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Experimental investigation for fracture analysis of solder joints in microelectronic and mems applications

: Walter, H.; Bombach, C.; Dudek, R.; Faust, W.; Michel, B.


Gdoutos, E.E.:
Fracture of Nano and Engineering Materials and Structures. Book + CD-ROM : Proceedings of the 16th European Conference of Fracture, Alexandroupolis, Greece, July 3-7, 2006
Dordrecht: Springer Netherland, 2006
ISBN: 978-1-402-04972-9
ISBN: 978-1-402-04971-2 (print)
European Conference on Fracture (ECF) <16, 2006, Alexandroupolis>
Fraunhofer IZM ()

In Microelectronic and MEMS (Micro-Electro-Mechanical Systems) applications the volume of solder joints decreases rapidly due to higher packaging density. In recent years, many solder alloys have been developed and used. Many of lead-free-solders show a complex material behaviour due to different mechanical and thermal properties throughout the sandwich which can influence the mechanical and thermal reliability as well as the life time. For this reason, ensuring the solder joint reliability is one of the most critical design aspects of electronic assemblies. To predict the failure of solder joints with the help of the FE-Simulations tools, the description of solder behaviour is needed. The mechanical behaviour of solder is non-linear and temperature dependent. Solder alloy in consideration are above 0.5 of their melting point at ? 40°C, so creep processes are expected. The failure behaviour of solder is a complex sequence and depends on microstructures, like grain coarsening, micro-voiding, recrystallization, micro-cracking and macro-cracking on alloying content, soldering temperature profile and dissolution of metallizations. Changes of the microstructure can significantly effect the mechanical properties of the solders. Inhomogeneity of solders, espacially of Sn-based lead free solders, can cause local fatigue driven multiple cracking. Furthermore, plate-like intermetallic compounds (IMC) may cause crack initiation and brittle fracture at interface to metallization, especially if the joint thickness becomes comperable to the IMC-thickness respectively.The failure criterion used for recording lifetime might vary based on either electrical failure or mechanical cracks. For the analysis the temperature and stress dependent inelastic behaviour (Creep and stress relaxation) no standard testing methods could be used for these applications. combination of numerical testing methods and experimental are necessary. A comprehensive mechanical characterization of solder alloy properties for tensile, shear, creep and fatigue properties were investigated. Thereby, this paper presents experimental test methods and results by means of modified grooved lap specimens. The analysis of shear stress and strain in solder joints is of interest for development of analytical models that describe the shear deformations of solder joints due to global thermal expansion mismatch between components and PCB. It could be seen, that a couple of Sn-based lead free solder shows both primary and secondary creep behaviour under cycling loading conditions. Primary creep and plastic strain may not be negligible in applications with high temperature ramp rate or under thermal cycling conditions with short dwell time. For the description of the low cycle fatigue behaviour of solder alloys different failure hypotheses are necessary. In addition to the defect free analysis concepts, a defect tolerant analysis concept can be used. The last concepts based on fracture and damage mechanics hypotheses. Fracture mechanics approaches using a critical crack tip parameters (J-Integral) with the assumes that fatigue is limited to propagation of micro cracks. Continuum damage approaches using a viscoplastic constitutive framework with damage evolution capabilities. The application of fracture mechanical concept enables an accurate interpretation of result of the FE-analysis to estimate the reliability and lifetime duration.