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COST Action 531: “Lead-Free Solder Materials”

COST Action 531: “Lead-Free Solder Materials”. Final Meeting 17-18 May 2007 Vienna, Austria. Bulk Embrittlement of Sn-Based Pb-Free Solder Alloys. Konstantina Lambrinou IPSI/REMO Group IMEC, Leuven, Belgium. Outline. General Introduction Brittle vs. Ductile Failure

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COST Action 531: “Lead-Free Solder Materials”

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

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

  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 Lambrinou Konstantina  imec restricted 2007

  4. Brittle vs. Ductile Fracture [1] [2] High toughness: combination of strength and ductility • 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 Lambrinou Konstantina  imec restricted 2007

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

  7. Dislocations and Plastic Deformation [3] [3] • 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 Lambrinou Konstantina  imec restricted 2007

  8. Crystal Structure and Fracture Resistance u A Al alloy (fcc) Steel (bcc) D F L 0 Small flaw Limit for large flaws Stress  C K bcc B E J H 5,000 psi NDT Flaw free NDT With flaw Temperature  bct (Sn) [3] [4] • 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 istemperature-dependent: it increases as the temperature decreases! • Yield strength and P-N stress are interrelated, and so are their temperature dependences Lambrinou Konstantina  imec restricted 2007

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

  10. Notches and Fracture Resistance SAC 405 TC: 0- 100oC [3] 50 m 10 m Ag3Sn IMC • Notchescreate 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’! Lambrinou Konstantina  imec restricted 2007

  11. Strain Rate, Temperature and Fracture Resistance [1] [1] Low yield strength steel • 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 Lambrinou Konstantina  imec restricted 2007

  12. Material Constraint and Fracture Resistance Plane-Stress Plane-Strain [2] [2] Min Constraint Max Constraint • Constraint:refers mainly to the transition fromplane-stresstoplane-straincondition 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 Lambrinou Konstantina  imec restricted 2007

  13. Material Constraint and Fracture Resistance [2] A283 steel • 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! 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 Lambrinou Konstantina  imec restricted 2007

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

  16. CVN Impact Tests of Bulk Sn-Based Solders 101055 mm3 5555 mm3 Notch: 2.5 mm [6, 7] [6, 7] 5 μm • 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:-195oC to +100oC Notch: 1.3 mm Notch: 2.5 mm • Behaviour of Sn37Pb: compromise between Pb-rich phase (fcc) and Sn-rich phase (bct) Lambrinou Konstantina  imec restricted 2007

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

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

  19. Results from Mini-Charpy Impact Tests on SAC 305 Solder Joints Test at 24C Test at 23C Test at -51C Test at -104C Test at -104C Test at -85C 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 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 andbct (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 sizeaffects the exact DBTT value! 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 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 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 10th International Conference on Lead-Free Electronic Components and Assemblies, October 17-19, 2005, Brussels, Belgium 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 Lambrinou Konstantina  imec restricted 2007

  26. Thank you! Lambrinou Konstantina  imec restricted 2007

  27. Elements of Fracture Mechanics Through-thickness crack Edge crack [2] [1] Mode I Mode II Mode III • Stress-intensity factor, KI:describes the stress field ahead of a sharp crack (in MPa·m1/2) KI is affected by the specimen geometry, the applied load, the shape and size of flaws in the material • Fracture toughness, Kc:critical KI value at failure; it • represents the material resistance to crack propagation • Kc is a material property; it is affected by temperature, • loading/strain rate, and material constraint Lambrinou Konstantina  imec restricted 2007

  28. Composition and Fracture Resistance [5, 6] Sn-0.5%Cu; aged at -18ºC • Phase transformation leading to embrittlement of Sn: -Sn (‘white’ Sn)-Sn (‘grey’ Snor‘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%) Lambrinou Konstantina  imec restricted 2007

  29. Second-Phase Particles and Fracture Resistance [2] 6061-T4 Al alloy • Brittle second-phase particles, likeIMCs, show a veryhigh probability of acting assites of crack nucleation • Crack nucleation occurs bydislocation 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 specificsize and spatial distributionof second-phase particles • Size distribution and spacing of IMCs: influence the fracture behaviour of solders! Lambrinou Konstantina  imec restricted 2007

  30. CVN Impact Tests of Bulk Sn-Based Solders [8] 5 m 5 m 5 m 5 m 150C, 100 h 150C, 1000 h 175C, 1000 h As-cast • Tested solder alloy: as-cast and annealed SAC 405 • Test temperature:-195oC to +100oC • Sample size: 101055 mm3 • Size distribution, spacing, and sharpness of IMCs: affect solder embrittlement! Lambrinou Konstantina  imec restricted 2007

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

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

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

  34. Mini-Charpy Results from SAC 405(Test close to -100C) Sn Sn Bond Pad Cu6Sn5 IMC Lambrinou Konstantina  imec restricted 2007

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

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