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Experimental determination of the Young's modulus of copper and solder materials for electronic packaging
Microelectronics Reliability 91 (2018) 251–256
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Experimental determination of the Young's modulus of copper and solder materials for electronic packaging
T
⁎
F. Kraemera, , M. Roelligb, R. Metaschb, J. Ahmara, K. Meierc, S. Wiesea a
Saarland University, Dept. Systems Engineering, Chair of Microintegration and Reliability, Campus C6 3, 66123 Saarbrucken, Germany Fraunhofer Institute for Ceramic Technologies and Systems, Dept. Testing of Electronics and Optical Methods, Maria-Reiche-Str. 2, 01109 Dresden, Germany c Technical University Dresden, Faculty of Electrical and Computer Engineering, Institute of Electronic Packaging Technology, Mommsenstr, 15, 01069 Dresden, Germany b
A B S T R A C T
The paper presents details about the adequate experimental determination of the Young's modulus on miniaturised specimens for material used in electronic packaging. The difficulty to determine accurately the Young's modulus is caused by the requirements of representative specimens for the area of electronic packaging. In many cases, such specimens e.g. solder balls are connected with the issues of inhomogeneous stress distributions, small dimensions, or special gripping requirements, that create a number of challenges to conduct mechanical experiments. In addition, there are problems that arise from the nonlinearities in the constitutive behaviour of the material to be characterised, such as creep deformation. Therefore, any attempt to accurately determine the Young's modulus needs a case to case consideration of the specific issues for the given specimen and material.
1. Introduction
2. Tensile test characteristics
The determination of Young's modulus is a widely discussed topic. The vast majority of papers concentrate on the experimental methodology for tiny specimens or structures. One of the most preferred methods to determine the Young's modulus is the use of indentation experiments using the evaluation technique introduced by Oliver and Pharr [1]. Most of the experiments focused on the area of thin films and soft materials [2–6], where other methods are cumbersome to apply. However, the degree of precise correlation to Young's modulus values was also discussed [7]. Another intensively regarded method for the determination of the Young's modulus in thin films is the bulge test [8–11]. Both methods are very sensitive to certain assumptions. Therefore, they carry some uncertainties if characteristic structural features of the sample are unknown. Moreover, both methods apply an inhomogeneous or very localised loading of the sample, which makes them prone to errors due to property gradients throughout the sample. Due to the described shortcomings of indentation and bulge test to determine the Young's modulus accurately, two other methods will be analysed, which seem to be more appropriate for the geometrical scale of characteristic structures in electronic assemblies. The first method is the classical tensile test, which is carried out on specially designed test fixtures for electronic packaging materials. The second method uses ultrasonic oscillations to account for the specific lattice stiffness in different crystallographic directions.
The use of a tensile test carries specific difficulties in determining the Young's modulus on representative specimens, which were specially designed for the area of electronic packaging, e.g. flip-chip-specimens [12] of FBGA-specimens [13], LIGA metal thin films [14, 15], etched copper traces [16]. These difficulties encompass:
⁎
(1) Insufficient strain measurement resolution (2) Inappropriate specimen design (3) Out of plane forces. The accuracy of the determined material model for the later use in FEM simulation depends on these aspects. There were some reliability issues in electronic packaging, where the accuracy of the Young's modulus influences the results of FEM calculations. For instance, analyses about the fatigue of copper metallisation in through hole connections of PCBs, which were carried out in [17, 18] show, that different assumptions of the crystal orientation of the copper metallisation in the PCB holes results in a 1:100 ratio in the fatigue cycles prognostics. Therefore, the existing practice of tensile testing had to be improved in order to achieve more precise results in the characterisation of the elastic response of the material. For this reason, test fixtures were designed for the specific needs and features of the specimens. Important design aspects concerned the grips and the displacement measurement.
Corresponding author. E-mail addresses: [email protected] (F. Kraemer), [email protected] (M. Roellig).
https://doi.org/10.1016/j.microrel.2018.06.002 Received 31 January 2018; Received in revised form 28 May 2018; Accepted 6 June 2018 Available online 27 October 2018 0026-2714/ © 2018 Elsevier Ltd. All rights reserved.
Microelectronics Reliability 91 (2018) 251–256
F. Kraemer et al.
axis was used in order to be able to observe the sample deformation from the topside using a CCD-camera connected to a telecentric lens. The CCD-camera recorded the changes in surface pattern of the sample, which was used as an input for the DIC algorithm, in order to calculate the strain of the sample. A well-defined projection of the sample's surface pattern to the pixels of the camera chip was enabled by the use of telecentric lens having a fixed magnification and working distance. The lightening of the sample was optimised by a pair of LED spotlights, which were adjusted by flexible arms, in order to achieve an optimum illumination and contrast for the display of the surface patterns. Vibrations of the optical systems were widely suppressed by the application of an optical rod [20]. The sample was fixed by grips onto the test fixture. One grip was moved by x-axis translation stage, which was actuated by a stepper motor. The other grip was mounted to a 3-D force sensor, which was fixed on the y-z-stage. The y-z-stage was mounted on x-z-bracket and was adjusted by micrometre heads, in order to align the grips to each other. The frame of the test fixture was realised by an optical base plate [17, 19]. 3.3. Results
Fig. 1. Specimen design: Geometry of the dog bone specimen, which is milled from a copper sheet (left). Cross-section through the specimen, showing the inhomogeneous thickness, due to the electrolytic deposition process (right).
