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Research Papers

Reliability of Fine Pitch Sn–3.8Ag–0.7Cu Flip Chip Solder Joints With Different Connection Pads on PCB

[+] Author and Article Information
Dezhi Li1

Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdomd.li@lboro.ac.uk

Changqing Liu, Paul P. Conway

Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom

1

Corresponding author.

J. Electron. Packag 130(1), 011005 (Jan 31, 2008) (7 pages) doi:10.1115/1.2837522 History: Received January 05, 2007; Revised July 12, 2007; Published January 31, 2008

The reliability of fine pitch Sn–3.8Ag–0.7Cu flip chip solder joints with three different pads, i.e., bare pads, pads with solder masks, and pads with microvia, on printed circuit boards (PCBs) was studied through thermal cycling. After assembly, (Au,Ni)Sn4 intermetallics (IMCs) formed both in the bulk solder and at the interfaces due to the immersion-Au finish on the PCB side. The (Au,Ni)Sn4 IMCs formed in the solder joints on the pads with microvia were more abundant than those formed in the solder joints on the pads without microvia. The results showed that the solder joints on the pads with a microvia had poor reliability due to the insufficient solder volume and the formation of large amounts of (Au,Ni)Sn4 IMCs. The main crack initiation position was the corner of solder joint at the chip side. For the pads with microvia, the main location of failure was at the (Au,Ni)Sn4/solder interface on the chip side, and for the solder joints on bare pads and pads with solder mask, the possible failure location was in the bulk solder.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

A testing PCB assembled with flip chips

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Figure 2

Temperature profile of thermal cycling

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Figure 3

Microvia with EN immersion-Au surface finish: (a) microstructure and (b) element mapping

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Figure 4

Microstructure of solder joints (bare pads on the PCB) after thermal cycling: (a) 100cycles, (b) 500cycles, (c) 1000cycles, and (d) 1500cycles.

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Figure 5

Microstructure of solder joints (pads with microvia on the PCB) after thermal cycling: (a) 100cycles, (b) 500cycles, (c) 1000cycles, and (d) 1500cycles

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Figure 6

Possible crack initiation positions during thermal cycling: (a) sharp points, (b) joint corner and (Au,Ni)Sn4/solder interface, (c) poor wetted area and voids, and (d) Ag3Sn/solder interface

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Figure 7

Irregular solder joint shapes after reflow soldering when pads with solder mask were used: (a) narrow neck and (b) UBM lift

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Figure 8

Failure modes of solder joints when pads with microvia were used: (a) failure at (Au,Ni)Sn4/solder interface and (b) failure at (Au,Ni)Sn4/solder interface and in bulk solder

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Figure 9

Possible failure mode for solder joints during thermal cycling: (a) on bare pads and (b) on pads with solder mask

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Figure 10

Crack propagation along the grain boundaries of β-Sn

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Figure 11

Ag3Sn IMCs in the way of crack propagation

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