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“Lead-Free Solder Materials”
Lambrinou Konstantina imec restricted 2007
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Bulk Embrittlement of Sn-Based Pb-Free Solder Alloys
Konstantina Lambrinou IPSI/REMO Group IMEC, Leuven, Belgium
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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
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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 [1] High toughness: combination of strength and ductility [2] Lambrinou Konstantina imec restricted 2007
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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
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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
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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] [3] Lambrinou Konstantina imec restricted 2007
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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 bct (Sn) bcc Al alloy (fcc) [3] Stress Temperature u 0 Small flaw Limit for large flaws NDT With flaw Flaw free 5,000 psi A B C D E F L H J K Steel (bcc) [4] Lambrinou Konstantina imec restricted 2007
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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 2007
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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 50 m TC: oC SAC 405 10 m Ag3Sn IMC [3] IMCs: internal ‘notches’! Lambrinou Konstantina imec restricted 2007
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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 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 2007
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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-Stress Plane-Strain Min Constraint Max Constraint [2] [2] Lambrinou Konstantina imec restricted 2007
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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 2007
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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
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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) [3] ‘Mini-Charpy’ setup: real solder joint sizes! IMEC, Belgium [] ASTM E [5] Lambrinou Konstantina imec restricted 2007
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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: -195oC to +100oC 101055 mm3 [6, 7] 5555 mm3 Notch: 2.5 mm 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 2007
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Results from CVN Impact Tests on Bulk Samples: SAC 405 vs. 99.99%Sn
Test at 20C Test at -75C SAC 405 Test at 20C Test at -190C 99.99% Sn Intergranular fracture Lambrinou Konstantina imec restricted 2007
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Results from Mini-Charpy Impact Tests on SAC 405 Solder Joints
Test at 23C Test at 23C Test at -41C Test at -88C Test at -78C Test at -88C Lambrinou Konstantina imec restricted 2007
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Results from Mini-Charpy Impact Tests on SAC 305 Solder Joints
Test at 24C Test at -104C Test at 23C Test at -51C Test at -104C Test at -85C Lambrinou Konstantina imec restricted 2007
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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
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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 2007
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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
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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 2007
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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
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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
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Thank you! Lambrinou Konstantina imec restricted 2007
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Elements of Fracture Mechanics
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 Mode I Mode II Mode III [2] Edge crack Through-thickness crack [1] 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
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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% [5, 6] Sn-0.5%Cu; aged at -18ºC Suppression by adding retardants: Sb (0.5%), Bi (0.3%), (Pb 5%) Lambrinou Konstantina imec restricted 2007
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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 6061-T4 Al alloy [2] Size distribution and spacing of IMCs: influence the fracture behaviour of solders! Lambrinou Konstantina imec restricted 2007
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CVN Impact Tests of Bulk Sn-Based Solders
Tested solder alloy: as-cast and annealed SAC 405 Test temperature: -195oC to +100oC Sample size: 101055 mm3 5 m As-cast 150C, 100 h 150C, 1000 h 175C, 1000 h [8] Material Condition DBTT (C) As-cast -28 6 100 h at 150C -42 5 1000 h at 150C -40 5 1000 h at 175C -48 5 Size distribution, spacing, and sharpness of IMCs: affect solder embrittlement! Lambrinou Konstantina imec restricted 2007
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Mini-Charpy Results from SAC 305 (Test at Room Temperature)
Lambrinou Konstantina imec restricted 2007
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Mini-Charpy Results from SAC 305 (Test close to -100C)
Sn Lambrinou Konstantina imec restricted 2007
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Mini-Charpy Results from SAC 405 (Test at Room Temperature)
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Mini-Charpy Results from SAC 405 (Test close to -100C)
Cu6Sn5 IMC Sn Bond Pad Lambrinou Konstantina imec restricted 2007
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Mini-Charpy Results from Sn-37%Pb (Test at Room Temperature)
Lambrinou Konstantina imec restricted 2007
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