Experiments were carried out at temperatures of 25 °C, 60 °C, 100 °C and 150 °C. Samples were loaded with a constant strain rate of approx. 10−4 s−1. Alternating loading and unloading phases formed a triangular wave type loading function. The load profile was similar to the profile shown in Fig. 7. The slope of the strain-stress-functions was evaluated for every loading and unloading phase. The evaluation showed that the unload phases have a smaller distribution between estimated values for Young's modulus. Yet it is not quite clear, if the better quality of Young's modulus data from the unload phase was due to plastic deformation, which occurred during the loading phase. Table 1 shows the determined Young's modulus at temperatures of 25 °C, 60 °C, 100 °C. The Young's modulus could not be determined from experiments at 150 °C, because of significant contributions of plastic deformation. Fig. 3 shows the results of tensile tests on copper specimens at a temperature of 25 °C. Due to the scatter within the data, more experiments need to be undertaken, in order to come to clear conclusions about the scatter of elastic properties between similar manufactured copper materials.
3. Tensile tests on PCB copper traces 3.1. Specimens In order to allow high precision displacement measurement a dog bone specimen design was chosen. Specimen dimensions are shown in Fig. 1. The large gauge zone of 4 mm × 10 mm enabled precise optical deformation measurements by DIC. The broad sample ends 8 mm × 7 mm reducing impact of the grip forces on the stress distribution along the specimen. The specimens were milled out of approx. 30 μm thick copper sheets to realise the complex geometry. The crosssection of the specimen (Fig. 1) shows an inhomogeneous thickness, which supposedly results from the electrolytic deposition process [19]. 3.2. Test setup Fig. 2 shows a detail of the test setup. A horizontally orientated load
4. Tensile test on bulk solder specimens 4.1. Bulk specimens The elastic modulus was measured using an in-house developed tensile tester. Bulk solder specimens were manufactured by an optimised preparation using a remelting process in a tightly temperature controlled casting mould [21]. Fig. 4 shows the tensile specimen geometry for bulk solder investigations. This geometry type had an elongation length of 45 mm with a constant cross section area of 15.14 mm2 (rectangle shape with rounded edges). The sample preparation of casted solder tensile specimens used a Table 1 Young's modulus of electrolytic deposited copper of approx. 30 μm thickness determined by a tensile test at various temperatures.
Fig. 2. Detailed view on the position of the telecentric lens above the thermal chamber of the test setup for experiments on representative copper stripe specimens. 252
Temperature
25 °C
60 °C
100 °C
Unload 1 Unload 2 Unload 3 Unload 4 Average Standard deviation
115.8 GPa 109.4 GPa 105.1 GPa 109.1 GPa 109.8 GPa ± 4.4 GPa
97.2 GPa 90.2 GPa 96.9 GPa 88.8 GPa 93.3 GPa ± 4.4 GPa
91.1 GPa 94.0 GPa 90.2 GPa 89.8 GPa 91.3 GPa ± 1.9 GPa
Microelectronics Reliability 91 (2018) 251–256
F. Kraemer et al.
Fig. 6. View into the temperature chamber of the tensile tester with a linear variable displacement transducer (LVDT) fixed on a clamped solder specimen.
4.3. Measurement procedure Fig. 3. Determination of Young's modulus stress-strain-curve at 25 °C.
The measurement of solder elasticity and Young's modulus in a tensile test was challenging, because the time-dependent creep behaviour dominates the deformation behaviour also at low deformations over the whole investigated temperature range. In order to minimise the influence of plastic strains during the tests very low displacements of only 10 μm, which was less than 0.025% strain, with a strain rate of 10−4 s−1 had to be applied to the bulk specimen. That leads to forces of up to 130 N, which was less than 10 MPa stress. In order to reduce the variation of results in every temperature state, ten cycles of loading and unloading experiments were performed, as seen in Fig. 7. It should be noted that the specimen length by the LVDT includes a part of the transition area between clamping and strain area. This arrangement increased the measurement accuracy and stability, because it avoided local necking effects of the very ductile solder specimen, especially at high temperature. However, this leads to a systematic error, which was corrected by finite element simulation.
Fig. 4. Solder bulk specimen geometry.
4.4. Results Under these test conditions, an increased temperature leads to a decreased stress during loading and unloading phases. The reason was a change of the solder elasticity dependent on the change of temperature, see Fig. 8. A finite element simulation was used to calculate stress-strain behaviour from the raw force-displacement data, in order to correct for geometry effects due to the specific clamp position of the LVDT on the
Fig. 5. Coquille of the casting mould for the solder bulk specimen production.
two-part casting mould (see Fig. 5). In the first production step the casting mould was heated up over the melting point of the used solder alloy. After the coquille was filled up with liquid solder the cool down step starts. Due to the temperature gradients of the heated mould, a directional solidification occurred after the heating control was switched off. This started at the bottom side towards the top side and avoided cracks due to volume contraction of the solder [21].
4.2. Test setup The displacement measurement was directly realised on the specimen by a LVDT sensor, in order to avoid any deformation influences by finite machine stiffness, see Fig. 6. This offered the ability to measure the elasticity with low systematic error influences. The mechanical measurements were conducted within a thermal chamber under isothermal conditions at temperatures between 25 °C and 125 °C. The forces were measured with a 6-wire load cell, which was placed in alignment to the specimen outside of the chamber. Fig. 7. Measured loading profile for determination of Young's modulus for solder in a tensile test. 253
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Fig. 8. Temperature dependent stress-strain data between 25 °C up to 125 °C (only initial load curves).
Fig. 9. Schematic drawing of measurement setup for materials characterisation by scanning acoustic microscopy.
