Mechanical deformation-induced Sn whiskers growth on electroplated films in the advanced flexible electronic packaging
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Yuhi Yorikado, Junxiang Jiang, Keun-Soo Kim, and Katsuaki Suganumaa) Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
Sinn-wen Chenb) Department of Chemical Engineering, National Tsing Hua University, Hsin-chu 300, Taiwan
Masanobu Tsujimoto and Isamu Yanada C. Uyemura & Co., Ltd., Osaka, Osaka, Japan (Received 8 January 2007; accepted 29 March 2007)
In this study, we investigated mechanical deformation-induced Sn whisker growth, which is frequently encountered in advanced flexible substrate packaging. Concentrated compressive stresses are introduced around the leads and solder surface finish joints connected by compression fixing. Six types of pure Sn thin films were electroplated on Ni-protected Cu substrates. These were 2- and 6-âŪm-thick Sn films electroplated with three different current densities: 2, 10, and 20 A/dm2. These films were compressed at room temperature and ambient humidity. The surface and cross-sectional grain morphologies of the films were examined by scanning electron microscopy and focused ion beam spectroscopy, respectively. The grain orientations of the electroplated Sn films were analyzed by x-ray diffraction and electron backscatter diffraction. After compression, nodule hillocks and whiskers were found around the indents. Beneath the indents, the original columnar Sn grains were deformed, and recovery and recrystallization processes occurred. Rapid whisker formation was observed. The whiskers induced by mechanical deformation are closely related to the grain microstructures, and the initial compression stresses are critical to the types and distribution of whiskers as well.
I. INTRODUCTION
Flexible printed circuits (FPCs) have been extensively used in microelectronic and optoelectronic products, such as notebook computers, digital cameras, liquid crystal displays (LCDs), foldable cellular phones, and CD/ DVD drives, and more and more potential applications of FPCs are proposed.1â4 In FPC assembly technology, pins of flexible flat cable (FFC)/FPC connectors and electroplated solder finishes are connected by mechanical interlocking. The connections bring concentrated compres-
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2007.0232 J. Mater. Res., Vol. 22, No. 7, Jul 2007
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sive stresses around the joints. Compressive stress has been proved to be the main driving force for the formation and growth of Sn whiskers.5â18 Long Sn whiskers may result in short circuits. The Sn whisker growth issue has recently raised serious reliability concerns again in the flexible electronic packaging industry primarily due to the development of fine-pitch FPC connectors and the emerging of new Pb-free solders. Spontaneous formation of Sn whiskers was first reported in 1951.5 After decades of intensive investigation, it has been concluded that the phenomenon is caused by a compressive stress relaxation process.5â18 T
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