Formation of Sn-rich phases via the decomposition of Cu 6 Sn 5 compounds during current stressing C.K. Lin, Chien-Ming Liu, Chih Chen n Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu 30010, Taiwan, ROC article info Article history: Received 5 March 2014 Accepted 13 March 2014 Available online 21 March 2014 Keywords: Phase transformation Metallic composites Intermetallic compound Solder Electromigration abstract This study examined the formation of Sn-rich phases in the matrix of Cu–Sn–Ni intermetallic compounds (IMCs) after current stressing of 1.2 Â 10 4 A/cm 2 at 160 1C. The Sn-rich phases were formed at the cathode end of the solder joints with Cu metallization, and this formation was attributed to the decomposition of Cu 6 Sn 5 IMCs. When the Cu 6 Sn 5 IMCs were transformed into Cu 3 Sn during current stressing, Sn atoms were released. When the supply of Cu atoms became deficient, Sn atoms accumulated to form Sn-rich phases among the Cu–Sn–Ni IMCs. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Electromigration has been one of the most persistent reliability issues in microelectronic devices [1–3]. As the dimensions of devices continue to shrink, solder joints must be reduced in size accordingly. In addition, with higher performance required of devices, the operating current in each solder joint increases progressively, leading to a dramatic increase in the current density in the joint. Therefore, electromigration has become a critical reliability issue [4,5]. Many efforts have been devoted to understanding electromi- gration behavior [6–12]. Electromigration may induce void forma- tion in solder [13–15]. In addition, electron flow may enhance the dissolution of under bump metallization (UBM) and cause the extensive formation of intermetallic compounds (IMCs) [16–20]. Both mechanisms can lead to an open circuit of solder joints. Pb-free solders have been adopted to replace Pb-containing solders [21,22]. Compared with Pb-containing solder, Pb-free solders possess higher reaction rates with Cu and Ni UBMs [7,17,23,24]. Therefore, extensive Cu–Sn IMCs are formed during electromigration in Pb-free solders with Cu UBMs [9,10]. Both Cu 3 Sn and Cu 6 Sn 5 IMCs are formed after current stressing. In addition to these two IMCs, Sn-rich phases have also been frequently observed at the cathode end [10,25,26]. However, the reason for the formation of Sn-rich phases among the Cu–Sn IMCs after electromigration remains unclear. This study investigated electromigration in Pb-free SnAg solder joints containing Cu/Ni UBMs. Sn-rich phases were formed after current stressing of 1.2 Â 10 4 A/cm 2 at 150 1C. A mechanism was proposed to explain this interesting phenomenon. 2. Experimental Sample dimensions and test conditions: Flip-chip Sn-2.3Ag (wt%) solder joints were used for the electromigration tests. Fig. 1 (a) presents a schematic structure for the test layout. Currents were applied at Nodes N2 and N3. The voltage was measured at Nodes N1 and N4. Fig. 1(b) presents a schematic drawing of the joint and Fig. 2(a) presents a cross-sectional SEM image of the joint. The thickness of the Cu traces on the chip side was 5 μm, whereas the thickness was 20 μm on the FR-5 substrate side. The 500-Å-thick Ti seed layer was between the Cu trace and the Cu UBM. The UBM on the chip side comprised electroplated 5-μm Cu and 3-μm Ni. On the substrate side, the metallization was electro- less Ni. For the as-fabricated sample, the interfacial IMCs were Ni 3 Sn 4 on both the chip and substrate sides. Kelvin bump struc- tures were employed to measure the individual bump resistance [15]. The solder joints were stressed at a current density of 1.2 Â 10 4 A/cm 2 at 150 1C. The actual stressing temperature was calibrated using the temperature coefficient of resistivity of the Cu trace on the chip side [27]. The actual stressing temperature increased to 160 1C due to the Joule heating effect in the solder joints [28]. Analytical procedures: As the resistance reached a desired value, the current stressing was terminated, and the sample was polished for cross-sectional observation using a JEOL 6700 scanning elec- tron microscope (SEM). Energy dispersive spectrometry (EDS) was Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.03.071 0167-577X/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ886 3 5731814; fax: þ886 3 5724727. E-mail addresses: chih@mail.nctu.edu.tw, chihchen98@yahoo.com (C. Chen). Materials Letters 124 (2014) 261–263