Table 2 Results of approximated Young's modulus by tensile measurement and by recalculation with finite element simulation. Temperature
Young's modulus
25 °C 50 °C 75 °C 100 °C 125 °C
57.6 GPa 54.6 GPa 51.5 GPa 48.5 GPa 45.5 GPa
The ultrasonic measurement setup basically measured the wave propagation time of transversal and longitudinal elastic wave through the specimen material. The runtime of ultrasonic signal reflection at the material interfaces were recorded and allowed the determination of the period of time for the propagation of elastic waves (longitudinal tL = tLL − t0 and transversal tT = tTL − t0). Typically, the specimen was mounted in water, see Fig. 9. Water as coupling medium was required for the low signal damping. Knowing the thickness d of the sample, the velocities of elastic waves were calculated by:
sample. The results are shown in Table 2. Each data entry represents the average of twenty determined elasticity values from a load-unload cycle. The standard deviation of one single value was 0.4 GPa. 5. Ultrasonic measurements on solder material
cL =
2⋅d tL
(1)
cT =
d 3⋅tL + tT − 1.5⋅2⋅tL
(2)
cL was the velocity of longitudinal wave and cT was the velocity of the transversal wave. The elastic modulus was calculated incorporating the material density ζ.
5.1. Background
c 2 − 2⋅cT2 ⎞ E = 2⋅ς⋅cT2 ⎜⎛1 + L 2 ⎟ 2. cL − 2⋅cT2 ⎠ ⎝
The tensile measurement principle came with two disadvantages, which limited the precision of the determination of Young's modulus: 1) A limited clamping stiffness and the limited measurement resolution. 2) Materials with high homologous temperature (as solders) additionally deform inelastic.
(3)
5.3. Sample preparation of plated metal specimen In advance of the US-measurements a specimen setup for metal based materials had been created. The basis work for the realisation of the homogenous metal specimen was published in [22]. In this procedure, different metal and metal alloys were mechanically prepared. At first metal foils were manufactured by a stepwise rolling process to flatten the material out of the cast material. The stepwise decreasing foil thickness reached 250 μm finally. Thereafter, the material specimen was manufactured by punching out a round shaped particle with a diameter of 2.5 mm. The actual thickness of the individual specimen was measured with a micrometre calliper. For each particular specimen the thickness was determined, because the ultrasonic velocity and the propagation length highly depended on each other in terms of echo runtime measurement. Furthermore, the mass and the outline geometry were measured in order to determine the material density.
5.2. Principle of ultrasonic measurements In contrast to the tensile measurement based elastic modulus determination, the ultrasonic measurement principle is an alternative method. Ultrasonic wave propagates as elastic wave through ridged materials by interacting with the materials micro structure (metal lattice). The advantages of the SAM (scanning acoustic microscopy) approach are as follows: - Contactless measurement, - Multiple measurement events on one specimen due to areal scanning option, - Totally elastic material reaction, without any inelastic overlapping effects, - Determination of Poisson constant and Elastic Module at one measurement.
5.4. US-microscopy and results of elastic properties of solder material The specimen was assembled on a ceramic sheet by an adhesive tape, see Fig. 10. The acoustical interfaces of soft and ridged material 254
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packaging. In many cases such specimens, e.g. solder balls, are connected with the issues of inhomogeneous stress distributions, small dimensions, or special gripping requirements, which in turn created a number of challenges to conduct mechanical experiments. These aspects contributed to the accuracy of the latter material model for the use in FEM simulation. There are some characteristic reliability issues in electronic packaging, where the accuracy of the Young's modulus matters for the results of the affiliated FEM simulations. Therefore the paper highlights the connection between specimen design, strain measurement methodology and imposed loading conditions with respect to the effects on the results of the mechanical tests for representative specimens. In the first example, the Young's modulus of copper was determined on thin (approx. 30 μm thick) dog bone specimens which represented copper traces on a PCB. The key to the accurate determination of the Young's modulus was the application of an optical strain measurement using a DIC methodology. This way the strain measurements were corrected for specimen slip and other sources of error. In the second example the Young's modulus of SnAg3.5 was determined on bulk dog bone specimens using LVDT for strain measurement. In the third example the solder Young's modulus of SnAg3.5 was determined on a cold rolled solder sheet (diameter = 2.5 mm, thickness = 250 μm) using the interaction with ultrasonic waves to calculate the elastic constants of the specimen. The Young's modulus determined by the ultrasonic measurement was higher than that determined by the tensile test. The three examples show the specific approaches that had to be taken for the different materials in electronic packaging, which demanded specialized test setups and experimental methodologies, and hence are different from the standard methodologies of mechanically testing materials.
Fig. 10. Image of a solder specimen prepared for US-microscopy to determine elastic properties by acoustic wave propagation. Table 3 Material SnAg3.5 averaged elastic modulus over temperature [23]. Solder
Temp.
E-modulus
SnAg3.5 SnAg3.5 SnAg3.5 Function [T in °C]
5 °C 63.9 GPa 21 °C 61.5 GPa 50 °C 60.1 GPa E(T) = −0,0798 T + 63.9 GPa
allowed the determination of the signal echoes reflecting from the interfaces in a very distinctive way. The US-measurements head was chosen to operate at 50 Hz. The specimen surface area was scanned and the acoustic echoes, in particular the time of flight of the waves determined. The longitudinal and the transversal wave propagation were inspected separately. For the temperature dependency, measurement the water temperature, as a coupling medium, was varied between 5 °C and 50 °C. For each specimen at a particular temperature a set of many signal velocities were evaluated and averaged. Based on the previous equations the elastic moduli were determined as a function of temperature. The authors published US-measurement based elastic properties in [23] as example. There the specimen of solder type SnAg3.5 was scanned acoustically and the propagation time ranges were measured. Table 3 summarises the recalculated results of elastic modulus and velocities of transversal and longitudinal elastic waves. The acoustically determined elastic modulus decreased with increasing temperature. This material effect was an expected property of metal based materials. Furthermore, there was a nearly linear dependency on temperature visible in the inspected range.
Acknowledgments The authors would like to acknowledge the work of Dr. Andreas Ruh for preparing metallographic samples and for performing the microscopic imaging for the study. The authors would also like to thank for the fruitful discussions with Dr. Rainer Dudek from Fraunhofer ENAS and Prof. Bernhard Wunderle from TU Chemnitz. References [1] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [2] W.D. Nix, Elastic and plastic properties of thin films on substrates: nanoindentation techniques, Mater. Sci. Eng. A 234 (1997) 37–44. [3] H. Li, J.J. Vlassak, Determining the elastic modulus and hardness of an ultra-thin film on a substrate using nanoindentation, J. Mater. Res. 24 (2009) 1114–1126. [4] H. Zheng, X.J. Zheng, S.T. Song, J. Sun, F. Jiao, W. Liu, G.Y. Wang, Evaluation of the elastic modulus of thin film considering the substrate effect and geometry effect of indenter tip, Comput. Mater. Sci. 50 (2011) 3026–3031. [5] Y. Kamran, P.-L. Larsson, Second-order effects at microindentation of elastic polymers using sharp indenters, Mater. Des. 32 (2011) 3645–3653. [6] C. Basaran, J.B. Jiang, Measuring intrinsic elastic modulus of Pb/Sn solder alloys, Mech. Mater. 34 (2002) 349–362. [7] C. Heermant, D. Dengel, Microhardness and estimated Youngs's Modulus: problems of ascertainment using Force-Indentation Depth-Values, Proceedings of the Micro Materials '95 Conference, Berlin, 1995, Nov. 28–29, pp. 537–544. [8] Y. Xiang, T.Y. Tsui, J.J. Vlassak, The mechanical properties of freestanding electroplated Cu thin films, J. Mater. Res. 21 (2006) 1607–1618. [9] J.J. Vlassak, W.D. Nix, A new bulge test technique for the determination of Young's modulus and Poisson's ratio of thin films, J. Mater. Res. 7 (1992) 3242–3249. [10] X.H. Ju, F.Z. Ren, G.S. Zhou, Inquire into the method of measuring elastic modulus of polymer and metal films in bulge test, Rare Metal Mater. Eng. 32 (2003) 313–316. [11] M.K. Small, B.J. Daniels, B.M. Clemens, W.D. Nix, The elastic biaxial modulus of Ag–Pd multilayered thin films measured using the bulge test, J. Mater. Res. 9 (1994) 25–30. [12] S. Wiese, E. Meusel, Characterization of lead-free solders in flip chip joints, J. Electron. Packag. 125 (2003) 531–538. [13] M. Roellig, R. Dudek, S. Wiese, B. Boehme, B. Wunderle, K.-J. Wolter, B. Michel, Fatigue analysis of miniaturized lead-free solder contacts based on a novel test concept, Microelectron. Reliab. 47 (2007) 187–195. [14] E. Mazza, J. Dual, G. Schiltges, Mechanical properties of microstructures – theory
5.5. Comparison with literature data A comparison of the measured elastic modulus based on ultrasonic method of the SnAg3.5 solder with literature data resulted in some differences. At room temperature Young's modulus values determined by tensile tests ranging from 40 GPa to 50 GPa [24–26]. Indentation methods resulted in a Young's Modulus of 35.0 GPa [27]. The measured ultrasonic values in this work were larger than the literature data. This might be due to small scale deformations associated with the ultrasonic method, which could not be achieved by tensile tests. However, earlier ultrasonic measurements conducted by Roellig and Gust [28] using a different methodology [29] resulted in even higher values of 70.5 GPa for the Young's modulus of SnAg 3.5. 6. Conclusions The paper presents various experimental methods to determine the Young's modulus of materials, which are used in electronic packaging, namely electrolytic deposited copper and lead free solder. The difficulty to determine accurately the Young's modulus was caused by the requirements of representative specimens for the area of electronic 255
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[15] [16] [17]
[18]
[19]
[20]
[21]
[22]
and experiment, Proceedings of the Micro Materials '95 Conference, Berlin, 1995, Nov. 28–29, pp. 125–130. E. Mazza, S. Abel, J. Dual, Experimental determination of mechanical properties of Ni and Ni-Fe microbars, Microsyst. Technol. 2 (1996) 197–202. G. Simons, Ch. Weippert, J. Dual, J. Villain, Size effects in tensile testing of thin cold rolled and annealed Cu foils, Mater. Sci. Eng. A 416 (2006) 290–299. S. Wiese, F. Kraemer, Adequate mechanical copper modelling for 2nd level interconnect structures, IEEE-Proceedings of the 16th International Conference on Thermal, Mechanical and Multi-Physics Simulation, and Experiments (EuroSimE), Budapest, 2015, April 20–22 (7 pp.). B.E. Abali, P. Lofink, W.H. Müller, Untersuchung der Variation der Materialkennwerte zur Lebensdauerabschätzung für Durchkontaktierungen in Leiterplatten, DVS-Berichte 301 (2014) 297–303. S. Wiese, D. Bruch, M. Elasmi, F. Kraemer, Experimental design for tensile tests on PCB copper traces for board level packaging, in: J. Ahmar (Ed.), IEEE-Proceedings of the 17th International Conference on Thermal, Mechanical and Multi-Physics Simulation, and Experiments (EuroSimE), Montpellier, 17–20 April 2016 (7 pp.). M. Elasmi, Entwurf und Aufbau einer Experimentalvorrichtung zur Bestimmung werkstoffmechanischer Eigenschaften von Verkapselungsmaterialien für PVModule, Bachelor Thesis, Saarland University, (2013). R. Metasch, J.C. Boareto, M. Roellig, S. Wiese, K.-J. Wolter, Primary and tertiary creep properties of eutectic SnAg3.8Cu0.7 in bulk specimens, IEEE-Proceeding of the 10th International Conference on Thermal, Mechanical and Multi-Physics simulation and Experiments in Microelectronics and Microsystems, Eindhoven, 26–29 April 2009, pp. 322–329. M. Roellig, S. Wiese, K. Meier, K.-J. Wolter, Creep measurements of 200 μm–400 μm
