Research Papers

Reliability Assessment of Wafer Level Packages With Novel FeNi Under Bump Metallization

[+] Author and Article Information
Jia Xi, Xinduo Zhai, Jun Wang, Donglun Yang, Mao Ru

Department of Materials Science,
Fudan University,
Shanghai 200433, China

Fei Xiao

Department of Materials Science,
Fudan University,
Shanghai 200433, China
e-mail: feixiao@fudan.edu.cn

Li Zhang, Chi Ming Lai

Jiangyin Changdian Advanced
Packaging Co., Ltd.,
Jiangyin 214431, China

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received February 5, 2015; final manuscript received June 30, 2015; published online July 21, 2015. Assoc. Editor: Yi-Shao Lai.

J. Electron. Packag 137(3), 031016 (Jul 21, 2015) Paper No: EP-15-1016; doi: 10.1115/1.4030974 History: Received February 05, 2015

FeNi alloy is considered a possible substitute for Cu as under bump metallization (UBM) in wafer level package (WLP) since it forms very thin intermetallic compound (IMC) layer with Pb-free solder in the reflow process. In this paper, WLPs with FeNi and Cu UBM were fabricated and their board level reliabilities were studied comparatively. The WLP samples assembled on the printed circuit board (PCB) were subjected to temperature cycling and drop tests according to JEDEC standards. The results showed that the reliability of WLP with FeNi UBM was a little lower than that with Cu UBM. The main failure modes for both FeNi and Cu UBM samples in temperature cycling test were the crack in IMC or solder ball on PCB side. And detachments between UBM and the redistribution layer (RDL) were also observed in Cu UBM WLPs. In drop test, the crack of RDL was found in all failed FeNi UBM samples and part of Cu UBM ones, and the primary failure mode in Cu UBM samples was the crack of IMC on PCB side. In addition, the finite element analysis (FEA) was carried out to further understand the difference of the failure modes between the FeNi UBM samples and the Cu UBM samples. The high stress was observed around the UBM and the pad on PCB in the temperature cycling model. And the maximum stress appeared on the RDL in the drop simulation, which was obviously larger than that on the pad. The FEA results showed that the introduction of FeNi UBM increased the stress levels both in temperature cycling and drop tests. Thus, the FeNi alloy cannot simply replace Cu as UBM in WLP without further package structural optimization.

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Fig. 1

Schematic processes of WLP with FeNi UBM/Cu UBM

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Fig. 2

Test board with samples assembled

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Fig. 3

Interfacial morphology of solder joints with different UBM after reflow

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Fig. 4

Weibull distribution of WLPs in temperature cycling tests

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Fig. 5

Failure modes of FeNi UBM samples in temperature cycling tests: (a) crack in IMC and (b) crack in solder ball

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Fig. 6

Crack in solder from the edge of UBM in Fe-64Ni WLPs

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Fig. 7

Detachment between UBM and RDL in Cu UBM samples

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Fig. 8

Weibull distribution of WLPs in drop tests

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Fig. 9

Failure modes in drop tests: (a) break of RDL and (b) crack in IMC

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Fig. 10

Finite element mesh: (a) temperature cycling model and (b) drop model

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Fig. 11

Equivalent stress distribution of FeNi UBM sample in temperature cycling simulation (unit in Pa)

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Fig. 12

Equivalent stress distribution of Cu UBM sample in temperature cycling simulation (unit in Pa)

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Fig. 13

Equivalent stress distribution of FeNi UBM sample in the drop test (unit in kPa)

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Fig. 14

Equivalent stress distribution of Cu UBM sample in the drop test (unit in kPa)

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Fig. 15

Stress–time curves at maximum stress position on the RDL and the PCB pad




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