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  • Microstructure evolution and thermomechanicalfatigue of solder materials

  • This research was financially supported by the technology foundation STW, the appliedscience division of NWO and the technology programme of the Ministry of EconomicAffairs under grant STW EWT 4923.

    CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN

    Matin, M. A.

    Microstructure evolution and thermomechanical fatigue of solder materials /by M. A. Matin. -Eindhoven: Technische Universiteit Eindhoven, 2005.Proefschrift. ISBN 90-386-2887-0NUR 978Subject headings: lead-free solders / thermomechanical fatigue / anisotropy/Ostwald-ripening / coalescence / scaling / distribution / Electron Backscattering Diffraction(EBSD) / misorientation / strain localization / grain boundaries / crystallography

    Printed by the Universiteitsdrukkerij, TU Eindhoven, The Netherlands.Cover design by Paul Verspaget (Grafische Vormgeving-Communicatie)

    Cover illustration: SAC solder after thermal fatigue, Inverse pole figure (IPF) map, PLMmicrograph after fatigue, Von Mises stress field-FE simulation, misorientation anglesbetween adjacent grains.This thesis was prepared with LATEX 2.

    Copyright c 2005 by M. A. MatinAll rights reserved. No parts of this publication may be reproduced or utilized in any formor by any means, electronic or mechanical, including photocopying, recording, or by anyinformation storage and retrieval system, without prior written permission of the copyrightholder.

  • Microstructure evolution andthermomechanical fatigue of

    solder materials

    PROEFSCHRIFT

    ter verkrijging van de graad van doctor aan deTechnische Universiteit Eindhoven

    op gezag van de Rector Magnificus, prof.dr.ir.C.J.vanDuijn,voor een commissie aangewezen door het College voor Promoties

    in het openbaar te verdedigen opwoensdag 16 november 2005 om 16.00 uur

    door

    Md. AbdulMatin

    geboren te Pabna, Bangladesh

  • Dit proefschrift is goedgekeurd door de promotor:

    prof.dr.ir. M.G.D. Geers

    Copromotor:dr.ir. W.P. Vellinga

  • Contents

    Summary vii

    Samenvatting ix

    1 Introduction 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Aims, contributions and outline of the thesis . . . . . . . . . . . . . . . . . . 3

    2 Evolution of microstructure in eutectic Sn-Pb solder 52.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    2.1.1 Coarsening in solders . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Experimental Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

    2.3.1 Microstructural evolution . . . . . . . . . . . . . . . . . . . . . . . . 92.3.2 Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3.3 Scaling and coalescence . . . . . . . . . . . . . . . . . . . . . . . . 122.3.4 Domain size distribution . . . . . . . . . . . . . . . . . . . . . . . . 142.3.5 Kinetics of domain coarsening . . . . . . . . . . . . . . . . . . . . . 16

    2.4 Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    3 Correlation between localized strain and damage in shear-loaded Pb-free solders 193.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.2 Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.3 Results and dicussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    3.3.1 Microstructure characterization . . . . . . . . . . . . . . . . . . . . 243.3.2 Shear tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.3.3 Evolution of strain-field . . . . . . . . . . . . . . . . . . . . . . . . 27

    3.4 Correlation between local strain and damage . . . . . . . . . . . . . . . . . . 283.5 Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

    4 Correlation between thermal fatigue and thermal anisotropy in a Pb-free solder 374.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374.2 Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

  • 4.3.1 Damage evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 394.3.2 Finite element modeling . . . . . . . . . . . . . . . . . . . . . . . . 41

    4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

    5 Microstructure evolution in a Pb-free solder alloy during mechanical fatigue 475.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.2 Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

    5.3.1 Crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505.3.2 Fatigue tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.3.3 Strain localization and damage evolution . . . . . . . . . . . . . . . 525.3.4 Elastic anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555.3.5 Plastic anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

    5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

    6 Damage evolution in SAC solder joints under thermomechanical fatigue 676.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676.2 Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686.3 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

    6.3.1 Microstructure characterization . . . . . . . . . . . . . . . . . . . . 696.3.2 Damage characterization . . . . . . . . . . . . . . . . . . . . . . . . 72

    6.4 Discussion and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

    7 Conclusions 87

    Bibliography 91

    Acknowledgements 97

    About the author 99

  • Summary

    The microelectronics industry is confronted with the new challenge to produce joints withlead-free solder materials replacing classical tin-lead solders in devices used in many fields(e.g. consumer electronics, road transport, aviation, space-crafts, telecommunication). Inservice, solder materials experience a complex thermomechanical load which may result inmicrostructure evolution, and strain localization. These phenomena may lead to the forma-tion of macroscopic cracks causing premature failure of components and functional loss ofdevices.

    Tin crystals are anisotropic, both mechanically and thermally, the effects of which arecompensated in tin-lead solders by the presence of the relatively soft isotropic lead (Pb). Snis the main constituent in the proposed lead-free alloys (e.g. Sn-Ag, Sn-Cu, Sn-Ag-Cu, Sn-Bi,Sn-Zn, Sn-Zn-Bi, Sn-Ag-Bi). For the safe use of any of these alloys, a thorough understand-ing of their behavior is required. With this in mind this thesis addresses the microstructureevolution and thermo-mechanical fatigue of eutectic Sn-Pb and Pb-free alternatives employ-ing a variety of microscopic techniques and numerical simulation.

    The coarsening of Pb-rich -Pb domains in eutectic Sn-Pb solder during isothermal an-nealing has been studied in detail. The importance of anisotropy and of coalescence eventsand the occurrence of a dynamic scaling regime are analyzed. Orientation imaging mi-croscopy revealed the presence of distinct crystallographic orientations between -Pb and-Sn lamellae in quenched eutectic Sn-Pb solder. The domain size distribution function isfound to approach a dynamic scaling regime and coalescence of domains is shown to be thedominant mechanism for the growth of domains larger than the mean domain size.

    Strain field localization and its evolution were measured in a number of Sn-based Pb-free solder interconnections which were mechanically shear loaded. The local strain wasfound to differ significantly from the applied global strain. Strain localization was shown todepend on the geometry of the samples as well as on the microstructure (at a grain level)of the solder. Strain field localization parallel to the solder-Cu interface was evident andfailure typically occurred along these regions. The exact location of damage however wasnot at the intermetallic layer-solder interface, but rather within the solder itself. Cracks alsoformed along grain boundaries irrespective of the solder type, indicating the importance ofmicrostructure in damage initiation. The junctions of grain boundaries with the interfaceare the typical locations of strain concentration in the examined Pb-free solder. A goodcorrelation has been established between the calculated strain fields and observed failures.

    Next, the effects of the intrinsic thermal anisotropy of Sn were studied in mechanicallyunconstrained SAC alloy under thermal fatigue. Damage was localized mainly along high

  • viii

    angle tilt Sn grain boundaries. It has been demonstrated from a combination of OrientationImaging Microscopy and Finite Element Modelling that encountered fatigue damage andstresses resulting from the thermal anisotropy of Sn are highly correlated.

    Microstructure evolution in a Pb-free SAC solder alloy was studied during low cycle me-chanical fatigue. Digital Image Correlation was employed to measure the strain-field local-ization during fatigue. Fatigue damage is correlated well with the measured localized strains.The effect of the elastic (i.e. mechanical) anisotropy on the onset of microscopic slip wasfound to be small (as shown by elasticity-based finite element calculations). Grain bound-aries were not particular

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