1. Lambrinou Konstantina  imec restricted 2007 1 COST Action 531: “Lead-Free Solder Materials” Final Meeting 17-18 May 2007 Vienna, Austria

2. Bulk Embrittlement of Sn-Based Pb-Free Solder Alloys Konstantina Lambrinou IPSI/REMO Group IMEC, Leuven, Belgium

3. Lambrinou Konstantina  imec restricted 2007 3 Outline  General Introduction Brittle vs. Ductile Failure Factors Affecting the Fracture Resistance  Impact Testing of Sn-Based Pb-Free Solders  Impact Testing of Bulk Solder Specimens  Impact Testing of Solder Joints  Conclusions  References & Acknowledgments

4. Lambrinou Konstantina  imec restricted 2007 4 [1] Brittle vs. Ductile Fracture Brittle fracture: occurs with little or no plastic deformation prior to failure, and at high speeds (e.g. 2000 m/s in steels) Ductile fracture: characterised by appreciable plastic deformation prior to failure, and high energy consumption Toughness: ability of a material to resist fracture Ductility: ability of a material to deform plastically without fracturing High toughness: combination of strength and ductility [2]

5. Lambrinou Konstantina  imec restricted 2007 5 Factors Affecting the Fracture Resistance Material Composition Crystal structure Microstructure Conditions Processing Conditions Service Conditions Properties Strength, E, Fracture Behaviour/Resistance, Ductile-to-Brittle Transition, etc. ‘Intrinsic’ Factors ‘Extrinsic’ Factors

6. Lambrinou Konstantina  imec restricted 2007 6 Factors Affecting the Fracture Resistance Composition (alloy selection, addition of elements that increase toughness or removal of those that degrade it) Microstructure (grain size, size and spatial distribution of second-phase particles, orientation of flaws) Crystal structure, nature of electron bond, atomic order Presence of notches (internal, external) Service conditions (temperature, strain rate, constraint)

7. Lambrinou Konstantina  imec restricted 2007 7 Dislocations and Plastic Deformation Dislocations enable the plastic deformation of metallic materials by means of a process known as slip Slip is the process of dislocation motion that results in the plastic deformation of crystalline materials [3]

8. Lambrinou Konstantina  imec restricted 2007 8 Crystal Structure and Fracture Resistance Peierls-Nabarro stress (P-N stress): stress required to move dislocations through a crystal lattice Close-packed materials, like fcc and hcp metals, exhibit lower P-N stress than bcc and bct metals P-N stress is temperature-dependent: it increases as the temperature decreases! Yield strength and P-N stress are interrelated, and so are their temperature dependences Al alloy (fcc) [3] Stress  Temperature  uu 00 Small flaw Limit for large flaws NDT With flaw NDT Flaw free 5,000 psi A B C D E F L H J K Steel (bcc) [4] bct (Sn) bcc

9. Lambrinou Konstantina  imec restricted 2007 9 Crystal Structure and Fracture Resistance Relative change of yield to tensile strength in bcc (and bct) metals leads to low-temperature embrittlement! [2] Ductile-to-Brittle Transition Temperature (DBTT)

10. Lambrinou Konstantina  imec restricted 2007 10 Notches and Fracture Resistance Notches create a triaxial stress state in the material Notch toughness: the ability of a material to absorb energy in the presence of a sharp notch  IMCs: internal ‘notches’! [3] 50  m TC: 0 - 100 o C SAC 405 10  m Ag 3 Sn IMC

11. Lambrinou Konstantina  imec restricted 2007 11 Strain Rate, Temperature and Fracture Resistance Slow loading rate: max load in  10 s; d / dt  10 -5 s -1 Intermediate loading rate: max load in 1 s; d / dt  10 -3 s -1 Dynamic loading rate: max load in  0.001 s; d / dt  10 s -1  Fracture toughness of bcc/bct metals: increases with increasing temperature and decreasing loading rate Low yield strength steel [1]

12. Lambrinou Konstantina  imec restricted 2007 12 Material Constraint and Fracture Resistance Constraint: refers mainly to the transition from plane- stress to plane-strain condition in the material Plane-stress: stress is zero in the thickness direction Plane-strain: strain is zero in direction normal to both axis of applied stress and direction of crack growth Plane-StressPlane-Strain Min ConstraintMax Constraint [2]

13. Lambrinou Konstantina  imec restricted 2007 13 Material Constraint and Fracture Resistance Change in the sample thickness changes the degree of constraint: plane-stress to plane-strain condition  Change in the sample thickness may shift the DBTT! [2] A283 steel

