Technical Brief

Structural Size Effect on Mechanical Behavior of Intermetallic Material in Solder Joints: Experimental Investigation

[+] Author and Article Information
Leila Ladani

Mechanical Engineering Department,
University of Connecticut,
Storrs, CT 06269-3139,
e-mail: lladani@engr.uconn.edu

Ousama Abdelhadi

Mechanical Engineering Department,
University of Connecticut,
Storrs, CT 06269-3139

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received October 7, 2013; final manuscript received June 29, 2014; published online October 6, 2014. Assoc. Editor: Felix Chen.

J. Electron. Packag 137(1), 014501 (Oct 06, 2014) (6 pages) Paper No: EP-13-1116; doi: 10.1115/1.4027992 History: Received October 07, 2013; Revised June 29, 2014

The elastic and plastic mechanical properties of intermetallic compound (IMC) phases of a lead-free Sn3.5Ag/Cu-substrate soldering system are investigated in different sized joints using nano-indentation. The specimens were prepared using solid–liquid interdiffusion soldering process with joint sizes ranging from 15 to 450 μm. Solder joints were subjected to 360 °C soldering temperature for 20 min and then air cooled to room temperature to create testable IMCs thicknesses. Nano-indentation was used to extract the elastic and plastic properties of Cu6Sn5, Cu3Sn, and Ag3Sn IMCs and β-tin and copper materials. Cu–Sn IMCs formed in specimens with smaller joint size show higher elastic modulus, hardness, and yield strength and lower work hardening exponent. This was attributed to the dimensional constraints associated with decreasing joint size and the local stresses developed during fabrication in joints with different sizes. Local elastic modulus and hardness of single grains of Cu6Sn5 were obtained using a combination of nano-indentation and electron backscatter diffraction (EBSD) techniques. Grains with c-axis at a 45 deg angle with respect to the nano-indentation loading direction show higher elastic modulus (∼8.70% higher) and hardness (8.85% higher) compared to the grains that have a c-axis that is almost perpendicular to the nano-indentation loading direction.

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Fig. 1

Nano-indentation load versus displacement curves of different materials. (a) Cu6Sn5 IMC, (b) Cu3Sn IMC, (c) eutectic solder, and (d) Ag3Sn IMC.

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Fig. 2

SEM micrographs show representative nano-indentation tests in (a) SS1 specimen, (b) SS2 specimen, (c) LS specimen, (d) Ag3Sn region, and (e) eutectic solder region

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Fig. 3

Elastic modulus and hardness versus joint thickness for (a) and (b) Cu6Sn5 IMC and (c) and (d) Cu3Sn IMC

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Fig. 4

Yield strength and work hardening exponent versus joint thickness for (a) and (b) Cu6Sn5 IMC, and (c) and (d) Cu3Sn IMC

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Fig. 5

(a) and (b) SEM images of two different areas of solder joints with two Cu6Sn5 grains, (c) and (d) band contrast (top) and orientation maps for Cu6Sn5 grains in (a) and (b), (e) and (f) phase maps for Cu6Sn5 grains in (a) and (b), (g) shows the inverse pole figures for orientation maps in (c) and (d), and (h) shows the color code in (e) and (f)



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