[23]
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solder joints, IEEE-Proceeding of the 8th International Conference on Thermal, Mechanical and Multi-Physics Simulation Experiments in Microelectronics and Micro-Systems, London, 16–18 April 2007, pp. 255–263. M. Roellig, R. Metasch, K. Meier, F. Alt, Characterization methods for determination of temperature depended electrical, thermal, mechanical and fatigue properties of SnAg3.5 solder, Proc. Of the 3rd Electronic System-Integration Technology Conference (ESTC), Berlin, 13–16 Sept. 2010, p. 11. N. Chawla, F. Ochoa, V.V. Ganesh, X. Deng, M. Koopman, K.K. Chawla, S. Scarritt, Measurement and prediction of Young's modulus of a Pb-free solder, J. Mater. Sci. Mater. Electron. 15 (2004) 385–388. C. Andersson, D.R. Andersson, P. Tegehall, J. Liu, Effect of different temperature cycle profiles on the crack propagation and microstructural evolutionof lead free solder joints of different electronic components, IEEE-Proceedings of the 5th International Conference on Thermal, Mechanical and Multi-Physics Simulation, and Experiments (EuroSimE), 10–2 May 2004, pp. 455–464 (Brussels). C. Kanchanomai, Y. Miyashita, Y. Mutoh, C-parameter approach to low cycle fatigue crack growth of solders, IEEE Proceedings of 4th Electronics Packaging Technology Conference (EPTC 2002), Singapore, 10–12 Dec. 2002, pp. 385–390. T. Ogawa, R. Kaga, T. Ohsawa, Microstructure and mechanical properties predicted by indentation testing of lead-free solders, J. Electron. Mater. 34 (2005) 311–317. M. Roellig, N. Gust, Laborbericht zu Ultraschallmessungen an Lotkugeln, Technische Universität Dresden, Institut für Aufbau- und Verbindungstechnik, (2007). N. Gust, E. Kühnicke, D. Breuer, Material characterization with the ultrasonic microscope, IEEE Proceedings of the 31st International Spring Seminar on Electronics Technology, Budapest, 7–11 May 2008, pp. 91–95.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Experimental determination of the Young's modulus of copper and solder materials for electronic packaging
T
⁎
F. Kraemera, , M. Roelligb, R. Metaschb, J. Ahmara, K. Meierc, S. Wiesea a
Saarland University, Dept. Systems Engineering, Chair of Microintegration and Reliability, Campus C6 3, 66123 Saarbrucken, Germany Fraunhofer Institute for Ceramic Technologies and Systems, Dept. Testing of Electronics and Optical Methods, Maria-Reiche-Str. 2, 01109 Dresden, Germany c Technical University Dresden, Faculty of Electrical and Computer Engineering, Institute of Electronic Packaging Technology, Mommsenstr, 15, 01069 Dresden, Germany b
A B S T R A C T
The paper presents details about the adequate experimental determination of the Young's modulus on miniaturised specimens for material used in electronic packaging. The difficulty to determine accurately the Young's modulus is caused by the requirements of representative specimens for the area of electronic packaging. In many cases, such specimens e.g. solder balls are connected with the issues of inhomogeneous stress distributions, small dimensions, or special gripping requirements, that create a number of challenges to conduct mechanical experiments. In addition, there are problems that arise from the nonlinearities in the constitutive behaviour of the material to be characterised, such as creep deformation. Therefore, any attempt to accurately determine the Young's modulus needs a case to case consideration of the specific issues for the given specimen and material.
1. Introduction
2. Tensile test characteristics
The determination of Young's modulus is a widely discussed topic. The vast majority of papers concentrate on the experimental methodology for tiny specimens or structures. One of the most preferred methods to determine the Young's modulus is the use of indentation experiments using the evaluation technique introduced by Oliver and Pharr [1]. Most of the experiments focused on the area of thin films and soft materials [2–6], where other methods are cumbersome to apply. However, the degree of precise correlation to Young's modulus values was also discussed [7]. Another intensively regarded method for the determination of the Young's modulus in thin films is the bulge test [8–11]. Both methods are very sensitive to certain assumptions. Therefore, they carry some uncertainties if characteristic structural features of the sample are unknown. Moreover, both methods apply an inhomogeneous or very localised loading of the sample, which makes them prone to errors due to property gradients throughout the sample. Due to the described shortcomings of indentation and bulge test to determine the Young's modulus accurately, two other methods will be analysed, which seem to be more appropriate for the geometrical scale of characteristic structures in electronic assemblies. The first method is the classical tensile test, which is carried out on specially designed test fixtures for electronic packaging materials. The second method uses ultrasonic oscillations to account for the specific lattice stiffness in different crystallographic directions.
