Electromigration and thermomigration behavior of flip chip solder joints in high current density packages
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M. Pecht Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, Maryland 20742 (Received 31 December 2007; accepted 11 March 2008)
The electromigration and thermomigration behavior of eutectic tin-lead flip chip solder joints, subjected to currents ranging from 1.6 to 2.0 A, at ambient temperatures above 100 °C, was experimentally and numerically studied. The temperature at the chip side was monitored using both a temperature coefficient of resistance method and a thermal infrared technique. The electron wind force and thermal gradient played the dominant role in accelerated atomic migration. The atomic flux of lead due to electromigration and thermomigration was estimated for comparison. At the current crowding region, electromigration induced a more serious void accumulation as compared with thermomigration. Also, because of different thermal dissipations, a morphological variation was detected at different cross-sectional planes of the solder joint during thermomigration.
I. INTRODUCTION
The pursuit of greater performance in microelectronic power devices has led to shrinkage of solder joint size and a significant increase in current. This has resulted in a dramatic increase in the current density passing through the solder joints and has placed challenges on product reliability. According to the 2006 International Technology Roadmap for Semiconductors (ITRS), electromigration in solder, defined as the solid-state atomic movement because of momentum transfer from flowing electrons, is becoming a limiting factor for the further miniaturization of packages due to the increase of current density and the shrinkage of solder joints.1 Electromigration-related issues, such as current crowding, Joule heating, and mechanical behavior, have been widely studied since 1998.2–5 Chiang et al. have demonstrated that current crowding occurred in solder joints, and could enhance the local atomic flux along with the electron flow and expedite the migration damage.6 Current crowding was expected to accelerate void formation and propagation along the interface between the intermetallic compound (IMC) and the solder.7 Furthermore, the crowding region could produce local Joule heating, which also accelerates electromigration-induced failures.8 Recently, thermomigration has been regarded as an a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0305 J. Mater. Res., Vol. 23, No. 9, Sep 2008
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additional reliability concern for solder joints. Thermomigration is triggered by thermal gradients across the solder joint. For a first-level solder interconnect, this type of thermal gradient is more evident due to Joule heating that accumulates at the chip side when the cross-sectional area of conductive lines on the chip is gradually decreased. The earliest report concerning the combined effect of thermomigration and electromigration has been given by Ye et al.9 More recently, the individual contribution of
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