Open ended microwave oven for flip-chip assembly

A novel open-ended waveguide cavity resonator for the microwave curing of bumps, underfills and encapsulants is described. The open oven has the potential to provide fast alignment of devices during flip-chip assembly, direct chip attach, surface mount assembly or wafer-scale level packaging. The prototype microwave oven was designed to operate at X-band for ease of testing, although a higher frequency version is planned. The device described in the paper takes the form of a waveguide cavity resonator. It is approximately square in cross-section and is filled with a low-loss dielectric with a relative permittivity of 6. It is excited by end-fed probes in order to couple power preferentially into the TM3,3,k mode with the object of forming nine 'hot-spots' in the open end. Low power tests using heat sensitive film demonstrate clearly that selective heating in multiple locations in the open end of the oven is achievabl

Experimental investigation of open-ended microwave oven assisted encapsulation process

An open ended microwave oven is presented with improved uniform heating, heating rates and power conversion efficiency. This next generation oven produces more uniform EM fields in the evanescent region forming part of the heating area of the oven. These fields are vital for the rapid and uniform heating of various electromagnetically lossy materials. A fibre optic temperature sensor and an IR pyrometer are used to measure in situ and in real-time the temperature of the curing materials. An automatic computer controlled closed feedback loop measures the temperature in the curing material and drives the microwave components to obtain predetermined curing temperature cycles for efficient curing. Uniform curing of the lossy encapsulants is achieved with this oven with typical cure cycle of 270 seconds with a ramp rate of 1oC/s and a hold period of 2 minutes. Differential scanning calorimeter based measurement for the pulsed microwave based curing of the polymer dielectric indicates a ~ 100% degree of cure

Focussed microwave heating using degenerate and non-degenerate cavity modes

Microwave ovens have long been recognised as a means of reducing heating times versus conventional convection ovens. The principle design feature is based on the procurement of uniform heating within any material placed in the interior of the microwave cavity oven. Materials within the oven are subjected to a degree of heating dependent on their electromagnetic properties. For many applications, it is desirable to maintain control over the distribution of heat deposition. This can be achieved through focussing of the electromagnetic field within the cavity. Two new mechanisms are identified where an increased level of control over the heating pattern and its location could be advantageous. The research described within this thesis aims to improve heating selectivity in microwave cavity ovens by the identification and enhanced control of modal patterns in degenerate and non-degenerate resonators. This is achieved through the analysis of two novel oven arrangements. The first of these addresses the requirement for highly selective heating in hyperthermia treatment. It is demonstrated that proper selection of a forced degenerate mode set can lead to an enhancement in field focussing within the centre of the cavity through constructive and destructive interference of the fields in each mode pattern. It is found that a highly selective peak of field can be produced within the centre of a large cylindrical waveguide cavity for the purpose of hyperthermia treatment. The peak is produced using a quasi degenerate mode set excited at approximately 1:3GHz. The second example presents an open oven design for the curing of epoxy and encapsulant materials within the micro-electronics packaging industry. It is intended that the oven be placed on the arm of a precision alignment machine such that the curing and placement stages of production be combined, suggesting an increase in production efficiency. Two excitation schemes are presented based on the coupling of quasi degenerate mode sets through a wide frequency range and the excitation of a single high order mode enabling uniform field distribution for heating of encapsulant material and increased selective heating through spatial alignment of modal field peaks, respectively. Experimental results demonstrate the viability of the open-ended microwave oven for curing. Both proposed excitation methods within the open oven design are investigated with results presented. Optimisation of the heating fields is achieved through inclusion of lowloss materials within the oven. Curing of an encapsulant material covering a commercial chip package is achieved and the overall design validated

Reliability testing and stress measurement of QFN packages encapsulated by an open-ended microwave curing system

