Farhad Sarvar

Thermal Interface Materials - A Review of the State of the Art

The past few decades have seen an escalation of power densities in electronic devices, and in particular in microprocessor chips. Together with the continuing trend of reduction in device dimensions this has led to dramatic increase in the thermal issues within electronic circuits. Thermal management is therefore becoming increasingly more critical and fundamental to ensuring that electronic devices operate within their specification. Although a thermal management system may make use of all modes of heat transfer to maintain temperatures within their appropriate limits and to ensure optimum performance and reliability, conductive heat transfer is typically used to spread the heat out from its point of generation and into the extended surface area of a heat sink. To minimise the contact resistance, thermal interface materials (TIMs) are introduced to the joint to fill the air gaps and are an essential part of an assembly when solid surfaces are attached together. This paper reviews the conventional interface materials and then goes on to present a comprehensive review of the emerging state-of-the-art research in the use of carbon nanotube based materials. The paper also outlines the advantages and disadvantages of each TIM category and the factors that need to be considered when selecting an interface material

Application of adhesives in MEMS and MOEMS assembly: a review

This paper presents a review of the recent literature on the use of adhesives in MEMS packaging applications. The aim of this review has been to establish the current applications of adhesives in MEMS and MOEMS assembly and to investigate the limitations and future requirements of these materials. The review has shown that while there is a wealth of information available on the packaging of MEMS devices, there is very limited detail available within the public domain regarding the specific uses of adhesives and in particular exactly which products are in use. The paper begins with an overview of the uses of adhesives in MEMS packaging, subdivided into sections on structural adhesives, adhesives for optical applications and other applications. The paper then describes methods for adhesive dispensing and issues with adhesive use which affect the reliability of the package. The reliability of MEMS devices assembled using adhesives is a challenging issue, being more than a simple combination of electrical, mechanical and material reliability. Many failure modes in MEMS devices can be attributed to the adhesives used in the assembly; for example, thermal expansion mismatches can cause stress in the die attach, while outgassing from epoxies can cause failure of sealed devices and contamination of optical surfaces.

Effective modeling of the reflow soldering process: basis, construction, and operation of a process model

Thermal history variation within printed circuit assemblies (PCAs) during reflow soldering is considered one of the main drivers for manufacturing defects. It is recognized that predictive tools could be used to identify the temperature variations that arise during the reflow process and, in conjunction with experimentally derived data, determine their impact on manufacturing quality. This paper describes the development of representative process models of the reflow soldering of PCAs and outlines some of the more important parameters to consider for accurate simulation of the reflow process. This first part also presents results which illustrate the variation with temperature of critical properties for electronic materials, such as for common substrate and epoxy based packaging materials.

Effective modeling of the reflow soldering process: use of a modeling tool for product and process design

The increasing component packing density and consequent reduction in feature size in printed circuit assemblies (PCAs) continues to place manufacturers under extreme pressures. The most significant demand is for improved first-off process yields because of the high cost and technical difficulty of rework processes and concerns over the reliability of reworked products. The dominant process for the production of PCAs is reflow soldering of a stencil/screen printed solder paste to form the interconnection between the component termination and the substrate. It is crucial for the manufacturer to ensure that each termination experiences a suitable thermal history throughout the reflow cycle. Despite the advances in processes to cope with complex product features, such as increasing the uniformity and amount of heat transfer in the process, ensuring right-the-first-time is still a problem leading to increased lead-times, reduced yields and the scrapping of assemblies used to establish the ideal process parameters for each particular product. This paper describes the utilization of a predictive model as a tool for the off-line determination of the most appropriate process and its specific set-up for a PCA. Results are also presented where PCA design is altered to improve thermal mass distribution.

A modelling tool for the thermal optimisation of the reflow soldering of printed circuit assemblies

Thermal history variation within printed circuit assemblies (PCAs) during reflow soldering is considered one of the main drivers for manufacturing defects. It is recognised that predictive tools could be used to identify the temperature variations that arise during the reflow process and, in conjunction with experimentally derived data, determine their impact on manufacturing quality. A predictive model would also be useful to a designer for rearranging component placement for thermal mass distribution, hence enabling the optimisation of the design for manufacture prior to final design commitment. Likewise, such a predictive tool could be utilised for off-line optimisation of reflow oven profiles and in the design of more thermally efficient production equipment. This paper describes the development of representative process models of the reflow soldering of PCAs and outlines some of the more important parameters to consider for accurate simulation of the reflow process. Furthermore, the utilisation of the predictive model is presented as a tool for a number of end uses applicable to different application domains, namely: process configuration for any given PCA; selection of the most appropriate process and as a product design or verification tool to improve thermal mass distribution and hence temperature history during processing.

