Conor Joseph Slater

FireBird: PowerPC e200 based SoC for high temperature operation

PowerPC Architecture microcontrollers are commonly used in embedded applications. In this work we present FireBird, the first PowerPC based SoC for reliable operation beyond 200°C. Designing SoCs for reliable operation at high temperatures is a significant challenge, due to increased static leakage current, reduced carrier mobility, and increased electro-migration. To alleviate the consequences of high temperatures, this paper proposes to customize a PowerPC e200 based SoC by using a dynamically reconfigurable clock frequency, exhaustive clock gating, and electromigration-resistant power supply rings. A 20×9 mm2 chip implementing this design has been fabricated in 0.35 μm CMOS technology. The custom testing procedure showed the expected maximum operating frequency reduction from 38MHz at room-temperature to 30 MHz at 200°C, which illustrates the importance of an adaptable clock frequency under temperature variation. At 200°C, the maximum power dissipation at 3.3 V supply voltage was 1.2W and the idle state static leakage current was 3.4 mA. Silicon measurements proved that this design outperforms PowerPC based SoCs available in the high-temperature microcontrollers market which are not operational at temperatures above 125°C.

Packaging technologies for high temperature control electronics

Current low temperature electronics ( 175°C) for prolonged periods of time require packaging technologies that have to tackle many new problems. At high temperatures traditionally used materials such as organic circuit boards, adhesives and standard solders degrade rapidly or undergo changes in structure and properties. An even more critical issue than high-temperature survivability is resistance to temperature cycling. Thermal mismatch between organic boards and semiconductor dies leads to high thermomechanical strains during swings from high to low temperature extremes, which can make an otherwise high temperature resistant assembly fail after a relatively low number of cycles. This work focuses on the packaging technologies for high temperature control modules, those with logical and signal conditioning applications. Although control modules share many similarities with power modules, they present their own unique design challenges, such as significantly higher complexity and a limitation of compatible materials. Here, recent research on substrates, die attach technologies and wirebond interconnects suited for high temperature ICs are presented along with packaging technologies for discrete components (capacitors and resistors) with the aim of identifying the current best solutions. Test vehicles for the various technologies were constructed and were subjected to high temperature storage at temperatures higher than 200°C. They were analysed in terms of degradation (i.e. loss in shear strength, pull strength, change in resistance, etc.). In parallel, a separate set of samples were subjected to temperature cycles from -20°C to 180°C and then analysed using the same tests as before for comparison. The combined data allow a recommendation to be made on how to assemble a viable control module such as one based on an SOI microcontroller designed at EPFL to operate at high temperatures. Keywords: High temperature packaging, CPUs and MCUs, Reliability

Effects of Thermal Losses on the Heating of a Multifunctional LTCC Module for Atomic Clock Packaging

An innovative multifunctional LTCC module has been designed for miniature atomic clock packaging. Efficient packaging and interconnection of the atomic clock packaging is a critical issue and a precise temperature control is required for some components, such as mini-cell and light source. The great advantage of using LTCC technology for this application is that it allows the integration of different functions, such as heaters and PTCs resistors for temperature measurement and control, and optionally other active elements. In this research, a platform for measuring the thermal conductivity of materials has been developed in order to perform precise thermal studies on the packaging. The relationship between achieved temperature and power dissipated for the heating of the LTCC module has been calculated in different experimental configurations, in order to determine the effects of conduction and convection on the heating and estimate the thermal losses that they introduce into the system.

Thermal characterization of an LTCC module for miniature atomic clock packaging

This paper presents the design and thermal characterization of a 15 × 22 mm2 LTCC module dedicated to the packaging of the elements of a miniature atomic clock. This module acts as a carrier for the components of the clock as well as temperature controller; this is particularly attractive since each component of the atomic clock requires precise and well defined working temperatures. For the thermal characterization of the designed module, various thermal simulations have been performed, by finite-element modelling using the software ANSYS; in each simulation a different experimental heating configuration was hypothesized, in order to determine the power required for achieving the desired temperature in the ideal case (module in perfect vacuum with no losses rather than conduction towards the external “cold” zone, through two small bridges) , and also for estimating the thermal losses that convection introduces into the system. The simulations were then experimentally validated by realizing and characterizing the different configurations, yielding results that were consistent with the simulations. The best experimental configuration was found, in which the heating performance was fairly close to the ideal one. The results show that LTCC is a very attractive technology for atomic clock packaging also in terms of dissipated power, provided the system is efficiently insulated.

