1. Dr. Jin Liang, Dr. Nader Dariavach
Microstructure & Creep Resistance of Lead-Free Solder under Different Solidification Conditions
Dr. Jin Liang, Dr. Nader Dariavach
EMC Corp.
Dongkai Shangguan
Flextronics Corp.
Enter speaker notes here.
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

2. 2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA
Problem Statement
Long term solder joint reliability is one of outmost important concerns for transition to the Pb-Free soldering process.
Pb-Free solder metallurgical and processing research:
Wettability
Creep at various loading and temperature conditions;
Low cycle fatigue under various strain rates and temperatures;
Deformation and fracture at variable strain rate loading conditions
Solidification rate (cooling rate) effects on microstructure and mechanical behavior.
The current viable leadfree solders are based on the near-eutectic composition of the Sn-Ag-Cu ternary system with or without additions of other alloying elements.
Among the potential candidates to replace Pb-containing solders, ternary Sn-Ag-Cu (SAC) alloys have been considered the most promising substitutes because of their relatively low melting temperature, reasonable wettability, and superior mechanical properties.1-3 However, the solidification characteristics and microstructure and mechanical properties of these alloys are quite different from traditional SnPb solder alloys.4-9 The cooling rate in the reflow process has been found to have a significant effect on the microstructure of SAC alloys during solidification,9,10 although creep behavior was not strongly dependent on the microstructure formation.10
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

3. Ternary Diagram for SAC alloys
A projection of the liquidus surfaces for the Sn-Ag-Cu phase diagram showing the ternary eutectic reaction
Liquidus surfaces of the Sn-Ag-Cu phase diagram at the Sn-rich corner
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

4. Macrostructure of Pb-free Solder Joints
Microstructure of Sn-Ag-Cu solder joint for a chip capacitor. The light etching regions are primary β dendrites, while the grey regions are eutectic
Macrostructure of Sn-Ag-Cu solder for a BGA solder ball. The light etching regions are primary β Sn dendrites, while the grey regions are eutectic.
Microstructure of Sn-Ag-Cu solder joint for a chip capacitor.
Macrostructure of Sn-Ag-Cu solder for a BGA solder ball.
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

5. 2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA
Pb-free Creep Samples
Alloy: SAC 387
Cast at 300oC
Mold size: 4” x 3/8” x 3/8”
Sac385 alloy
Mold
Aluminum Block
Water bath
Enter speaker notes here.
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

6. Creep samples Casting at Variable Cooling Rates
Sample ID
Cooling rate, deg./sec.
at oC 5
0.29 1
2.86 2
8.00 3
12.50 4
16.67
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

7. Creep Curves for SAC387 Solder Alloy
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

8. Strain Rate as Function of Stress for the SAC387 Alloy
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

9. Strain Rate as function of Cooling Rate for the SAC387 Alloy
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

10. Microstructure Analysis
0.3 deg/sec
2.9 deg/sec
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

11. Microstructure Analysis (cont.)
16.7 deg/sec
12.5 deg/sec
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

12. EDAX Analysis (cooling rate 0.3 deg/sec)
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

13. EDAX Analysis (cooling rate 8 deg/sec)
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

14. EDAX Analysis (cooling rate 8 deg/sec)
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

15. EDAX Analysis (cooling rate 16.5 deg/sec)
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA

16. 2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA
Conclusion
2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA