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Evolution of curvature under thermal cycling in sandwich assembly bonded by sintered nanosilver paste

Yunhui Mei (Tianjin Key Laboratory of Advanced Joining Technology and School of Materials Science and Engineering, Tianjin University, Tianjin, China)
Gang Chen (School of Chemical Engineering and Technology, Tianjin University, Tianjin, China)
Xin Li (Key Laboratory of Advanced Joining Technology and School of Materials Science and Engineering, Tianjin University, Tianjin, China)
Guo‐Quan Lu (Department of Materials Science and Engineering and Bradley Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, Virginia, USA)
Xu Chen (School of Chemical Engineering and Technology, Tianjin University, Tianjin, China)

Soldering & Surface Mount Technology

ISSN: 0954-0911

Article publication date: 5 April 2013

597

Abstract

Purpose

The purpose of this paper is to determine: how much the residual curvature could be formed in sintered nano‐silver assembly when it is cooled to room temperature from the sintering temperature (normally 275°C); how the cyclic temperature load affects the residual curvature or stresses in sintered joint. Then the stress level and the reliability of sintered nano‐silver for high‐temperature applications can be understood.

Design/methodology/approach

5 mm * 2.5 mm silicon chip was bonded with 96 per cent Al2O3 substrate by sintering nanosilver paste. An optical system was developed to measure the curvature of the sintered assemblies. Reliability of the sintered assemblies was evaluated by temperature cycling of −40∼125°C. Finite element analysis was employed to simulate the behavior of the joint subjected to the temperature cycling from −40°C to 125°C by ANSYS. SEM images were taken to investigate the impact of temperature cycling on the reliability of sintered silver attachment.

Findings

This residual bending at room temperature was found concave towards the substrate (alumina) side. Also, with the bondline thickness increasing, the residual curvature decreases obviously. The severity of the residual bending in all the structures was mitigated to some extent with increasing number of cycles. There is no crack in the joint with the thickness of 25 μm. The drop of the residual curvature of the samples with bondline of 25 μm is caused mainly by stress relaxation in sintered silver before 300 cycles. Sample with thicker bondline is more susceptible to thermal cycling for the structure bonded with nanosilver than that with thinner bondline. The poor quality of bonding is due to the thicker sintered joint, which means that sintered nanosilver is not suitable for die‐attachment requiring thick bondline.

Originality/value

The paper describes: how a precise optical system was developed to measure the residual curvature of the sintered assemblies; how the evolution of the residual curvature of the sintered assembly with the temperature cycling was obtained by both experiment and simulation; and how microstructures of the sintered silver joint were analyzed for as‐sintered assembly and the sintered assembly after temperature cycling.

Keywords

Citation

Mei, Y., Chen, G., Li, X., Lu, G. and Chen, X. (2013), "Evolution of curvature under thermal cycling in sandwich assembly bonded by sintered nanosilver paste", Soldering & Surface Mount Technology, Vol. 25 No. 2, pp. 107-116. https://doi.org/10.1108/09540911311309077

Publisher

:

Emerald Group Publishing Limited

Copyright © 2013, Emerald Group Publishing Limited

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