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Low-Cycle Fatigue Behavior of Lead-Free Solder

Low-Cycle Fatigue Behavior of Lead-Free Solder . EM388F Final Presentation Kuan (Gary) Lu Materials Science & Engineering April 30 2008. Outline. Introduction Solder Alloys Fatigue behavior of Solder Case study Frequency effect Temperature effect Solder composition Solder joint.

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Low-Cycle Fatigue Behavior of Lead-Free Solder

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  1. Low-Cycle Fatigue Behavior of Lead-Free Solder EM388F Final Presentation Kuan (Gary) Lu Materials Science & Engineering April 30 2008

  2. Outline • Introduction • Solder Alloys • Fatigue behavior of Solder • Case study • Frequency effect • Temperature effect • Solder composition • Solder joint

  3. Underfill Solder bump Silicon Chip Plastic substrate Introduction: Solder in Microprocessor • Solder provides electrical, mechanical and thermal interconnects. • Thermal-mechanical fatigue of solder joints during normal usage. (Strain controlled fatigue) CTE: 2.6ppm/oC for Si; ~15ppm/oC for PCB

  4. Introduction: Solder Alloys • Eutectic solders: • 63Sn-37Pb • 96.5Sn-3.5Ag (Pb-free) • Small addictives: Cu, Bi, Sb, Zn,... • Some Thermal-mechanical properties: • High homologous temperature @ RT (~0.6Tm); • Time dependent creep deformation; • Low-Cycle Fatigue (Nf<104).

  5. Solder under Cyclic Load (strain control) • Strain softening in the beginning: • Stress amplitude approaches a steady state. • Stress drops dramatically at the end of fatigue life. • Coffin-Manson eqn.: J.A. Bannantine, Fundamentals of Metal Fatigue Analysis. 1990. C. Andersson, Mater. Sci. Eng.A 394 2005

  6. General Approaches in Solder Fatigue Study • Strain controlled (0.1~10% total strain); • Bulk solder (regulated by ASTM) or solder joints; • Isothermal fatigue test or TMF test; • Failure criteria: 25% or 50% of stress drop; • Variables to study: Solder composition, Temperature, frequency, solder joint geometry and metallization,…. • Strain-Nf log plot, derive Coffin-Manson coefficients; • Failure analysis. Dogbone bulk solder (unit: mm)

  7. Case Study: Frequency Effect • Bulk 96.5Sn-3.5Ag • Isothermal (20oC) C. Kanchanomai, Mater. Sci. Eng.A 345 2003

  8. Case Study: Frequency Effect • Coffin-Manson relationship: C. Kanchanomai, Mater. Sci. Eng.A 345 2003

  9. Case Study: Frequency Effect • Eckel relationship: • Frequency-modified Coffin-Manson relationship: C. Kanchanomai, Mater. Sci. Eng.A 345 2003

  10. Case Study: Temperature Effect • Bulk 96.5Sn-3.5Ag • Isothermal (20oC, 85oC, 120oC) • Frequency: 0.1Hz C. Kanchanomai, Mater. Sci. Eng.A 381 2004

  11. Case Study: Temperature Effect C. Kanchanomai, Mater. Sci. Eng.A 381 2004

  12. 3.5Ag Case Study: Solder composition • Bulk solder • Isothermal (RT) C. Andersson, Mater. Sci. Eng.A 394 2005

  13. Case Study: Solder Joint • Shear test @ RT; • Hourglass shape solder joint; • Cu/Ni/Au metallization; • Frequency: 0.2Hz. (mm) C. Andersson, Mater. Sci. Eng.A 394 2005

  14. Case Study: Solder Joint Low strain level High strain level C. Andersson, Mater. Sci. Eng.A 394 2005

  15. Complexity of Solder Joint Study • Sample geometry effect; • Intermetallic compound (IMC); • Crack path may depend on: • Solder composition; • Under bump metallization (UBM); • Strain range.

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