The use of a tensile test carries specific difficulties in determining the Young's modulus on representative specimens, which were specially designed for the area of electronic packaging, e.g. flip-chip-specimens [12] of FBGA-specimens [13], LIGA metal thin films [14, 15], etched copper traces [16]. These difficulties encompass:
⁎
(1) Insufficient strain measurement resolution (2) Inappropriate specimen design (3) Out of plane forces. The accuracy of the determined material model for the later use in FEM simulation depends on these aspects. There were some reliability issues in electronic packaging, where the accuracy of the Young's modulus influences the results of FEM calculations. For instance, analyses about the fatigue of copper metallisation in through hole connections of PCBs, which were carried out in [17, 18] show, that different assumptions of the crystal orientation of the copper metallisation in the PCB holes results in a 1:100 ratio in the fatigue cycles prognostics. Therefore, the existing practice of tensile testing had to be improved in order to achieve more precise results in the characterisation of the elastic response of the material. For this reason, test fixtures were designed for the specific needs and features of the specimens. Important design aspects concerned the grips and the displacement measurement.
Corresponding author. E-mail addresses: [email protected] (F. Kraemer), [email protected] (M. Roellig).
https://doi.org/10.1016/j.microrel.2018.06.002 Received 31 January 2018; Received in revised form 28 May 2018; Accepted 6 June 2018 Available online 27 October 2018 0026-2714/ © 2018 Elsevier Ltd. All rights reserved.
Microelectronics Reliability 91 (2018) 251–256
F. Kraemer et al.
axis was used in order to be able to observe the sample deformation from the topside using a CCD-camera connected to a telecentric lens. The CCD-camera recorded the changes in surface pattern of the sample, which was used as an input for the DIC algorithm, in order to calculate the strain of the sample. A well-defined projection of the sample's surface pattern to the pixels of the camera chip was enabled by the use of telecentric lens having a fixed magnification and working distance. The lightening of the sample was optimised by a pair of LED spotlights, which were adjusted by flexible arms, in order to achieve an optimum illumination and contrast for the display of the surface patterns. Vibrations of the optical systems were widely suppressed by the application of an optical rod [20]. The sample was fixed by grips onto the test fixture. One grip was moved by x-axis translation stage, which was actuated by a stepper motor. The other grip was mounted to a 3-D force sensor, which was fixed on the y-z-stage. The y-z-stage was mounted on x-z-bracket and was adjusted by micrometre heads, in order to align the grips to each other. The frame of the test fixture was realised by an optical base plate [17, 19]. 3.3. Results
Fig. 1. Specimen design: Geometry of the dog bone specimen, which is milled from a copper sheet (left). Cross-section through the specimen, showing the inhomogeneous thickness, due to the electrolytic deposition process (right).
Experiments were carried out at temperatures of 25 °C, 60 °C, 100 °C and 150 °C. Samples were loaded with a constant strain rate of approx. 10−4 s−1. Alternating loading and unloading phases formed a triangular wave type loading function. The load profile was similar to the profile shown in Fig. 7. The slope of the strain-stress-functions was evaluated for every loading and unloading phase. The evaluation showed that the unload phases have a smaller distribution between estimated values for Young's modulus. Yet it is not quite clear, if the better quality of Young's modulus data from the unload phase was due to plastic deformation, which occurred during the loading phase. Table 1 shows the determined Young's modulus at temperatures of 25 °C, 60 °C, 100 °C. The Young's modulus could not be determined from experiments at 150 °C, because of significant contributions of plastic deformation. Fig. 3 shows the results of tensile tests on copper specimens at a temperature of 25 °C. Due to the scatter within the data, more experiments need to be undertaken, in order to come to clear conclusions about the scatter of elastic properties between similar manufactured copper materials.
3. Tensile tests on PCB copper traces 3.1. Specimens In order to allow high precision displacement measurement a dog bone specimen design was chosen. Specimen dimensions are shown in Fig. 1. The large gauge zone of 4 mm × 10 mm enabled precise optical deformation measurements by DIC. The broad sample ends 8 mm × 7 mm reducing impact of the grip forces on the stress distribution along the specimen. The specimens were milled out of approx. 30 μm thick copper sheets to realise the complex geometry. The crosssection of the specimen (Fig. 1) shows an inhomogeneous thickness, which supposedly results from the electrolytic deposition process [19]. 3.2. Test setup Fig. 2 shows a detail of the test setup. A horizontally orientated load
4. Tensile test on bulk solder specimens 4.1. Bulk specimens The elastic modulus was measured using an in-house developed tensile tester. Bulk solder specimens were manufactured by an optimised preparation using a remelting process in a tightly temperature controlled casting mould [21]. Fig. 4 shows the tensile specimen geometry for bulk solder investigations. This geometry type had an elongation length of 45 mm with a constant cross section area of 15.14 mm2 (rectangle shape with rounded edges). The sample preparation of casted solder tensile specimens used a Table 1 Young's modulus of electrolytic deposited copper of approx. 30 μm thickness determined by a tensile test at various temperatures.
Fig. 2. Detailed view on the position of the telecentric lens above the thermal chamber of the test setup for experiments on representative copper stripe specimens. 252
Temperature
25 °C
60 °C
100 °C
Unload 1 Unload 2 Unload 3 Unload 4 Average Standard deviation
115.8 GPa 109.4 GPa 105.1 GPa 109.1 GPa 109.8 GPa ± 4.4 GPa
97.2 GPa 90.2 GPa 96.9 GPa 88.8 GPa 93.3 GPa ± 4.4 GPa
91.1 GPa 94.0 GPa 90.2 GPa 89.8 GPa 91.3 GPa ± 1.9 GPa
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Fig. 6. View into the temperature chamber of the tensile tester with a linear variable displacement transducer (LVDT) fixed on a clamped solder specimen.
