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

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

Lambrinou Konstantina  imec restricted 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 [1] Brittle vs. Ductile Fracture Brittle fracture: occurs with little or no plastic deformation prior to failure, and at high speeds (e.g 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]

Lambrinou Konstantina  imec restricted 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

Lambrinou Konstantina  imec restricted 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 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]

Lambrinou Konstantina  imec restricted 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

Lambrinou Konstantina  imec restricted 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)

Lambrinou Konstantina  imec restricted 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: o C SAC  m Ag 3 Sn IMC

Lambrinou Konstantina  imec restricted Strain Rate, Temperature and Fracture Resistance Slow loading rate: max load in  10 s; d / dt  s -1 Intermediate loading rate: max load in 1 s; d / dt  s -1 Dynamic loading rate: max load in  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]

Lambrinou Konstantina  imec restricted 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]

Lambrinou Konstantina  imec restricted 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

Lambrinou Konstantina  imec restricted 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 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 [5]  ‘Mini-Charpy’ setup: real solder joint sizes! IMEC, Belgium [3]

Lambrinou Konstantina  imec restricted 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)

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

Lambrinou Konstantina  imec restricted 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

Lambrinou Konstantina  imec restricted 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

Lambrinou Konstantina  imec restricted 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 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!

Lambrinou Konstantina  imec restricted 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 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] [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

Lambrinou Konstantina  imec restricted 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

Lambrinou Konstantina  imec restricted 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 Thank you!

Lambrinou Konstantina  imec restricted 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]

Lambrinou Konstantina  imec restricted 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

Lambrinou Konstantina  imec restricted 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]

Lambrinou Konstantina  imec restricted 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  h at 150C-42  h at 150C-40  h at 175C-48  5  Size distribution, spacing, and sharpness of IMCs: affect solder embrittlement!

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

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

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

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

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