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Research Papers

Influence of Thermomigration on Lead-Free Solder Joint Mechanical Properties

[+] Author and Article Information
Mohd F. Abdulhamid

Electronic Packaging Laboratory, University at Buffalo, SUNY, Buffalo, NY 14260

Cemal Basaran1

Electronic Packaging Laboratory, University at Buffalo, SUNY, Buffalo, NY 14260cjb@buffalo.edu

1

Corresponding author.

J. Electron. Packag 131(1), 011002 (Feb 11, 2009) (12 pages) doi:10.1115/1.3068296 History: Received September 25, 2007; Revised April 17, 2008; Published February 11, 2009

Thermomigration experiments were conducted to study the change in mechanical properties of 95.5Sn–4Ag–0.5Cu (SAC405) lead-free solder joint under high temperature gradients. This paper presents some observations on samples that were subjected to 1000°C/cm thermal gradient (TG) for 286 h, 712 h, and 1156 h. It was observed that samples subjected to thermal gradient did not develop a Cu3Sn intermetallic compound (IMC) layer, and we observed disintegration of Cu6Sn5 IMC. On the other hand, samples subjected to isothermal annealing exhibited IMC growth. In samples subjected to thermomigration, near the cold side the Cu concentration is significantly higher compared with hot side. Extensive surface hardness testing showed an increase in hardness from the hot to cold sides, which possibly indicates that Sn grain coarsening is in the same direction.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

(a) Front view of the test vehicle sandwiched between heating and cooling plates. (b) Partial side view of test apparatus showing 3 out of 4 possible test vehicles. In both views, the thermocouple probe locations are indicated.

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Figure 2

Indentation marks on tested solder joint. Bottom side is the hot side.

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Figure 3

FEM heat transfer analysis results show that the highest (bottom) temperature is 155°C while the lowest (top) temperature is 55°C, creating a temperature gradient of 1000°C/cm

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Figure 4

Copper plate/solder joint interface for the as-flowed sample

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Figure 5

Cold side copper plate-solder joint interface in thermomigration samples showing no development of Cu3Sn. Bottom row shows the outline of the Cu6Sn5 layer.

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Figure 6

Hot (bottom) side copper plate-solder joint interface in thermomigration samples showing no development of Cu3Sn. Bottom row shows the outline of the Cu6Sn5 layer.

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Figure 7

Hot interface of TG sample after 286 h of annealing showing the nonuniformity and waviness of the IMC

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Figure 8

Top side of isothermal (55°C) samples showing the development of only the Cu6Sn5 IMC

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Figure 9

Bottom side of isothermal (170°C) samples showing the development of Cu3Sn IMC between Cu plate and Cu6Sn5 IMC

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Figure 10

A typical load/unloading curve for an indentation point. The curve is used to determine the hardness and elastic modulus.

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Figure 11

A plot of hardness versus indentation depth for specimen 1 for the thermal gradient experiment after 712 h at a normalized distance of 0.95 from the hot side

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Figure 12

A plot of elastic modulus versus indentation depth for specimen 1 for thermal gradient experiment after 712 h at a normalized distance of 0.95 from the hot side

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Figure 13

Average measured surface hardness of thermomigration and isothermal samples across the solder height (TG: thermal gradient, IT: isothermally annealing)

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Figure 14

Average calculated elastic modulus of thermomigration and isothermal samples across the solder height (TG: thermal gradient, IT: isothermally annealing)

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Figure 15

The process of “old” Cu6Sn5 layer disintegration into “new” layer of Cu6Sn5

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Figure 16

SEM image of the cold interface after 712 h. Points 1–3 are identified as the Cu6Sn5 IMC. Points 4–6 show the increase in Cu concentration as points are closer to the interface.

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Figure 17

Pb-grain coarsening reported by Ye at al (36). At the chip (hot) side, the Pb-grain size is larger compared with the substrate (cold) side.

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