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.