Hee-Ra Roh

Hanyang University, Ansan, Gyeonggi, South Korea

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Publications (5)5.24 Total impact

  • Young Min Kim · Hee-Ra Roh · Sungtae Kim · Young-Ho Kim ·
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    ABSTRACT: The growth kinetics of an intermetallic compound (IMC) layer formed between Sn-3.5Ag-0.5Cu (SAC) solders and Cu-Zn alloy substrates was investigated for samples aged at different temperatures. Scallop-shaped Cu6Sn5 formed after soldering by dipping Cu or Cu-10 wt.%Zn wires into the molten solder at 260°C. Isothermal aging was performed at 120°C, 150°C, and 180°C for up to 2000 h. During the aging process, the morphology of Cu6Sn5 changed to a planar type in both specimens. Typical bilayer of Cu6Sn5 and Cu3Sn and numerous microvoids were formed at the SAC/Cu interfaces after aging, while Cu3Sn and microvoids were not observed at the SAC/Cu-Zn interfaces. IMC growth on the Cu substrate was controlled by volume diffusion in all conditions. In contrast, IMC growth on Cu-Zn specimens was controlled by interfacial reaction for a short aging time and volume diffusion kinetics for a long aging time. The growth rate of IMCs on Cu-Zn substrates was much slower due to the larger activation energy and the lower layer growth coefficient for the growth of Cu-Sn IMCs. This effect was more prominent at higher aging temperatures.
    Journal of Electronic Materials 12/2010; 39(12):2504-2512. DOI:10.1007/s11664-010-1379-x · 1.80 Impact Factor
  • Young Min Kim · Chang-Yul Oh · Hee-Ra Roh · Young-Ho Kim ·
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    ABSTRACT: The excessive growth of intermetallic compound (IMC) and the formation of microvoids in the interface between Sn-Ag-Cu (SAC) solder and Cu substrate have been a serious problem which may degrade the solder joints reliability. Recently, a new Cu-Zn alloy solder wetting layer for the Pb-free solders has been developed to reduce the IMC growth rate. In this paper, the kinetics of IMC growth and the shear strength of SAC solder joints formed on electroplated Cu-Zn wetting layer were investigated. And, drop impact reliability of SAC/Cu-Zn joints was evaluated. The solder joints were fabricated by reflowing Sn-4.0Ag-0.5Cu (SAC 405) solder balls on electroplated Cu-20 wt% Zn wetting layers. An experiment of the SAC solder joints on electroplated Cu layers was also carried out for comparison. After soldering, Cu<sub>6</sub>Sn<sub>5</sub> IMC formed on Cu or Cu-Zn wetting layer. The typical double layer of Cu<sub>6</sub>Sn<sub>5</sub> and Cu<sub>3</sub>Sn with microvoids formed during aging at the solder/Cu interfaces. On the contrary, only Cu<sub>6</sub>Sn<sub>5</sub> formed at the SAC/Cu-Zn interfaces with aging. Neither Cu<sub>3</sub>Sn nor microvoids were found in these specimens. The growth rate of IMCs at the SAC/Cu-Zn interfaces was much slower than that at the SAC/Cu interfaces. Cu-Zn wetting layer was very effective to reduce the IMC growth. IMC growth was controlled by volume diffusion. The shear test was carried out with SAC 405 solder bumps formed on Cu or Cu wetting layers. The shear strength of SAC solder bumps on Cu or Cu-Zn layers decreased with aging. The shear strength of SAC solder on Cu-Zn pads was slightly higher than that of SAC solder on Cu layer at the same aging time. Since all specimens showed ductile fracture in the solders, the shear strength of the solder bumps depended on the strength of solder ball. Drop test were conducted with the assembly which was fabricated using SAC 405 solder balls and two printed circuit boards. The mean number of drops- to failure of Cu/SAC/Cu and Cu-Zn/SAC/Cu-Zn specimens was about 150 and about 350 drops, respectively. Though the mean number of drops to failure decreased with increasing aging time, the mean number of drops to failure of Cu-Zn/SAC/Cu-Zn specimens was higher than that of Cu/SAC/Cu specimens.