4.3. Measurement procedure Fig. 3. Determination of Young's modulus stress-strain-curve at 25 °C.
The measurement of solder elasticity and Young's modulus in a tensile test was challenging, because the time-dependent creep behaviour dominates the deformation behaviour also at low deformations over the whole investigated temperature range. In order to minimise the influence of plastic strains during the tests very low displacements of only 10 μm, which was less than 0.025% strain, with a strain rate of 10−4 s−1 had to be applied to the bulk specimen. That leads to forces of up to 130 N, which was less than 10 MPa stress. In order to reduce the variation of results in every temperature state, ten cycles of loading and unloading experiments were performed, as seen in Fig. 7. It should be noted that the specimen length by the LVDT includes a part of the transition area between clamping and strain area. This arrangement increased the measurement accuracy and stability, because it avoided local necking effects of the very ductile solder specimen, especially at high temperature. However, this leads to a systematic error, which was corrected by finite element simulation.
Fig. 4. Solder bulk specimen geometry.
4.4. Results Under these test conditions, an increased temperature leads to a decreased stress during loading and unloading phases. The reason was a change of the solder elasticity dependent on the change of temperature, see Fig. 8. A finite element simulation was used to calculate stress-strain behaviour from the raw force-displacement data, in order to correct for geometry effects due to the specific clamp position of the LVDT on the
Fig. 5. Coquille of the casting mould for the solder bulk specimen production.
two-part casting mould (see Fig. 5). In the first production step the casting mould was heated up over the melting point of the used solder alloy. After the coquille was filled up with liquid solder the cool down step starts. Due to the temperature gradients of the heated mould, a directional solidification occurred after the heating control was switched off. This started at the bottom side towards the top side and avoided cracks due to volume contraction of the solder [21].
4.2. Test setup The displacement measurement was directly realised on the specimen by a LVDT sensor, in order to avoid any deformation influences by finite machine stiffness, see Fig. 6. This offered the ability to measure the elasticity with low systematic error influences. The mechanical measurements were conducted within a thermal chamber under isothermal conditions at temperatures between 25 °C and 125 °C. The forces were measured with a 6-wire load cell, which was placed in alignment to the specimen outside of the chamber. Fig. 7. Measured loading profile for determination of Young's modulus for solder in a tensile test. 253
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Fig. 8. Temperature dependent stress-strain data between 25 °C up to 125 °C (only initial load curves).
Fig. 9. Schematic drawing of measurement setup for materials characterisation by scanning acoustic microscopy.
Table 2 Results of approximated Young's modulus by tensile measurement and by recalculation with finite element simulation. Temperature
Young's modulus
25 °C 50 °C 75 °C 100 °C 125 °C
57.6 GPa 54.6 GPa 51.5 GPa 48.5 GPa 45.5 GPa
The ultrasonic measurement setup basically measured the wave propagation time of transversal and longitudinal elastic wave through the specimen material. The runtime of ultrasonic signal reflection at the material interfaces were recorded and allowed the determination of the period of time for the propagation of elastic waves (longitudinal tL = tLL − t0 and transversal tT = tTL − t0). Typically, the specimen was mounted in water, see Fig. 9. Water as coupling medium was required for the low signal damping. Knowing the thickness d of the sample, the velocities of elastic waves were calculated by:
sample. The results are shown in Table 2. Each data entry represents the average of twenty determined elasticity values from a load-unload cycle. The standard deviation of one single value was 0.4 GPa. 5. Ultrasonic measurements on solder material
cL =
2⋅d tL
(1)
cT =
d 3⋅tL + tT − 1.5⋅2⋅tL
(2)
cL was the velocity of longitudinal wave and cT was the velocity of the transversal wave. The elastic modulus was calculated incorporating the material density ζ.
5.1. Background
c 2 − 2⋅cT2 ⎞ E = 2⋅ς⋅cT2 ⎜⎛1 + L 2 ⎟ 2. cL − 2⋅cT2 ⎠ ⎝
The tensile measurement principle came with two disadvantages, which limited the precision of the determination of Young's modulus: 1) A limited clamping stiffness and the limited measurement resolution. 2) Materials with high homologous temperature (as solders) additionally deform inelastic.
(3)
5.3. Sample preparation of plated metal specimen In advance of the US-measurements a specimen setup for metal based materials had been created. The basis work for the realisation of the homogenous metal specimen was published in [22]. In this procedure, different metal and metal alloys were mechanically prepared. At first metal foils were manufactured by a stepwise rolling process to flatten the material out of the cast material. The stepwise decreasing foil thickness reached 250 μm finally. Thereafter, the material specimen was manufactured by punching out a round shaped particle with a diameter of 2.5 mm. The actual thickness of the individual specimen was measured with a micrometre calliper. For each particular specimen the thickness was determined, because the ultrasonic velocity and the propagation length highly depended on each other in terms of echo runtime measurement. Furthermore, the mass and the outline geometry were measured in order to determine the material density.
5.2. Principle of ultrasonic measurements In contrast to the tensile measurement based elastic modulus determination, the ultrasonic measurement principle is an alternative method. Ultrasonic wave propagates as elastic wave through ridged materials by interacting with the materials micro structure (metal lattice). The advantages of the SAM (scanning acoustic microscopy) approach are as follows: - Contactless measurement, - Multiple measurement events on one specimen due to areal scanning option, - Totally elastic material reaction, without any inelastic overlapping effects, - Determination of Poisson constant and Elastic Module at one measurement.
