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

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

3. Introduction: Solder in Microprocessor
Solder provides electrical, mechanical and thermal interconnects.
Thermal-mechanical fatigue of solder joints during normal usage. (Strain controlled fatigue)
Silicon Chip
Plastic substrate
Solder bump
Underfill
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 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
C. Andersson, Mater. Sci. Eng.A

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

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

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

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

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

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

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

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

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.