A new solder wetting layer for Pb-free solders
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A new solder wetting layer for Pb-free solders Chang Yul Oh, Hee-Ra Roh, Young Min Kim, Jin Soo Lee, Hae Young Cho, and Young-Ho Kima) Division of Materials Science & Engineering, Hanyang University, Seoul 133-791, Korea (Received 30 June 2008; accepted 27 October 2008)
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. Sn-based solders with alloying elements such as Cu, Ag, and Bi have been developed to replace Pb–Sn solders because of environmental concerns and regulations around the world.1–4 However, the fast growth of intermetallic compounds (IMCs) and the formation of microvoids in the interface between Sn-based solder and Cu, which has been widely used as a solder wetting layer, have been serious problems since thick IMC layers and microvoids degrade solder joint reliability.5–7 Also, the formation of large Ag3Sn plates in Sn–Ag–Cu (SAC) solders poses a reliability concern.5,6,8,9 Recently, modification of Sn-based solders has been attempted by adding a small amount of Zn.10 In these solders, the growth of IMC layers and the formation of microvoids were depressed during isothermal aging. Furthermore, the minor Zn addition to the SAC solders was shown to suppress the formation of large Ag3Sn plates.11 However, Sn-based solders containing Zn were questionable for application in electronic packaging because of a high tendency to oxidize and corrode.12 In this study, we added Zn to the Cu wetting layer, instead of adding Zn to the Sn-based solders. Cu–Zn solder wetting layers were fabricated by electroplating, and the growth behavior of interfacial IMCs was investigated during aging. Also, the result of the impact reliability of solder joints is presented. Cu/bismaleimide–triazine (BT) substrates with a circular opening of 650 mm in diameter, as shown in Fig. 1, were prepared to study the IMC growth behavior. Cu– 20wt%Zn layers were fabricated on Cu by electroplating. The thickness of electroplated Cu–Zn layers was controlled by changing the electroplating time. Cu was also electroplated on the same substrates to compare with the Cu–Zn wetting layer. The additive such as bis (3-sulfopropyl) disulfide (SPS) was not included in the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0047 J. Mater. Res., Vol. 24, No. 2, Feb 2009
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electroplating solution. Sn–4.0wt%Ag–0.5wt%Cu (SAC 405) solder balls with a diameter of 760 mm were attached to the substrates after applying a rosin mildly activated flux and reflowed in a conventional oven with a peak temperature of 260 C for 20 s. Then these specimens were aged at 150 C for up to 1000 h. The microstructure of the solder joints before and after aging treatment were characterized u
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