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Lead-free soldering alloys: microstructure optimization for electronic applications

By Sergey Belyakov

Abstract

As a result of environmental issues, manufacturing of lead-free electronics has become a global trend since July 1, 2006. Due to the considerable toxicity of lead leading to health and environmental concerns, new legislations have been imposed in its use. Efforts have been made to replace the traditional soldering alloys with new compositions, but the reliability of the new materials requires further investigation. \ud The purpose of this work is investigations of: \ud 1. Growth of intermetallic phases (IMCs) during soldering and service life. \ud 2. Investigation of effects of solder compositions, cooling kinetics and ultrasound vibration on the microstructure of crystallizing solders. \ud 3. Nucleation and growth of bulk intermetallic precipitates and interfacial intermetallic layer in the "solder-joint" system. \ud 4. Mechanisms of solder-joint failure. \ud 5. Stability of microstructures over time. \ud 6. Crystallographically-faceted void formation phenomenon. \ud \ud The solder alloy compositions used: SAC405 (Sn95.5–Ag4.0–Cu0.5), SAC305 (Sn96.5–Ag3.0–Cu0.5), CASTIN (Sn96.2–Ag2.5–Cu0.8–Sb0.5) and SN100C (Sn–Cu0.7–Ni0.05+Ge) were chosen due to their preferability for lead-free electronics applications. \ud Investigations carried out revealed a strong dependence of the lead-free alloy microstructure on composition and solidification conditions (cooling rates, ultrasound). Lower amounts of alloying elements (< 2.5% of Ag) result in an increase in solder microstructural integrity. Cooling rates and ultrasound modify phase size and distribution. For instance, a cooling rate of 24 °C/sec (SAC405 alloy) lowered the maximum IMC needles length to 2μm compared to 44 μm at 1 °C/sec. Ultrasound of 30 kHz increases the microstructural homogeneity of bulk solders and lowers the amount of undercooling (from 24 °C to 7 °C for SAC 405 alloy). However, it deteriorates the strength characteristics of a solder-joint due to intensification of interfacial diffusional process, causing more intensive growth of the interfacial IMC layer. It was shown that solder-joint tensile strength is highly dependent on the IMC layer which has an optimal width of about 2μm. Thermal-cycling revealed differences in evolution of solder-joints and an average 7% strength reduction was obtained for samples subjected to 100 thermal cycles. Ageing resulted in rapid coarsening of a solder-joint microstructure and preferable IMC growth along the β-Sn grain boundaries. The interfacial IMC layer separates into two strata: Cu3Sn and Cu6Sn5 as it develops into the solder. Crystallographically-facetted voiding was observed in solder-joints in the as 'soldered' state and during subsequent ageing. The voids were found to be strongly dependent on the width of Cu substrate. \ud A fundamental goal of this research is to help to meet the future requirements of reliable lead-free electronic products

Publisher: University of Leicester
Year: 2009
OAI identifier: oai:lra.le.ac.uk:2381/7880

