Margaret Stern

Indium solder as a thermal interface material using fluxless bonding technology

The capability and diversity of high performance microprocessors is increasing with each process technology generation in order to meet increasing application demand. The cooling designs for these electronic chips have to deal with larger temperature gradients across the die than previously. The key to thermal management is to dissipate the thermal energy from a heat-generating device to a heat sink via conduction through a thermal interface material (TIM). The TIM must also relieve the mechanical stress and absorb strain caused by coefficient of thermal expansion (CTE) mismatch between heat spreader lid and silicon die during field operation. Low modulus TIM is excellent at strain absorption and relieves stress from CTE mismatch of different materials. In this study we explore the fluxless bonding of Indium as a candidate TIM for high performance microprocessors due to its high thermal conductivity, low melting temperature and low tensile strength and its ‘green’ness (non-hazardous material, minimal waste and ease of product reworkability). Challenges in the development process include: controlling bond line thickness, fillet extent and controlling voids in the TIM assembly at reflow temperatures. This paper aims to investigate these key challenges and provide some general recommendations.

Sequential deposition and electroforming of metal-ceramic composites for thermal management applications

Abstract Three approaches for the fabrication of ceramic–metal composites for thermal management of electronic circuitry are studied both theoretically and experimentally. The first approach includes the fabrication of Cu/Al2O3 coatings by the sequential electrophoretic deposition (EPD) – electroplating method. The kinetics of EPD, suspension stability are analysed. The SEM characterisation of the produced deposits shows good bonding between Al2O3 inclusions and copper matrix. An alternative approach includes the electroforming of copper ‘rivets’ on plastic substrate and on a porous SiC skeleton. The residual stress distribution at the ceramic/metal interface was simulated using finite element analysis. The third approach involves the fabrication of metal–ceramic foams as a basis for integrated thermal sinks. Copper–silicon carbide foam is considered as a model and experimental system for this approach. The modelling results provide recommendations on the optimal thicknesses of the deposited ceramic and metal layers to achieve the best heat conductivity with minimal thermal expansion mismatch.

Fabrication of functionally graded 3A/5A zeolites by electrophoretic deposition

Abstract Functionally graded zeolites of molecular sieve type 3A and 5A are deposited by electrophoretic deposition (EPD) from acetone suspension with 8% volume concentration of n-butylamine as particle charging agent. The EPD characteristics of both 3A and 5A suspensions are studied. Functionally graded zeolite 3A/5A deposits are obtained at 200 V DC. Energy dispersive X-ray dispersion (EDX) analysis results confirm the graded structure. The deposited zeolites are also analysed by scanning electron microscopy (SEM). The factors influencing the deposition process are discussed.

Evaluation of high performance thermal greases for CPU package cooling applications

High performance thermal greases have been evaluated in three separate environments: ideal laboratory, in situ laboratory, and system mockup testing to better understand how bulk and interfacial thermal properties, in combination with the test vehicles used, effect the resultant thermal performance. The three methodologies are described and measurements on a baseline material reported.

Evaluation of and Inspection Metrology for Lid Attach for Advanced Thermal Packaging Materials

We have evaluated a new Ag-filled silicone thermal interface material (TIM) for its sensitivity to lid finish and impact on imaging discontinuities in the die/lid (TIM1) layer, in conjunction with two high performance lid materials, as a part of our advanced packaging technology development effort. Thermal and mechanical (shear stress and lid pull) measurements have been carried out on a number of different lid finishes to optimize thermal performance and adhesion at the TIM1/lid interface. This silicone TIM1 is found to be sensitive to the type of Ni-plating and plating bath chemistry. Nondestructive and destructive metrology has been carried out on flip chip (FC) packages using Ag-filled silicone TIM1 and either Cu or AlSiC lids. A number of silicone formulations have been investigated to assess their impact on surface acoustic microscopy (SAM) and X-ray imaging. Nondestructive evaluation (NDE) by real time X-ray and SAM has identified artifacts that make it difficult to unambiguously detect voids and delamination in the TIM1 layer. A “dark ring” or “picture frame” artifact is observed at the die perimeter in acoustic microscope images of packages with the Ag-filled TIM1. Detailed SEM cross-section and thermal mapping analyses on a number of specially constructed FC packages have been correlated with TIM1/lid delamination and voiding observed in SAM and X-ray images. Results of these studies point to changes in the TIM1 modulus during cure and post cure thermal excursions as the cause of the “dark ring” observed in the transmission SAM images rather than delamination at the TIM1/lid or TIM1/die interfaces. However, in the event that delamination is present at the edges it cannot be unambiguously deconvoluted from the “dark ring” artifact in the SAM images.Copyright

Measurements of mechanical coupling of non-curing high performance thermal interface materials

For high performance cooling applications, a separable thermal interface material (TIM) is inserted between the CPU lid (electrical component) and heat sink surface to reduce the thermal resistance. Non-curing TIMs, including thermal greases, phase change materials (PCMs), putty-like gap fillers, pads, and dry films are potential classes of TIMs that are used in this application. Materials that have higher effective thermal conductivity, such as filled thermal greases and PCMs tend to have lower interfacial contact resistance. The TIMs also mechanically couple the two interfaces through a weak adhesive bond. Data on the adhesive bonding properties of these materials is needed to predict how the TIM interface will behave when the system is subject to tensile forces during manual separation or dynamic loading induced by shock impulses. A series of controlled tests have been designed and implemented to measure the tensile pull strength of the adhesive bond. The suggested methodology, based on ASTM standards for adhesives, presents one approach for characterizing the adhesive TIM interface that could be readily adapted by others in the industry. These measurements complement in-house thermal testing of these thermal interface materials

Preliminary specification for a closed loop liquid cooling system product reliability test plan

Future high performance systems may incorporate liquid cooling to the board. These systems will be assembled using commercially available components and subsystems, which include the pump, liquid coolant, tubing and connections, heat exchanger, and cold plate. Liquid cooling systems are known to be susceptible to more degradation and failure mechanisms than conventional solid metal heatsinks, heat pipe heatsinks or vapor chamber heatsinks. Component and system providers need guidelines to ensure that their products meet projected reliability requirements. A generic preliminary specification has been assembled based on industry standards and information collected from leading equipment suppliers.