14. Lambrinou Konstantina  imec restricted 2007 14 Outline  General Introduction Brittle vs. Ductile Failure Factors Affecting the Fracture Resistance  Impact Testing of Sn-Based Pb-Free Solders  Impact Testing of Bulk Solder Specimens  Impact Testing of Solder Joints  Conclusions  References & Acknowledgments

15. Lambrinou Konstantina  imec restricted 2007 15 Charpy V-Notch (CVN) Impact Testing  CVN Impact Testing: reproduces very strenuous service conditions (high strain rates, triaxial stress state due to the presence of sharp notches, and low temperatures) [] ASTM E 23-06 [5]  ‘Mini-Charpy’ setup: real solder joint sizes! IMEC, Belgium [3]

16. Lambrinou Konstantina  imec restricted 2007 16 CVN Impact Tests of Bulk Sn-Based Solders  Tested solder alloys: SAC 305, SAC 405, 99.99%Sn, Sn-5%Ag, Sn-0.7%Cu, Sn-0.7%Cu-0.1%Ni, Sn-37%Pb Test temperature: -195 o C to +100 o C Notch: 2.5 mm 101055 mm 3 [6, 7] 5555 mm 3 [6, 7] Notch: 1.3 mm Notch: 2.5 mm 5 μm  Behaviour of Sn37Pb: compromise between Pb-rich phase (fcc) and Sn-rich phase (bct)

17. Lambrinou Konstantina  imec restricted 2007 17 Results from CVN Impact Tests on Bulk Samples: SAC 405 vs. 99.99%Sn Intergranular fracture Test at 20C Test at -75C SAC 405 Test at 20C Test at -190C 99.99% Sn

18. Lambrinou Konstantina  imec restricted 2007 18 Results from Mini-Charpy Impact Tests on SAC 405 Solder Joints Test at 23C Test at -88C Test at 23C Test at -41C Test at -88C Test at -78C

19. Lambrinou Konstantina  imec restricted 2007 19 Results from Mini-Charpy Impact Tests on SAC 305 Solder Joints Test at 24C Test at -104C Test at 23CTest at -51C Test at -104C Test at -85C

20. Lambrinou Konstantina  imec restricted 2007 20 Outline  General Introduction Brittle vs. Ductile Failure Factors Affecting the Fracture Resistance  Impact Testing of Sn-Based Pb-Free Solders  Impact Testing of Bulk Solder Specimens  Impact Testing of Solder Joints  Conclusions  References & Acknowledgments

21. Lambrinou Konstantina  imec restricted 2007 21 Conclusions  The fracture behaviour of Sn-based Pb-free solders is very similar to that of bcc metals, due to the similarity of the bcc and bct (Sn) crystal structures  The fracture behaviour of Sn-based solder alloys is affected by: the service conditions (temperature, strain rate, and degree of material constraint) the size distribution, spacing, and acuity of IMCs  At low temperatures, embrittlement of Sn is a fact!  When testing a Sn-based solder alloy with a certain composition in impact, it is important to realise that the sample size affects the exact DBTT value!

22. Lambrinou Konstantina  imec restricted 2007 22 Outline  General Introduction Brittle vs. Ductile Failure Factors Affecting the Fracture Resistance  Impact Testing of Sn-Based Pb-Free Solders  Impact Testing of Bulk Solder Specimens  Impact Testing of Solder Joints  Conclusions  References & Acknowledgments

23. Lambrinou Konstantina  imec restricted 2007 23 References (1) [1] J.M. Barsom, S.T. Rolfe, “Fracture and Fatigue Control in Structures: Applications of Fracture Mechanics”, ASTM Manual Series: MNL41, West Conshohocken, PA, USA, 1999 [2] R.W. Hertzberg, “Deformation and Fracture Mechanics of Engineering Materials”, John Wiley & Sons, Inc., New York, USA, 1996 [3] D.R. Askeland, P.P. Phulé, “The Science and Engineering of Materials”, Thomson, Toronto, Canada, 2006 [4] http://www.key-to-steel.com [5] ASTM E 23-06: “Standard Test Methods for Notched Bar Impact Testing of Metallic Materials”, ASTM International, 2006 [6] P. Ratchev, T. Loccufier, B. Vandevelde, B. Verlinden, S. Teliszewski, D. Werkhoven, B. Allaert, “A Study of Brittle to Ductile Fracture Transition Temperatures in Bulk Pb-Free Solders”, Proceedings of EMPC 2005 (IMAPS-Europe), June 12-15, 2005, Brugge, Belgium, pp. 248-252