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Citations

  1. (2003). (Unitive, Inc.), presented at the Peaks
  2. (1988). A microstructural study of the thermal fatigue failures of 60Sn-40Pb solder joints, doi
  3. (2002). A study of the effects of solder volume on the interfacial reactions in solder joints using the differential scanning calorimetry technique, doi
  4. (1996). A thermodynamic study on the Ag-Sb-Sn system, doi
  5. (1994). A Viable. Tin-Lead Solder Substitute: SnAg-Cu, doi
  6. (2002). Ag3Sn plate formation in the solidification of near ternary eutectic Sn-Ag-Cu alloys, doi
  7. (2000). Analysis of ring and plug shear strengths for comparison of lead-free solders, doi
  8. (2005). Avoiding Microvoids,‖ Assembly (on-line magazine),
  9. (1996). B.J.Lee, D.N.Lee, Thermodynamic database for phase diagrams in micro-soldering alloys,
  10. Ceramic Ball Grid Array Solder Joint Thermal Fatigue Life Enhancement,
  11. (2002). Characterization of lead-free solders and under bump metallurgies for flip-chip package, doi
  12. (1966). Clinical Pharmacology, 3rd edn.,
  13. (2004). Controlling Ag3Sn Plate Formation in Near-Ternary-Eutectic Eutectic SnAg-Cu Solder by Minor Zn Alloying, doi
  14. Cooling rate influence on CF voiding. It was shown that quenching suppresses the CFVs formation, although they develop during subsequent ageing. This experiment needs to be carried out more precisely widening the range of cooling rates.
  15. (2009). Cracking and Phase Stability in Reaction Layers between Sn-Cu-Ni Solders and Cu Substrates, doi
  16. (1940). Crystallization of organic substances in ultrasonic field,
  17. (1939). Crystallization of piperylpiperidine in ultrasonic field,
  18. (1937). Die Wirkung intensiven Schalls auf
  19. (2009). Effect of Ag, Fe, Au and Ni on the growth kinetics of Sn–Cu intermetallic compound layers, doi
  20. (2004). Effect of thermal aging on board level drop reliability for Pb-free BGA packages, doi
  21. (2005). Effect of Voiding on Lead-Free Reliability,
  22. (2004). Effect of volume in interfacial reaction between eutectic Sn–Pb solder and Cu metallization in microelectronic packaging, doi
  23. (2002). Effects of cooling speed on microstructure and tensile properties of Sn-Ag-Cu alloys, doi
  24. (2003). Effects of intermetallic compounds on properties of Sn–Ag–Cu lead-free soldered joints, doi
  25. (2003). Effects of mechanical deformation and annealing on the microstructure and hardness of Pb-free solders, doi
  26. (2001). Electron Microscopy and analysis,
  27. (2004). Elevated temperature aging of solder joints based on Sn-Ag-Cu: Effects on joint microstructure and shear strength, doi
  28. (2004). Evaluation of thermal fatigue life and failure mechanisms of Sn-Ag-Cu solder joints with reduced Ag contents, doi
  29. (2008). Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate, doi
  30. (2000). Experimental and thermodynamic assessment of Sn-Ag-Cu solder alloys, doi
  31. (1959). Experimental and Thermodynamic Assessment of Sn-AgCu Solder Alloys,
  32. (2004). Impact reliability of solder joints, doi
  33. (2004). Interfacial reactions and bump reliability of various Pb-free solder bumps on electroless Ni-P UBMs, doi
  34. (1996). Interfacial reactions during soldering with lead-tin eutectic and lead (Pb)-free, tin-rich solders, doi
  35. (2002). Interfacial reactions, microstructure and mechanical properties of Pb-free solder joints in PBGA laminates, doi
  36. (2004). J-STD-020C, Moisture/Reflow Sensitivity Classification for Non-hermetic Solid State Surface Mount Devices, Revision C,
  37. (2004). Lead-free flip chip interconnect reliability for DCA and FC-PBGA packages, doi
  38. (2001). Lead-Free Technology,
  39. (1998). Materials interfaces in flip chip interconnects for optical components; performance and degradation mechanisms, doi
  40. (1936). Metallforschung mit Ultraschall,
  41. (1989). Metastable phase equilibria in the lead-tin alloy system, doi
  42. (2002). Microstructural modifications and properties of Sn-Ag-Cu solder joints induced by alloying, doi
  43. (2001). Microstructure and intermetallic growth effects on shear and fatigue strength of solder joints subjected to thermal cycling aging, doi
  44. (2005). Microstructure and mechanical properties of lead-free solders and solder joints used in microelectronic applications, doi
  45. NEMI's Lead-Free Alloy,
  46. (2001). Pb-Free Solders for Flip-Chip Interconnects, doi
  47. (2002). Physical and mechanical properties of intermetallic compounds commonly found in solder, Metallurgy Division NIST,
  48. (1970). Physical principles of the ultrasound technology, doi
  49. (2005). presented at the 2005 TMS Annual Meeting,
  50. (2003). presented at the TMS
  51. (1962). Pressure impact on new phase nuclei sites formation,
  52. (1939). Rates of Diffusion of Copper and Zinc in Alpha Brass,
  53. (2005). Report on Effect of Voiding on Lead-Free Reliability,
  54. (2007). Ripening-assisted void formation in the matrix of Pb-free solder joints during solid-state aging, doi
  55. (2001). Sandia National Laboratories, doi
  56. (2003). Scanning Electron Microscopy and X-Ray microanalysis. 3 ed., doi
  57. (1963). Science 141, X-Ray diffraction studies on tin at high pressure and high temperature, doi
  58. (2001). Shear deformation in Sn-3.5Ag and Sn-3.6Ag-1.0Cu solder joints subjected to asymmetric four-point bend tests, doi
  59. Solder volume/ distance between Cu substrates influence on CF voiding during ageing.
  60. (2007). Solidification characteristics of Pb-Sb hypereutectic alloy within ultrasonic field, doi
  61. (2002). Studies of the mechanical and electrical properties of lead-free solder joints, doi
  62. (2003). Study of IMC morphologies and phase characteristics affected by the reactions of Ni and Cu metallurgies with Pb-free solder joints, doi
  63. (2004). Study of spalling behavior of intermetallic compounds during the reaction between electroless Ni-P metallization and lead-free solders, doi
  64. Substrate material/IMC layer composition influence on CF voiding. Experiments with Ni substrates and Au-coated Cu substrates are in initial stages and will be continued.
  65. Substrate thickness influence on CF voiding. In the present study only two available substrate widths were used. Dependence of the CF voids development on the substrate geometrical parameters need to be investigated more precisely.
  66. (1968). Tables of Physical and Chemical Constants, Longmans, doi
  67. (2003). The compression stress-strain behavior of Sn-Ag-Cu solder, doi
  68. (1987). The effect of Cu6Sn5 whisker precipitates in bulk 60Sn-40Pb solder, doi
  69. (1954). The growth rate of dendrites in undercooled tin, doi
  70. (2004). The Microstructure of Sn in near eutectic Sn-Ag-Cu alloy solder joints and its role in thermomechanical fatigue, doi
  71. (2004). The microstructure of Sn in near-eutectic Sn–Ag–Cu alloy solder joints and its role in thermomechanical fatigue, doi
  72. The ratio solder/IMCs volume fraction influence on CF voiding. As it was shown, the CFVs are correlated with volume fraction of Su6Sn5 IMCs. The same correlation needs to be determined for Ag3Sn and Cu3Sn IMCs.
  73. (1936). The ultrasonic influence on chemical reactions,
  74. (2001). Tin pest in doi
  75. (2000). Tin-silver-copper eutectic temperature and composition, doi
  76. (1962). Ultrasound influence on crystallization kinetics, doi
  77. (2005). Voids in solder joints,

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