5.4. US-microscopy and results of elastic properties of solder material The specimen was assembled on a ceramic sheet by an adhesive tape, see Fig. 10. The acoustical interfaces of soft and ridged material 254
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packaging. In many cases such specimens, e.g. solder balls, are connected with the issues of inhomogeneous stress distributions, small dimensions, or special gripping requirements, which in turn created a number of challenges to conduct mechanical experiments. These aspects contributed to the accuracy of the latter material model for the use in FEM simulation. There are some characteristic reliability issues in electronic packaging, where the accuracy of the Young's modulus matters for the results of the affiliated FEM simulations. Therefore the paper highlights the connection between specimen design, strain measurement methodology and imposed loading conditions with respect to the effects on the results of the mechanical tests for representative specimens. In the first example, the Young's modulus of copper was determined on thin (approx. 30 μm thick) dog bone specimens which represented copper traces on a PCB. The key to the accurate determination of the Young's modulus was the application of an optical strain measurement using a DIC methodology. This way the strain measurements were corrected for specimen slip and other sources of error. In the second example the Young's modulus of SnAg3.5 was determined on bulk dog bone specimens using LVDT for strain measurement. In the third example the solder Young's modulus of SnAg3.5 was determined on a cold rolled solder sheet (diameter = 2.5 mm, thickness = 250 μm) using the interaction with ultrasonic waves to calculate the elastic constants of the specimen. The Young's modulus determined by the ultrasonic measurement was higher than that determined by the tensile test. The three examples show the specific approaches that had to be taken for the different materials in electronic packaging, which demanded specialized test setups and experimental methodologies, and hence are different from the standard methodologies of mechanically testing materials.
Fig. 10. Image of a solder specimen prepared for US-microscopy to determine elastic properties by acoustic wave propagation. Table 3 Material SnAg3.5 averaged elastic modulus over temperature [23]. Solder
Temp.
E-modulus
SnAg3.5 SnAg3.5 SnAg3.5 Function [T in °C]
5 °C 63.9 GPa 21 °C 61.5 GPa 50 °C 60.1 GPa E(T) = −0,0798 T + 63.9 GPa
allowed the determination of the signal echoes reflecting from the interfaces in a very distinctive way. The US-measurements head was chosen to operate at 50 Hz. The specimen surface area was scanned and the acoustic echoes, in particular the time of flight of the waves determined. The longitudinal and the transversal wave propagation were inspected separately. For the temperature dependency, measurement the water temperature, as a coupling medium, was varied between 5 °C and 50 °C. For each specimen at a particular temperature a set of many signal velocities were evaluated and averaged. Based on the previous equations the elastic moduli were determined as a function of temperature. The authors published US-measurement based elastic properties in [23] as example. There the specimen of solder type SnAg3.5 was scanned acoustically and the propagation time ranges were measured. Table 3 summarises the recalculated results of elastic modulus and velocities of transversal and longitudinal elastic waves. The acoustically determined elastic modulus decreased with increasing temperature. This material effect was an expected property of metal based materials. Furthermore, there was a nearly linear dependency on temperature visible in the inspected range.
Acknowledgments The authors would like to acknowledge the work of Dr. Andreas Ruh for preparing metallographic samples and for performing the microscopic imaging for the study. The authors would also like to thank for the fruitful discussions with Dr. Rainer Dudek from Fraunhofer ENAS and Prof. Bernhard Wunderle from TU Chemnitz. References [1] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [2] W.D. Nix, Elastic and plastic properties of thin films on substrates: nanoindentation techniques, Mater. Sci. Eng. A 234 (1997) 37–44. [3] H. Li, J.J. Vlassak, Determining the elastic modulus and hardness of an ultra-thin film on a substrate using nanoindentation, J. Mater. Res. 24 (2009) 1114–1126. [4] H. Zheng, X.J. Zheng, S.T. Song, J. Sun, F. Jiao, W. Liu, G.Y. Wang, Evaluation of the elastic modulus of thin film considering the substrate effect and geometry effect of indenter tip, Comput. Mater. Sci. 50 (2011) 3026–3031. [5] Y. Kamran, P.-L. Larsson, Second-order effects at microindentation of elastic polymers using sharp indenters, Mater. Des. 32 (2011) 3645–3653. [6] C. Basaran, J.B. Jiang, Measuring intrinsic elastic modulus of Pb/Sn solder alloys, Mech. Mater. 34 (2002) 349–362. [7] C. Heermant, D. Dengel, Microhardness and estimated Youngs's Modulus: problems of ascertainment using Force-Indentation Depth-Values, Proceedings of the Micro Materials '95 Conference, Berlin, 1995, Nov. 28–29, pp. 537–544. [8] Y. Xiang, T.Y. Tsui, J.J. Vlassak, The mechanical properties of freestanding electroplated Cu thin films, J. Mater. Res. 21 (2006) 1607–1618. [9] J.J. Vlassak, W.D. Nix, A new bulge test technique for the determination of Young's modulus and Poisson's ratio of thin films, J. Mater. Res. 7 (1992) 3242–3249. [10] X.H. Ju, F.Z. Ren, G.S. Zhou, Inquire into the method of measuring elastic modulus of polymer and metal films in bulge test, Rare Metal Mater. Eng. 32 (2003) 313–316. [11] M.K. Small, B.J. Daniels, B.M. Clemens, W.D. Nix, The elastic biaxial modulus of Ag–Pd multilayered thin films measured using the bulge test, J. Mater. Res. 9 (1994) 25–30. [12] S. Wiese, E. Meusel, Characterization of lead-free solders in flip chip joints, J. Electron. Packag. 125 (2003) 531–538. [13] M. Roellig, R. Dudek, S. Wiese, B. Boehme, B. Wunderle, K.-J. Wolter, B. Michel, Fatigue analysis of miniaturized lead-free solder contacts based on a novel test concept, Microelectron. Reliab. 47 (2007) 187–195. [14] E. Mazza, J. Dual, G. Schiltges, Mechanical properties of microstructures – theory
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