In this paper, the influence of microwave curing on the reliability of a representative electronic package is examined by reliability testing and measurement of residual stresses. A LM358 voltage regulator die was mounted to an open Quad Flat No-leads package (QFN) for reliability testing. For the stress measurement, a specifically designed stress measurement die was mounted to the QFN package. The chips were encapsulated with Hysol EO1080 thermosetting polymer material. Curing was performed using an open-ended microwave oven system equipped with in situ temperature control. Three different temperature profiles for microwave curing were selected according to the requested degree of cure and chemical composition of the cured material. A convection cure profile was selected for the control group samples. Thermal cycling and HAST tests were performed on a total number of 80 chips. 95 QFN packages with stress measurement chips were also manufactured. Increased lifetime expectancy of the microwave cured packaged chips was experimentally demonstrated and measured between 62% to 149% increased lifetime expectancy after Temperature Cycling Test (TCT), and between 63% and 331% after highly Accelerated Ageing Test (HAST) and TCT compared to conventionally cured packages. Analysis of specifically designed stress test chips showed significantly lower residual stresses ranging from 26 MPa to 58.3 MPa within the microwave cured packages compared to conventionally cured packaged chips which displayed residual stresses ranging from 54 MPa to 80.5 MPa. This article therefore provides additional confidence in the industrial relevance of the microwave curing system and its advantages compared to traditional convection oven systems

On variable frequency microwave processing of heterogeneous chip-on-board assemblies

Variable Frequency Microwave (VFM) processing of heterogeneous chip-on-board assemblies is assessed using a multiphysics modelling approach. The Frequency Agile Microwave Oven Bonding System (FAMOBS) is capable of rapidly processing individual packages on a Chip-On-Board (COB) assembly. This enables each package to be processed in an optimal manner, with temperature ramp rate, maximum temperature and process duration tailored to the specific package, a significant benefit in assemblies containing disparate package types. Such heterogeneous assemblies may contain components such as large power modules alongside smaller modules containing low thermal budget materials with highly disparate processing requirements. The analysis of two disparate packages has been assessed numerically to determine the applicability of the dual section microwave system to curing heterogeneous devices and to determine the influence of differing processing requirements of optimal process parameters

Modeling Solder Ball Array Interconnects for Power Module Optimization

PowerSynth is a software platform that can co-optimize power modules utilizing a 2D topology and wire bond interconnects. The novel 3D architectures being proposed at the University of Arkansas utilize solder ball interconnects instead of wire bonds. Therefore, they currently cannot be optimized using PowerSynth. This paper examines methods to accurately model the parasitic inductance of solder balls and ball grid arrays so they may be implemented into software for optimization. Proposed mathematical models are validated against ANSYS Electromagnetics Suite simulations. A comparison of the simulated data shows that mathematical models are well suited for implementation into optimization software platforms. Experimental measurements proved to be inconclusive and necessitate future work

Development of an open-ended microstrip stub apparatus and technique for the dielectric characterization of powders

A new apparatus and method to characterize the complex dielectric permittivity of powders is described. The apparatus and technique are used to determine the dielectric properties of detergent powder agglomerates at different conditions. The technique is based on the measurement of Scattering-parameters of an open circuit microstrip stub partly loaded with the test powder material. The scattering parameters relate the voltage waves incident on the ports of a microwave network to those reflected from the ports and can easily be measured with a vector network analyzer. A 3D finite element electromagnetic field simulation tool HFSS (High frequency structural simulator) is used to replicate the measured S-parameters and then extract the complex permittivity data from it. The method has been verified by measuring the dielectric properties of disks of known dielectric materials – specifically Duriod 5880 and Teflon. Results are in good agreement with manufacturer data sheets. The complex permittivity of a range of detergent powder agglomerates with different moisture levels, at ambient and elevated temperatures, has been determined using this technique. Results are consistent with predictions of how the water interacts with the different components of the detergent particles at these different conditions

Mathematical and Computer-Based Models for Optimizing Microwave Heating Processes of Frozen Oysters