Effective transient process modelling of the reflow soldering of printed circuit assemblies

Process modelling of the reflow soldering of Printed Circuit Assemblies (PCAs) requires complex thermal models incorporating a number of modes of heat transfer, including radiation (infra red) and (forced/free) convection. This paper describes the development of representative process models of the reflow soldering of PCAs and outlines some of the more important parameters to consider for accurate simulation of the reflow process. Results are also presented illustrating the variation with temperature of critical properties for electronic materials, such as for FR-4 and epoxy based packaging materials.

IGBT package design for high power aircraft electronic systems

This paper will discuss the design of semiconductor packages having integrated air cooled heatsinks for use in high power electronic systems. It will demonstrate how simple models of the heat transfer from the heatsink fins, which are based on empirical correlations, may be utilised in combination with either simple analytical models or two dimensional finite difference (FD) models of the heat conduction from the semiconductor die through the multilayer package structure to the base of the fins. These models allow the rapid evaluation of performance under both steady state and transient overload conditions, and can be used to rapidly explore a wide range of design options before selecting candidate layouts for more detailed evaluation using, for example, 3D FD analysis. Wind tunnel experiments, which will also be reported, have been carried out to verify the modelling results for different semiconductor device layouts. These trials demonstrate excellent agreement between the models and experimental results.

Thermal and thermo‐mechanical modelling of polymer overmoulded electronics

Purpose – The encapsulation of electronic assemblies within thermoplastic polymers is an attractive technology for the protection of circuitry used in harsh environments, such as those experienced in automotive applications. However, the relatively low‐thermal conductivity of the encapsulating polymer will introduce a thermally insulating barrier, which will impact on the dissipation of heat from the components and may result in the build‐up of stresses in the structure. This paper therefore seeks to present the results from computational models used to investigate the thermal and thermo‐mechanical issues arising during the operation of such electronic modules. In particular, a two‐shot overmoulded structure comprising an inner layer of water soluble and an outer layer of conventional engineering thermoplastics was investigated, due to this type of structures potential to enable the easy separation of the electronics from the polymer at the end‐of‐life for recycling.Design/methodology/approach – Represen...

Thermo-mechanical modelling of polymer encapsulated electronics

This paper reports on some initial results from a research project investigating a novel technology for the manufacture of recyclable polymeric modules with embedded electronic systems. The aim of this project is to develop a technology that fully encapsulates electronics for use in the demanding automotive environment. A two shot moulding technology protect delicate electronic circuitry mounted outside of the passenger compartment from extremes of temperature, vibration and humidity. The resultant components also be readily recyclable, making it possible to cost-effectively separate electronic components from the polymer at the end of vehicle life, allowing the recovery of high purity recyclate. The encapsulating polymers have low thermal conductivity, so the process of encapsulation introduce a thermally insulating barrier around the electronics, which impact on the dissipation of heat from the components. In addition, the thermal performance of the assembly is further affected by the high temperature environments within which some of these electronic modules have to operate, such as under the bonnet of a vehicle. This paper presents the results of preliminary models developed for investigating the thermal and mechanical issues arising during the operation of such encapsulated electronics. Analytical models and finite element techniques have been employed to simulate the thermo-mechanical behaviour of overmoulded printed circuit boards.

THERMAL DESIGN OF HIGH POWER SEMICONDUCTOR PACKAGES FOR AIRCRAFT SYSTEMS

Future aircraft containing a greater proportion of electrical systems will require more extensive use of power electronics, for which thermal management is a key issue. This paper will present an approach to semiconductor package design incorporating integrated air cooled heatsinks. The paper will show how simple models of the heat transfer from heatsink fins, which are based on well established empirical correlations, may be utilised in combination with simple models of the heat conduction from the semiconductor die through the multilayer package structure to the base of the fins. These models allow the generation of design curves which may be used to rapidly explore a wide range of design options before selecting potential designs for more detailed evaluation using 3-D FE analysis. This approach has been used to design a semiconductor package for a power converter where the semiconductor devices are switched at high frequency to ensure good input and output current waveforms. The power dissipated in the semiconductors, and therefore the heatsink weight however increases with the switching frequency, whereas the associated filtering components will be smaller and lighter at higher frequencies. The optimisation of the overall system weight therefore involves a trade-off between the heatsinking and filtering requirements rather than just determining the optimum heatsink design for a specific power dissipation.