    Electronic Components and Technology Conference, 2009. ECTC 2009. 59th; 06/2009
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    ABSTRACT: We developed a new Cu–Zn wetting layer for Pb-free solders. By adding Zn to the Cu wetting layer, intermetallic growth in the Sn–Ag–Cu (SAC) solder interfaces was delayed. Cu3Sn intermetallic compounds and microvoids were not observed in the SAC/Cu–Zn interfaces after aging. The drop reliability of the SAC solder/Cu–Zn joints was excellent.
    Journal of Materials Research 02/2009; 24(2):297-300. DOI:10.1557/jmr.2009.0047 · 1.65 Impact Factor
  • Hee-Ra Roh · Young-Ho Kim ·
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    ABSTRACT: We recently developed Cu-Zn alloy wetting layer for the SAC solder balls to improve the reliability. In this paper, IMC formation and solder joint reliability have been investigated in the solder joint between SAC solder balls and Cu-Zn alloy pads, and Cu pads were also used as a reference. Sn-4.0Ag-0.5 Cu solder balls were used and the substrates were Cu and 80 wt% Cu-20 wt% Zn pads which were defined with solder masks. The SAC solder balls on Cu or Cu-Zn pads were reflowed and the solder bump specimens were aged at 150degC up to 1000 h. The Cu<sub>6</sub>Sn<sub>5</sub> and large Ag<sub>3</sub>Sn plates in the solder ball were formed at the SAC/Cu interfaces after soldering. The well-known double layer structure of Cu-Sn IMCs (Cu<sub>6</sub>Sn<sub>5</sub> and Cu<sub>3</sub>Sn) and Kirkendall voids formed in the SAC/Cu interfaces during aging. On the SAC/Cu-Zn interfaces, Cu<sub>6</sub>Sn<sub>5</sub> formed after soldering and thickened with aging. However, Cu<sub>3</sub>Sn IMC was not formed on SAC/Cu-Zn interfaces after aging. Also, the growth rate of IMCs at the SAC/Cu-Zn interface was much slower than that at the SAC/Cu interface. The ball shear test was conducted after soldering and aging to evaluate solder joint strength. The fracture mainly occurred in the solder and the shear strength value decreased with increasing aging time on both SAC/Cu and SAC/Cu-Zn. The shear strength of SAC solder on Cu substrates was lower than that of SAC solder on Cu-Zn substrates during aging.
    Electronic Materials and Packaging, 2007. EMAP 2007. International Conference on; 12/2007
  • Seung-Hyun Lee · Hee-Ra Roh · Zhi Gang Chen · Young-Ho Kim ·
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    ABSTRACT: Solder joints were successfully bonded by joining Ag/Sn/Cu bumps and Ag/Sn/Cu layers at 200°C for 30 sec under 20 MPa, 40 MPa, and 80 MPa using thermo-compression bonder. The solder joints were aged at 150°C up to 1000 h. The strength of the solder joints was measured by the shear test and the contact resistance was measured using four-point probe method. The microstructure of the solder joints and the fracture modes after shear test were analyzed by scanning electron microscopy (SEM) with the energy-dispersive spectrometry (EDS). Results showed that the electrical resistance of the solder joints decreased, and the shear strength of the solder joints increased after aging treatment. The fracture modes were observed to move from the interfacial failure between solder and intermetallic compounds (IMCs) to the interfacial failure between IMCs. It was considered that the transition of fracture modes was closely related with the microstructure evolution of the solder joints, especially the transformation of IMC phases during the aging treatment.
    Journal of Electronic Materials 10/2005; 34(11):1446-1454. DOI:10.1007/s11664-005-0204-4 · 1.80 Impact Factor