24. Lambrinou Konstantina  imec restricted 2007 24 References (2) [7] P. Ratchev, B. Vandevelde, B. Verlinden, “Brittle to Ductile Fracture Transition in Bulk Pb-Free Solders”, in press for IEEE- Transactions on Components and Packaging Technologies [8] P. Ratchev, B. Vandevelde, B. Verlinden, “Effect of the Intermetallics Particle Size on the Brittle to Ductile Fracture Transition in a Bulk Sn-4wt%Ag-0.5wt%Cu Solder”, CD-ROM Proceedings of IPC/JEDEC 10 th International Conference on Lead- Free Electronic Components and Assemblies, October 17-19, 2005, Brussels, Belgium

25. Lambrinou Konstantina  imec restricted 2007 25 Acknowledgments  IMEC:Dr. Bart Vandevelde Paresh Limaye Frederic Duflos  K. U. Leuven:Prof. Bert Verlinden Wout Maurissen  Financial support by IWT (Flemish Government) in the framework of the ALSHIRA (Aspects of Lead-Free Soldering for High-Reliability Applications) Project

26. Lambrinou Konstantina  imec restricted 2007 26 Thank you!

27. Lambrinou Konstantina  imec restricted 2007 27 Stress-intensity factor, K I : describes the stress field ahead of a sharp crack (in MPa·m 1/2 ) K I is affected by the specimen geometry, the applied load, the shape and size of flaws in the material Elements of Fracture Mechanics Fracture toughness, K c : critical K I value at failure; it represents the material resistance to crack propagation K c is a material property; it is affected by temperature, loading/strain rate, and material constraint Mode IMode IIMode III [2] Edge crackThrough-thickness crack [1]

28. Lambrinou Konstantina  imec restricted 2007 28 Composition and Fracture Resistance  Phase transformation leading to embrittlement of Sn:  -Sn (‘white’ Sn)   -Sn (‘grey’ Sn or ‘tin pest’)  -Sn (bct structure)   -Sn (diamond cubic structure) Sluggish: 18 months incubation period 13.2ºC  V  + 26%  Suppression by adding retardants: Sb (0.5%), Bi (0.3%), (Pb  5%) [5, 6] Sn-0.5%Cu; aged at -18ºC

29. Lambrinou Konstantina  imec restricted 2007 29 Second-Phase Particles and Fracture Resistance Brittle second-phase particles, like IMCs, show a very high probability of acting as sites of crack nucleation Crack nucleation occurs by dislocation coalescence, since second-phase particles tend to ‘pin’ dislocations Dislocation ‘pinning’ limits the material’s ability for plastic deformation, and is often accompanied by strengthening (known as ‘precipitation hardening’) Dislocation ‘pinning’: for specific size and spatial distribution of second-phase particles  Size distribution and spacing of IMCs: influence the fracture behaviour of solders! 6061-T4 Al alloy [2]

30. Lambrinou Konstantina  imec restricted 2007 30 CVN Impact Tests of Bulk Sn-Based Solders  Tested solder alloy: as-cast and annealed SAC 405 Test temperature: -195 o C to +100 o C Sample size: 101055 mm 3 5  m As-cast 150  C, 100 h150  C, 1000 h175  C, 1000 h [8] Material Condition DBTT (C) As-cast -28  6 100 h at 150C-42  5 1000 h at 150C-40  5 1000 h at 175C-48  5  Size distribution, spacing, and sharpness of IMCs: affect solder embrittlement!

31. Lambrinou Konstantina  imec restricted 2007 31 Mini-Charpy Results from SAC 305 (Test at Room Temperature)

32. Lambrinou Konstantina  imec restricted 2007 32 Mini-Charpy Results from SAC 305 (Test close to -100C) Sn

33. Lambrinou Konstantina  imec restricted 2007 33 Mini-Charpy Results from SAC 405 (Test at Room Temperature)

34. Lambrinou Konstantina  imec restricted 2007 34 Mini-Charpy Results from SAC 405 (Test close to -100C) Cu 6 Sn 5 IMC Sn Bond Pad

35. Lambrinou Konstantina  imec restricted 2007 35 Mini-Charpy Results from Sn-37%Pb (Test at Room Temperature)