PhD Thesis and PostDoc


Thèse-300x257PhD title: Thermo-mechanical characterization and modelling of radiofrequency and microwave printed circuit boards for space applications

PhD student: Gautier GIRARD

Starting date: October 2015

Printed circuit boards play a key role in the working of electronic equipments. They enable the electrical and mechanical interconnection of various components assembled on the board in any electronic systems. For microwave applications, the PCB also have a critical signal transmission functionality. Therefore, the reliability of a PCB is an essential feature for any electronic devices, and has to be addressed as early as possible during the design stage.
A PCB is a multilayered structure. Layers are made of various materials (polymer, glass fiber composite, copper..). Such a complex structure with materials having different thermo-mechanical behaviors is really complex to model. For radiofrequency (RF) and microwave applications, the combination in the same PCB of specific low losses materials with thermoplastic and thermoset polymers are quite common. In addition, for applications in extreme conditions environment as it is considered in the present case, various failure modes can occur. So there is a strong necessity to develop knowledge on PCB for RF and microwave applications.
Over the past few years, the needs for larger electrical performance and for higher densification in multilayer PCB have led to the design of highly complex boards. In addition, a wide range of materials is proposed by suppliers for such microwave applications. At the same time, the number of PCB produced for that purpose is relatively small compared to PCB dedicated to low frequency applications. As a consequence, the development of a new generation of printed circuit boards for microwave applications with a high degree of reliability is very challenging.
The subject of the thesis addresses the thermo-mechanical behavior of PCB for microwave applications. The candidate will characterize, understand and model the behavior of the printed circuit board and evaluate crucial parameters affecting reliability. The results will be used to improve the design and reliability of future printed circuit boards for microwave applications used in space environment. The objective is to be able to determine potential critical configurations and to provide guidelines for the design.
Experimentally, two main failure modes exist: failure of electroplated copper in plated through holes and delamination at layer interfaces. To be able to characterize failure modes, an important point is to define the behavior of the base materials, for example the copper plating. Since PCB will experience thermal cycling during operation, it is important to perform mechanical cyclic tests to represent the behavior of materials under loading paths similar to that experienced by the materials in the PCB. Secondly, delamination of interfaces in the assembly will also be considered. Delamination of interfaces generates stress concentration. In delaminated area, the plastic deformation cumulated during cycles is strongly modified and therefore this affects the lifetime of the PCB.
In addition to characterization, finite element simulations will be performed. A PCB has a 3D structure and it is not possible to model the entire board. Critical configuration will be selected. 3D simulations, which are very time consuming, will be compared to 2D models, to determine whether 2D calculations can be relevant.
The research subject is proposed within a collaboration framework between academic / industrial partners. The candidate, working in LEM3 (Metz), will interact with industry partners : CIMULEC (SME, Printed Circuit Board manufacturer), Thales Alenia Space and CNES.

PHD Thesis H2020-MSCA-ITN-2015 Outcome Project

Thèse-300x257PhD title: Modelling of delamination and interface strength in printed circuit boards (PCBs)

PhD student: Essossinam SIMLISSI

Starting date: October 2016

A printed circuit board is a passive component which allows to interconnect electronic components soldered on the outer layers in order to realize a complex electronic system. It is a complex multi-layer assembly developed for a very specific goal which requires expertise in mechanical and material sciences. In order to reach the expected performance and lifetime in harsh environments dedicated high-performance base materials are required. During its lifetime, the PCB undergoes a large number of thermal cycles which can lead to the breakup of copper path. In addition, when active components are soldered on the PCB, a thermal shock is produced. It has been detected that the use of certain combination of based materials leads to delamination, which damages the PCB and limits its service-life. We will investigate the delamination process and the evolution of the strength of interfaces from both theoretical and experimental sides. Within this context, we will develop a research on the damage and failure processes of PCBs subjected to thermal cycling. A theoretical model will be specifically developed in order to predict the onset and evolution of damage and failure. The predictions of the theoretical model will be compared with own experiments specifically conducted for that purpose (to check the reliability of the model predictions). The planned experiments are peeling tests at various temperatures performed for various pairs of materials frequently used to manufacture the PCBs.

PostDoc subject: Flatness of PCBs under thermo-mechanical loadings

Thèse-300x257PhD: Hassan OBEID

Starting date: January 2016

A PCB is a multi-materials with a large number of layers. The board contains also a large number of plated through holes to ensure the electrical connectivity. During its lifetime, the board may sustain complex thermo-mechanical loadings. In addition, it has been designed for a specific application. Thus, an optimum design has to be found between all requirements.

To create the board, the layers are backed at elevated temperature. Due to curing of the resin, a shrinkage can occur which will generate residual stresses and strain. Since the pcb is built for a specific use and with a large number of layers, the composite structure does not always present the mirror symmetry. As a consequence, the flatness of the board after the curing is not ensured. The goal of the research is to develop a predictive tool to ensure that the board will remain flat, after processing but also during the thermal loading faced by the PCB. The board is not simply an assembly of layers but contains also PTH and sometimes passive components. All these ingredients must be accounted for in the modeling.