Comprehensive study of intermetallic compounds in solar cell interconnections and their growth kinetics

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1 Comprehensive study of intermetallic compounds in solar cell interconnections and their growth kinetics T. Geipel, M. Moeller, A. Kraft, U. Eitner Fraunhofer Institute for Solar Energy Systems ISE 6 th Metallization Workshop Constance,

2 Introduction Drawing of PV module Motivation Improve the reliability of PV modules Intermetallic compounds (IMC) negatively impact the reliability of solder joints [1] Increase the understanding of microstructure and growth kinetics Cross section through interconnection fracture Aims Characterization of solder bonds initially and thermal aging Modeling of IMC growth 10 µm Cross section of a fractured solder bond 2 [1] P. L. Tu et al, IEEE Trans. Comp., Packag., Manufact. Technol. B, vol. 20, no. 1, pp , 1997

3 Solder bonds in PV modules Sn60Pb40 bond in the initial state After thermal aging for 500 h at 130 C Thin and uniform IM Cs are necessary for a good metallurgical bond [2] Extended IM Cs may lead to fracture due to their brittle nature [3] [2] T. Laurila et al, Mater. Sci. Eng. R-Rep, vol. 49, no. 1-2, pp. 1 60, 2005 [3] D. R. Frear and P. T. Vianco, Metall. Mater. Trans. A, vol. 25, no. 7, pp ,

4 Experimental approach Semi-automatic soldering of front busbars of industrial Al-BSF cells Solders: Sn60-Pb40, Sn41-Bi57-Ag2 [4], Sn43-Bi57, Sn62-Pb36-Ag2, Sn91-Zn9 Metallographic cross sections Isothermal aging between 85 C to 150 C for 15 h to 155 h Measurement of layer growth with confocal laser microscopy Semi-automatic soldering 10 µm [4] M. M ccormack et al, J. Electron. Mater., vol. 26, no. 8, pp , 1997 Laser confocal microscope image with highlighted measurement lines 4

5 M icrost ruct ural changes and IMC growth Sn60Pb40 initial Sn41Bi57Ag2 5

6 M icrost ruct ural changes and IMC growth Sn60Pb40 initial 155 h at 130 C Sn41Bi57Ag2 6

7 M icrost ruct ural changes and IMC growth Sn60Pb40 initial 155 h at 130 C 750 h at 130 C Sn41Bi57Ag2 7

8 Crain coarsening Volumetric changes and generation of extended grain boundaries within the solder matrix (~ 10 nm) Negative impact on solder joint reliability assumed 8 Sn41Bi57Ag2 solder matrix after 155 h at 100 C Detail of Sn62Pb36Ag2 after 155 h at 100 C

9 Penetration of Sn into the busbar Overview 9 [4] Schmitt, P. et al., Energy Procedia 27, ,2012

10 Penetration of Sn into the busbar Ag 3 Sn ζ-(agsn) Overview Detailed view on Ag 3 Sn phase with SEM Norm. Atomic Content Ag Sn Position [µm] EDX line scan ζ-(agsn) is located around cavities and lead-glass deep within the busbar Sn penetration may be associated with metallization ablation [5] 10 [5] Schmitt, P. et al., Energy Procedia 27, ,2012

11 Kinetics of intermetallic phase growth Kinetics of IM C growth based on root mean square diffusion distance and Arrhenius [6] x x 0 t T D 0 Q R Descript ion IMC thickness Initial thickness Time Absolute temperature Pre-exponential factor Activation energy Gas constant 12 [6] R. J. Borg and G. J. Dienes, An Introduction to Solid State Diffusion. San Diego: Academic Press, 1988

12 Kinetics of intermetallic phase growth Kinetics of IM C growth based on root mean square diffusion distance and Arrhenius [6] Arrhenius plots to extract kinetic parameters x x 0 t T D 0 Q R Descript ion IMC thickness Initial thickness Time Absolute temperature Pre-exponential factor Activation energy Gas constant Lef t : Experimentally determined Ag 3 Sn thickness within Sn41Bi57Ag2 joints at various aging temperatures 13 [6] R. J. Borg and G. J. Dienes, An Introduction to Solid State Diffusion. San Diego: Academic Press, 1988

13 Kinetics of intermetallic phase growth Kinetics of IM C growth based on root mean square diffusion distance and Arrhenius [6] Arrhenius plots to extract kinetic parameters -12 x x 0 t T D 0 Q R Descript ion IMC thickness Initial thickness Time Absolute temperature Pre-exponential factor Activation energy Gas constant ln[x x 0 ] h 85h 155h Lef t : Experimentally determined Ag 3 Sn thickness within Sn41Bi57Ag2 joints at various aging temperatures Right: Arrhenius plot of the same data Temperature [1E-3/K] 14 [6] R. J. Borg and G. J. Dienes, An Introduction to Solid State Diffusion. San Diego: Academic Press, 1988

14 Simulation of intermetallic layer growth Simulation of IM C growth with temperature profiles from environmental chamber tests Sn41Bi57Ag2 Ag 3 Sn Sn60Pb40 Ag 3 Sn Sn41Bi57Ag2 Cu 6 Sn 5 &Cu 3 Sn Sn60Pb40 Cu 6 Sn 5 15 Temp [ C] Simulation for thermal cycling -40 to 85 C Time [h] 1.5 IM C Thickness [µm] Number of cycles

15 Simulation of intermetallic layer growth Simulation of IM C growth with temperature profiles from environmental chamber tests Significant layer growth projected during 85 C aging Sn41Bi57Ag2 Ag 3 Sn Sn60Pb40 Ag 3 Sn Sn41Bi57Ag2 Cu 6 Sn 5 &Cu 3 Sn Sn60Pb40 Cu 6 Sn 5 Temp [ C] Simulation for thermal cycling -40 to 85 C Time [h] 1.5 IM C Thickness [µm] IM C Thickness [µm] Simulation for 85 C Number of cycles Aging Time [h] 16

16 Prognosis of IMC growth in an outdoor location One-year temperature curve of a module in Freiburg (Germany) is iterated 25 times (~ 25 years) 17

17 Prognosis of IMC growth in an outdoor location One-year temperature curve of a module in Freiburg (Germany) is iterated 25 times (~ 25 years) IM C Thickness [µm] Time [years] Prognosis: 1000 h at 85 C would result in thicker IM Cs than 25 years in Freiburg IM C Thickness [µm] Sn60Pb40 - Ag 3 Sn Sn41Bi57Ag2 - Ag 3 Sn Time [years] 18

18 Conclusions Thin and uniform IM Cs are necessary Rapid growth of IM Cs within Bi-based solders observed that may cause lower fatigue life Local Sn accumulation around cavities in the busbar Microstructural ripening may be related to reported solder bond failures Simulation enables the prediction of phase growth under non-isothermal outdoor conditions (thermal cycling, outdoor temperature curves) Tailored accelerated aging tests for phase growth possible 19

19 Thank you for your attention! Fraunhofer Institute for Solar Energy Systems ISE Torsten Geipel 20