Low-Cycle Fatigue Behavior of Lead-Free Solder
EM388F Final PresentationKuan (Gary) Lu
Materials Science & EngineeringApril 30 2008
Outline• Introduction
– Solder Alloys– Fatigue behavior of Solder
• Case study– Frequency effect– Temperature effect– Solder composition– Solder joint
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
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).
Solder under Cyclic Load (strain control)
• Strain softening in the beginning:– Stress amplitude approach
es 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
CN fp
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, fre
quency, solder joint geometry and metallization,….• Strain-Nf log plot, derive Coffin-Manson coefficients;• Failure analysis.
Dogbone bulk solder
(unit: mm)
Case Study: Frequency Effect
• Bulk 96.5Sn-3.5Ag• Isothermal (20oC)
C. Kanchanomai, Mater. Sci. Eng.A 345 2003
Case Study: Frequency Effect
CN fp • Coffin-Manson relationship:C. Kanchanomai, Mater. Sci. Eng.A 345 2003
Case Study: Frequency Effect
• Eckel relationship:• Frequency-modified Coffin-Manson relationship:
C. Kanchanomai, Mater. Sci. Eng.A 345 2003
Case Study: Temperature Effect
• Bulk 96.5Sn-3.5Ag• Isothermal (20oC, 85oC, 120oC)• Frequency: 0.1Hz
C. Kanchanomai, Mater. Sci. Eng.A 381 2004
Case Study: Temperature Effect
C. Kanchanomai, Mater. Sci. Eng.A 381 2004
Case Study: Solder composition
• Bulk solder• Isothermal (RT)
3.5Ag
C. Andersson, Mater. Sci. Eng.A 394 2005
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
Case Study: Solder Joint
Low strain level High strain levelC. Andersson, Mater. Sci. Eng.A 394 2005
Complexity of Solder Joint Study• Sample geometry effect;• Intermetallic compound (IMC);• Crack path may depend on:
– Solder composition;– Under bump metallization (UBM);– Strain range.