Armelle Girard

Impact of voids in interconnection materials

This chapter deals with the study of heat transfer phenomena related to the dissipation of thermal energy in a power module and its subsequent effects. In the packaging process of a mechatronic module, defects may appear in the form of voids inside the interconnection material (ICM). By trapping thermal energy, these voids can act as a potential source of failure in the module. Indeed, they introduce an inhomogeneous character into the distribution of temperature at the interface of the different assembled components and within the ICM, and the resulting thermomechanical stresses weaken the packaging. Because a theoretical study of these effects is difficult, they are simulated using finite element methods (FEMs). The temperature distribution in the power module was determined by considering different types of ICMs, Sn60Pb40, Sn95,5Ag3Cu0,5, Ag and sintered Ag. Then, the thermomechanical stresses were simulated to determine the temperature effects on the packaging when voids are present in the ICM. This study aims to determine the influence of voids on the reliability of a module by using the parameters which are the maximum temperature of the chip and the thermomechanical stresses at the interfaces as indicators. This chapter gives an example of the importance of simulation in the design of a reliable mechatronic device. We not only obtain results which allow quick anticipation of the root cause of failures, but also significant data to optimize the bench tests to be implemented in order to improve the fabrication process and increase efficiency.

Weak-beam study of dislocations in D022-Al3Ti deformed at 400 C

Polycrystalline samples of Al3Ti with the D022-type structure have been plastically deformed at 400°C. Observations using weak-beam transmission electron microscopy show activation of dislocations with <100] or <110] Burgers vectors. The fine structure of [100] and [010] dislocations reveals strong stabilization effect on the pure edge character. [110] superdislocations, resulting from the interactions between [100] and [010] perfect dislocations, are dissociated in two identical superpartials separated by an antiphase boundary (APB). Their segmented shape together with the separation distances between the [110] superpartials point to a large anisotropy of APB energies whose estimated values on (001) and (111) are 13 ± 3 mJ/m2 and 64 ± 5 mJ/m2, respectively. The low mobility of <100] and <110] dislocations is highlighted and related to the low ductility of this material.

Reliability in Mechatronics Systems from TEM, SEM and SE Material Analysis

Mechatronic systems designed to comply to new EU directives are studied through interconnections by electronic or photonic probes, SEM, TEM, SE or 3D Tomography. Leaded and lead free modules assembled by standard interconnection technologies are studied for robustness relative to thermal accelerated life tests. Results obtained from JEOL 6060LV SEM and Optical Microscopy show that although slow growth rate of inter-metallics (IMC) is consistent with expected reliability, they are responsible for propagation of cracks especially in the presence of gold on PCB side. Innovative Low Temperature Joining (LTJ) technology applied to nano or micro silver pastes which should reduce IMC effects are tested on mechatronic systems. Results obtained from SEM, TEM and 3D Tomography will be shown as well as non destructive Spectroscopic Ellipsometry studies of samples. Pressureless LTJ technology is unsuitable for robust interconnection.