Validation of Novel Wobbe Index Sensor for Biogas Cogeneration

Biogas is a fuel made from the anaerobic digestion of organic material to form methane. It can be used to power a stationary engine to generate electricity making it a viable method of decentralised power generation from renewables. However, biogas is a mixture of methane and carbon dioxide, and other trace gases such as hydrogen, hydrogen sulphide and oxygen. As such the quality can vary and setting the air-fuel ratio for efficient combustion can be problematic under these conditions. The Wobbe Index, or Wobbe Number, is a quality of combustible gases that allows the air-fuel requirement to be determined. This work presents a novel type of Wobbe Index sensor based on a miniaturised capillary viscometer that can be used with biogas. The sensor is validated at a biogas cogeneration plant which uses a stationary engine and the results are compared to a methane sensor installed at the plant.

Fabrication and Test of High-Temperature Ceramic Transformer

This work describes the fabrication and test of a high temperature (+200°C) capable high frequency transformer. It was manufactured using Low Temperature Co-fired Ceramic (LTCC) technology, which allowed the complex multilayer structure of ceramic and metal windings to be formed. However, the selected LTCC composition is a free sintering ceramic and there is an interaction between the metal conductor and the ceramic substrate during lamination and firing that can lead to significant deformation, presenting a significant engineering challenge. Here the fabrication process for the LTCC is described (screen printing, collation, lamination and firing) for a number of iterations of the transformer design, each of which was analysed for deformation and subjected to electrical tests. In addition a silicone adhesive for assembling the LTCC with the transformer was analysed for high-temperature performance. A test vehicle was assembled and it was subjected to 1000 hours at 210°C. Shear tests were performed at intervals to quantify the loss in bond strength over time. After a good solution for manufacture was found, a batch of transformers was produced, characterized and tested to demonstrate a high reproducibility and manufacturing yield.

Test Vehicle for Studying Thermal Conductivity of Die Attach Adhesives for High Temperature Electronics

Polymer adhesives offer a viable method for mounting silicon dies for high temperature applications. Here a test vehicle for comparing the thermal conductivity of different die attach materials is presented. The setup can be used to determine the degree of degradation of polymers. It consists of a mock die that has an integrated thick film heater, which is mounted onto a substrate. In operation, the substrate is placed on a heatsink and the die is heated. When the temperature reaches equilibrium the heater is switched off and the temperature of the die is measured as it cools. The time constant of the temperature decay is calculated to give the thermal conductivity. In this paper the thermal conductivity of an epoxy die attach adhesive is compared to its shear strength.

Characterisation of test vehicle for in-situ measurement of die attach thermal conductivity

The continuing trend in the automotive and aviation industries to reduce complexity of electronic systems by removing cooling results in a need for high temperature electronics and associated packaging technologies. To ensure reliability over long periods of time the degradation of the packaging materials should be characterised. Epoxies show great promise as a reliable die attach solution for high temperature electronics due to their high bond strength, resistance to fatigue and chemical stability at temperatures up to 250°C. This work presents a method and test vehicle for measuring the thermal conductivity of an epoxy die attach. The test vehicle is constructed by using the epoxy under test to bond a die with an integrated PTC heater to an alumina substrate. To measure the thermal conductivity the heater heats the die for a few seconds after which the die allowed to cool down to the temperature of the substrate. The temperature of the cooling die is monitored and the time constant of the temperature decay is used to calculate the thermal conductivity of the die attach. Previous work demonstrated that this method can provide realistic information on the state of the die attach by relating the measured thermal conductivity to the shear strength of the die. Additionally the method is non destructive and can be used to monitor the degradation of the attach, such as fatigue cracking during thermal cycling. Here the test vehicle is modeled using the finite element method to get a better understanding of what temperatures the die attach is subjected to and to improve the thermal conductivity measurement.