100 um Fine Pitch Flip Chip/ 10-20 um Microbumps Issues

100 um Flip Chip Geometry and Assembly

Methodology

IMC Growth with Thermal Aging: 00 um to 0.5 mm Pitch Components

Introduction

Intermetallic Compounds and Reliability

Microstructure Evolution in Lead-Free Solder: BGAs and Flip Chips

Justin Chow, Ye Tian, Suresh K. SitaramanSchool of Mechanical Engineering, Georgia Institute of Technology and other institutions.

Reliability of lead-based solders is well understood. In the past

decade however, lead-free solder alloys are being used due to

environmental and health considerations. National Electronics

Manufacturing Initiative (NEMI, 2007) has recommended

Sn4.0Ag0.5Cu (SAC405) alloy and Japan Electronic Industry

Development Association (JEIDA) has recommended Sn3.0Ag0.5Cu

(SAC305) alloy. With the change in solder material the reliability of

lead free solder alloys needs to be understood.

A layer of Inter-Metallic Compounds (IMC’s) form at the

copper/solder interface during reflow and subsequent aging.

• IMC’s are more brittle than bulk solder

• Joints tend to fail at IMC region

What variables are important for growth?

Labie, R., et al. Electronic System-Integration Technology Conference 2010: IEEE.

Copper Pad

Bulk Solder

Cu6Sn5

Cu3Sn

BGA SAC Alloy Characterization Experiments

Characterize IMC Growth as a function of:

• Aging Time (12 hr, 24 hr, 1 week, 4 weeks)

• Temperature (25 C, 70 C, 100 C, 150C)

• Solder Metallurgy (SAC 105, 305, SnAg, SnPb)

• Surface Finishes (Ni, OSP)

• Geometry (Pitch, Size)

Phase 1: Unassembled Packages

Phase 2: Assembled Packages

Phase 3: Ex-situ Aging

With technologies and form factors constantly getting smaller,

solder joints and pitches are concurrently shrinking. At these

smaller scales, the atomic diffusion paths are shortened,

increasing the interfacial chip/substrate interaction. Additionally,

the IMC volume proportion increases dramatically. These

changes in microstructure have an important effect on the

reliability.

With small stand off heights, the

entire solder joint can become an

intermetallic.

Ex-Situ

1 Day: 3.94 um

1 Week: 4.77 um

4 Weeks: 5.87 um

y = 0.0912x + 3.5296

R² = 0.997

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30

Th

ick

ne

ss M

icro

ns

Time(hours)1/2

0.5 mm Pitch BGA IMC Growth

100 um Pitch Flip Chip Cross-Sections After Reflow 100 Hours at 150 C