Research Papers

Improvement of Thermo-Mechanical Reliability of Wafer-Level Chip Scale Packaging

[+] Author and Article Information
Lei Shi, Lin Chen

State Key Laboratory of ASIC and System,
School of Microelectronics,
Fudan University,
Shanghai 200433, China

David Wei Zhang

State Key Laboratory of ASIC and System,
School of Microelectronics,
Fudan University,
Shanghai 200433, China
e-mail: dwzhang@fudan.edu.cn

Evan Liu

Tongfu Microelectronics Co., LTD,
Nantong 226006, Jiangsu, China

Qiang Liu

Tongfu Microelectronics Co., LTD,
Nantong 226006, Jiangsu, China;
School of Microelectronics,
Tianjin University,
Tianjin 300072, China
e-mail: qiangliu@tju.edu.cn

Ching-I Chen

Mechanical Engineering,
Chung Hua University,
Hsinchu 30012, Taiwan

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received March 23, 2017; final manuscript received October 6, 2017; published online March 2, 2018. Assoc. Editor: Kaushik Mysore.

J. Electron. Packag 140(1), 011002 (Mar 02, 2018) (9 pages) Paper No: EP-17-1035; doi: 10.1115/1.4038245 History: Received March 23, 2017; Revised October 06, 2017

Due to low cost and good electrical performance, wafer-level chip scale packaging (WLCSP) has gained more attention in both industry and academia. However, because the coefficient of thermal expansion (CTE) mismatches between silicon and organic printed circuit board (PCB), WLCSP technology still faces reliability challenges, such as the solder joint fragile life issue. In this paper, a new WLCSP design (WLCSP-PN) is proposed, based on the structure of WLCSP with Cu posts (WLCSP-P), to release the stress on the solder joints. In the new design, there is a space between the Cu post and the polymer which permits NiSn coating on the post sidewall. The overcoating enhances the solder–post interface where cracks were initiated and enlarges the intermetallic compounds (IMC) joint area to enhance the adhesion strength. Design of experiment (DOE) with the Taguchi method is adopted to obtain the sensitivity information of design parameters of the new design by the three-dimensional (3D) finite element model (FEM), leading to the optimized configuration. The finite element analysis results demonstrate that compared to WLCSP-P, the proposed WLCSP-PN reduces the package displacement, equivalent stress, and plastic strain energy density and thus improves the fatigue life of solder joints.

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Fig. 1

Structures of (a) WLCSP-U (UBM), (b) WLCSP-P, and (c) WLCSP-PN

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Fig. 2

Scanning electron microscope images of TCT failure mode for WLCSP-P: (a) cracks occurring at both outer edges of solder–post interface, (b) the enlarged view of the crack showing the CuSn IMC/solder interface, (c) the fatigue crack path formed along a thin layer of solder connecting to the post, and (d) the equivalent stress distribution through FEM simulation

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Fig. 3

Detail structure of WLCSP-PN

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Fig. 4

WLCSP-PN process flow: (a) photo-lithography and Cu post electroplating, (b) photoresist stripping, (c) epoxy coating encapsulation Cu post and RDL, (d) epoxy-post planarization by grinding, (e) epoxy lithography-exposing, (f) epoxy developing, (g) Ni and Sn overcoating using electroless plating, (h) flux coating and solder ball drop, and (i) solder reflow forming final structure

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Fig. 5

The FEM mesh of one-quarter of the WLCSP-PN package: (a) top view, (b) front view, and (c) angle view of solder joints

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Fig. 6

One-quarter FEM of the WLCSP-PN package on board: (a) global model and submodel, showing the location of submodel corresponding to the global model, (b) cross section profile of global model, (c) cross section profile of submodel, and (d) boundary condition

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Fig. 7

Temperature cycling condition. Each cycle is 48 min, including the dwell time 9 min at the temperature extremes and ramp up and down time of 15 min, respectively.

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Fig. 8

Main effect plot of S/N ratio

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Fig. 9

Sum of square difference contribution percentage of each factor

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Fig. 10

Displacement distribution for WLCSP-P and WLCSP-PN

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Fig. 11

Equivalent stress distribution for WLCSP-P and WLCSP-PN

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Fig. 12

Plastic strain energy density

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Fig. 13

Fatigue life comparison with different configurations



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