Consumers with full-time jobs prefer microwavable-frozen-meals for convenience when lack time to cook. Microwaves do not require a medium for heat transfer and provide quick heating even in low thermal conductivity foods, which does not occur in conventional heating. The main problem in utilizing microwave-heating for cooking is the non-uniform temperature distribution in foods which may result in insufficient lethality of microorganisms in some part. This non-uniformity can be due to a number of factors including composition and geometry of food. The objectives of this study were to develop a mathematical-model based on Maxwell¡¯s equations for predicting the temperature distribution in frozen oyster- meats undergoing microwave-heating and to solve the mathematical-model of the microwave heating process for frozen-oysters with finite-element-software. Oyster meats were analyzed for proximate analysis. The thermal properties of oyster were determined by Choi and Okos¡¯s equation. The dielectric properties of oyster meat were measured by transmission line method. Oyster meats were frozen cryogenically at -20¡ÆC and weight, shape, and dimensions of the frozen-meats were measured. The frozen samples were placed in a laboratory Microwave-Workstation with a maximum power of 1200 W at the operational frequency of 2450MHz. Temperature sensors with fiber optic leads were used to minimize interactions with microwaves. The sensors were connected to a computer with a FISO commander direct acquisition system. Temperature profiles were plotted in real-time during microwave heating. A model based on the Maxwell¡¯s equations was developed and used to model the heat generation during microwave-heating. The model predicted hot spots and cold spots in the oysters. Fresh oysters were heated to 100¡ÆC within 12 sec with the microwave heating. Frozen oysters reached 100¡ÆC after 20 sec of microwave-heating. The temperatures of oysters immediately after microwave-cooking ranged from 85.9 to 100.3¡ÆC, which evidenced that microwave cooking creates non-uniform-heating. The root-mean-square-error of the predicted-temperature vs. actual experimental values at hot spots ranged from 0.23 to 5.47 ¢ªC. This is decent agreement, and thus provides confidence in the model¡¯s ability to predict temperature-profiles of frozen-oyster-meats during microwave-cooking. The models were also employed to predict the temperature distribution for oyster meat-contained microwavable instant meals

Development of a Microwave Instrumentation System for the Determination of Moisture Content in Oil Palm Fruits

This thesis describes the development of a microwave instrument for moisture content determination of oil palm fruits. The developed PC-based Six Port Reflectometer provides a simple, fast and accurate solution to the more expensive and bulky Vector Network Analyzer for monitoring the ripeness of oil palm fruit at various stages based on the moisture content measurement. The Microwave Office software (MWO) was used to design and analyzed the Six Port Reflectometer. Various coaxial sensors were simulated using FEMLAB to study the reflection coefficient of the sensor corresponding to the moisture content and ripeness stage of oil palm fruit. The developed Six Port Reflectometer operating at 2 GHz consists of a stripline ring junction, three diode detectors and an Analog to Digital converter. Two types of coaxial sensors were selected for this work: the open ended coaxial and monopole sensors. A computer software was developed using Agilent Visual Environment Engineering (VEE) graphical programming for hardware control and implement all computation work. The performance of the developed Six Port Reflectometer in reflection coefficient measurement was tested. The empirical equations which relate the measured reflection coefficient and moisture content were found. The comparison of a batch of 200 fruit samples was done and it is found that the empirical equations is in the best agreement with the samples follow by finite element method (FEM) simulation and capacitance model. The monopole sensor phase shift measurement was found to be the best for moisture content measurement of oil palm fruits with 3.5% mean error. In bunch measurement, the apical region has the highest accuracy of moisture content measurement with 2.7% mean error followed by equatorial region, 3.0%, and basal region, 5.8%. For whole bunch moisture content measurement, the mean error was found to be 3.8%. The uncertainty of the developed Six Port Reflectometer system is calculated to be 5.5% for reflection coefficient measurement. This study shows that the Six Port Reflectometer is suitable for moisture content measurement of oil palm fruit and bunch. It is simple, fast, accurate, and a portable instrumentation system. It is suitable for early quality check of fruit and bunch ripeness

Design against Food Poverty

What contribution can the discipline of Design make against food poverty? How can it improve access to food for the needy? How can it improve the use of food resources in socially marginalised contexts? Starting fromthese questions, the contribution illustrates four experimentations, based on a practical and participatory approach, carried out by the research team in a shelter for homeless people in the city of Turin