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RESEARCH PAPERS

Systematic Study on Thermo-Mechanical Durability of Pb-Free Assemblies: Experiments and FE Analysis

[+] Author and Article Information
Qian Zhang1

CALCE Electronic Products and Systems Center, Mechanical Engineering Department, University of Maryland, College Park, MD 20742

Abhijit Dasgupta

CALCE Electronic Products and Systems Center, Mechanical Engineering Department, University of Maryland, College Park, MD 20742

Dave Nelson, Hector Pallavicini

 Raytheon Company, McKinney, TX 75069

WEEE Directive, 2001.

1

Currently with Dell Inc., Round Rock, TX 78681.

J. Electron. Packag 127(4), 415-429 (Jan 06, 2005) (15 pages) doi:10.1115/1.2098812 History: Received April 12, 2004; Revised January 06, 2005

As the ban of the Pb use in electronics products is approaching due to the waste electrical and electronic equipment (WEEE) and restriction of hazardous substances (ROHS) directives, electronics companies start to deliver the products using the Pb-free solders. There are extensive databases of mechanical properties, durability properties (for both mechanical and thermal cycling), and micromechanical characteristics for Sn-Pb solders. But similar databases are not readily yet available for Pb-free solders to predict its mechanical behavior under environmental stresses. In this study, the thermo-mechanical durability of the Pb-free Sn3.8Ag0.7Cu solder is investigated by a systematic approach combining comprehensive thermal cycling tests and finite element modeling. A circuit card assembly (CCA) test vehicle was designed to analyze several design and assembly process variables when subjected to environmental extremes. The effects of mixed solder systems, device types, and underfill are addressed in the thermal cycling tests. The thermal cycle profile consisted of temperature extremes from 55to+125° Celsius with a 15min dwell at hot, a 10min dwell at cold, and a 5–10° Celsius per minute ramp. Thermal cycling results show that Sn3.8Ag0.7Cu marginally outperforms SnPb for four different components under the studied test condition. In addition, the extensive detailed three-dimensional viscoplastic finite element stress and damage analysis is conducted for five different thermal cycling tests of both Sn3.8Ag0.7Cu and Sn37Pb solders. Power law thermo-mechanical durability models of both Sn3.8Ag0.7Cu and Sn37Pb are obtained from thermal cycling test data and stress and damage analysis. The results of this study provide an important basis of understanding the thermo-mechanical durability behavior of Pb-free electronics under thermal cycling loading and environmental stresses.

Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

Test vehicle schematic

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Figure 2

Optical micrographs of SnAgCu solder joints; (a) PBGA352, (b) fleXBGA144, (c) TABGA96, (d) μBGA46, (e) μBGA48

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Figure 3

Optical micrographs of four solder systems of TABGA 96 component (label: Solder ball/paste)

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Figure 4

Optical micrograph of pure SnAgCu PBGA352 solder joint after etching

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Figure 5

Optical micrograph of pure SnAgCu PBGA352 solder joint after etching

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Figure 6

Optical micrograph of SnAgCu∕SnPb solder joint of μBGA48 device after etching

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Figure 7

Temperature profile

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Figure 8

Effects of mixed technology and component type on characteristic life of the solder joints

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Figure 9

Optical micrographs of four kinds of components after micro sectioning (SAC)

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Figure 10

Optical micrographs of μBGA46 with extremely early failure (SAC)

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Figure 11

Optical micrographs of PBGA352 solder joint (SAC)

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Figure 12

Optical micrographs of TABGA96 solder joint with voids (SAC)

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Figure 13

Optical micrographs of PBGA352 solder join (SnPb)

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Figure 14

Optical micrographs of TABGA96 solder joint (SnPb ball/SAC paste)

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Figure 15

Optical micrographs of PBGA352 solder joints (SAC ball/SnPb paste)

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Figure 16

Overall approach for thermomechanical durability analysis

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Figure 17

Schematic of thermal profile

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Figure 18

Finite element model of fleXBGA144 package

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Figure 19

Variation of equivalent creep strain range per cycle with mesh density; creep strain is averaged in the local neighborhood of the critical region in the critical solder joint

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Figure 20

Finite element meshing of the solder ball array for fleXBGA144

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Figure 21

Equivalent stress-strain hysteresis loops for fleXBGA1 package subjected to our thermal cycling

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Figure 22

Total work density contour of fleXBGA1 package at −125°C before dwell: Sn3.8Ag0.7Cu solder

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Figure 23

Total work density contours of fleXBGA2 package at 100°C before dwell: Sn3.8Ag0.7Cu solder

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Figure 24

Total work density contours of TABGA package at −125°C before dwell: Sn3.8Ag0.7Cu solder

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Figure 25

Comparison between thermal cycle tests and FE simulation of fleXBGA1: Sn3.8Ag0.7Cu solder

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Figure 26

Half layer used to assess average stress, strain and energy: TABGA96, Sn3.8Ag0.7Cu, 125°C before dwell

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Figure 27

Normal stress contour plot of the whole layer of the critical solder joint: TABGA96, Sn3.8Ag0.7Cu, 125°C before dwell

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Figure 28

Hysteresis loops of Sn3.8Ag0.7Cu and Sn37Pb solders: TABGA96

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Figure 29

Effect of thermal profile on hysteresis loop of SAC for fleXBGA2

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Figure 30

Power law durability model for Sn3.8Ag0.7Cu solder

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Figure 31

Power law durability model for Sn37Pb solder

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Figure 32

Comparison of Coffin-Manson type durability model between Sn3.8Ag0.7Cu solder and Sn37Pb solder

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