Fabrizio Vecchio

LTCC integrated miniature Rb discharge lamp module for stable optical pumping in miniature atomic clocks and magnetometers

We report here on an integrated mini-lamp module (15×26×4 mm3) with a microfabricated rf-powered Rb dielectric barrier discharge (DBD) lamp (10×10×3 mm3) positioned on top of a 0.6 mm thick (thickness adjustable) 4-layer LTCC (Low Temperature Co-fired Ceramic) stacked platform containing a serpentine heating resistor design with high heating capacity (up to several hundred degree Celsius) for lamp heating, a fast response DP 5092D PTC temperature sensor for temperature stabilization using PID feedback and a patterned pad layout for the drive circuit components and interconnects. This is the first report of an LTCC integrated Rb mini-lamp module. The novelties of this design include: (1) compact module and independent heating design with thermal isolation of the drive components, (2) very low capacitive interference of the heating elements on the lamp electrodes leading to lower power coupling losses and higher optical power stability during pumping operation, and (3) the components can be batch-fabricated and the module can be independently used for optical pumping in other applications including magnetometers and gyroscopes.

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.

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.