joints. This paper also highlights the concerns as regards the implementation of conventional thermal fatigue models for accurate life time prediction of large area solder joint

Thermal management materials for electronic control unit: trends, processing technology and R and D challenges

The development of advanced thermal management materials for Electronic Control Unit(ECU) is the key to achieving high reliability and thus safety critical operations in areas of ECU applications such as automotives and power systems. Thermal management issues associated with the operation of ECU at elevated temperature have accounted for some of the recent reliability concerns which have culminated in current systems failures in some automobiles. As the functions of ECU in systems have increased in recent times, the number of components per unit area on its board has also risen. High board density boosts internal heat generated per unit time in ECU ambient. The generated heat induces stress and strain at the chip interconnects due to variation in the Coefficient of Thermal Expansion (CTE) and thermal conductivity of different bonded materials in the assembly. Thermal degradation could become critical and impacts device’s efficiency. The life expectancy of electronic components reduces exponentially as the operating temperature rises thus making thermal management pivotal in electronic system reliability. Since materials’ properties vary with operating condition, material performance has become a major consideration in the design of heat dissipation mechanism in ECU. The development of advanced thermal management materials and hence improving the performance of ECU requires an in-depth understanding of the complex relationship between materials’ properties and their behaviours at elevated temperatures. The paper presents an overview of thermal management materials, review trends in material and processing technology. In addition, the paper outlines the crucial challenges in materials, cost and composite formulations and the outstanding R & D issues

Thermal interface materials for automotive electronic control unit: Trends, technology and R&D challenges

The under-hood automotive ambient is harsh and its impact on electronics used in electronic control unit(ECU)assembly is a concern. The introduction of Euro 6 standard (Latest European Union Legislation)leading to increase in power density of power electronics in ECU has even amplified the device thermal challenge. Heat generated within the unit coupled with ambient temperature makes the system reliability susceptible to thermal degradation which ultimately may result in failure. Previous investigations show that the technology of thermal interface materials (TIMs) is a key to achieving good heat conductions within a package and from a package to heat sinking device. With studies suggesting that current TIMs contribute about 60% interfacial thermal resistance, a review of engineering materials has become imperative to identify TIM that could enhance heat transfer. This paper critically reviews the state-of-theart in TIMs which may be applicable to automotive ECU. Our review shows that carbon-nanotube (CNT) when used as the structure of TIM or TIM filler could considerably advance thermal management issues by improving heat dissipation from the ECU. This search identifies chemical vapor deposition (CVD) as a low cost process for the commercial production of CNTs. In addition, this review further highlights the capability of CVD to grow nanotubes directly on a desired substrate. Other low temperature techniques of growing CNT on sensitive substrates are also presented in this paper

The effect of thermal constriction on heat management in a microelectronic application

Thermal contact constriction between a chip and a heat sink assembly of a microelectronic application is investigated in order to access the thermal performance. The finite element model (FEM) of the electronic device developed using ANSYS software was analysed while the micro-contact and micro-gap thermal resistances were numerically analysed by the use of MATLAB. In addition, the effects of four major factors (contact pressure, micro-hardness, root-mean-squared (RMS) surface roughness, and mean absolute surface slope) on thermal contact resistance were investigated. Two lead-free solders (SAC305 and SAC405) were used as thermal interface materials in this study to bridge the interface created between a chip and a heat sink. The results from this research showed that an increase in three of the factors reduces thermal contact resistance while the reverse is the case for RMS surface roughness. In addition, the use of SAC305 and SAC405 resulted in a temperature drop across the microelectronic device. These results might aid engineers to produce products with less RMS surface roughness thereby improving thermal efficiency of the microelectronic application