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62 SMT Magazine • January 2017 µm and 0.1 µm thick Pd films were (Cu, Ni)6Sn5 [Cu:Ni:Sn=23.5:18.1:57.4 wt%] and (Cu, Ni, Pd)6Sn5 [Cu:Ni:Pd:Sn=24.1:17.4:0.2:58.3 wt%], respectively. The cross-sectional views of the IMCs for various Pd thicknesses after thermal aging are shown in Figure 15. In the case of 0.1 µm thick electroless Pd film, (Cu, Ni, Pd)6Sn5 IMCs were formed at the solder joint interface, and the growth rate of (Cu, Ni, Pd)6Sn5 IMC layers was less than that of (Cu, Ni)6Sn5 IMCs with ther- mal aging. We inferred that the trace amounts of Pd prevented the growth of the IMCs. The IMCs are generally more brittle than the base metal, and it is reported that thick IMCs decrease sol- der joint reliability 20-22). We estimate that the thin layer of (Cu, Ni, Pd)6Sn5 IMCs with 0.1 µm thick Pd film after thermal aging is the cause of excellent solder ball joint reliability. Conclusion The influence of Pd film thickness in elec- troless Ni/Pd/Au plating on the solder ball joint reliability was investigated. The following con- clusions were obtained: (1) Based on the solder joint reliability ob- tained after multiple reflow cycles and thermal aging, the optimum thickness of Pd film was found to be 0.05–0.2µm. (2) The shape of the IMCs is considered to be one of the factors that influence the solder joint reliability after multiple reflow cycles. We estimated that the high adhesion at the den- drite layers of IMCs/solder interface resulted in excellent joints after multiple reflow cycles. (3) The thickness of the IMCs is considered to be one of the factors that influence the solder joint reliability after thermal aging. For (Cu, Ni, Pd)6Sn5 IMCs that contained trace amounts of Pd, the growth of the IMCs is prevented by Pd, resulting in excellent solder ball joint reliability after thermal aging. SMT References 1. Z. Mei, P. Callery, D. Fisher, F. Hau and J. Glazer, "Interfacial Fracture Mechanism of BGA Package on Elec- troless Ni/Au," Advances in Electronic Packaging 1997, Proc. Inter Pack '97, ASME International, New York, NY, 1997, pp. 1543~1550. 2. Z. Mei, M. Kaufman, A. Eslambolchi and P. John- son, "Brittle Interfacial Fracture of PBGA Packages Sol- dered on Electroless Nickel/Immersion Gold," Proc. 48th Electronic Components & Technology Conference, New York, NY, 1998, pp. 952~961. 3. H. Matsuki, H. Ibuka, H. Saka, Y. Araki and T. Kawa- hara, "TEM Observations of Solder Joints for Electronic Device," Proc. 3rd IEMT/IMC, IEEE, New York, NY, 1999, pp. 315~320. 4. N. Buinno, "A Root Cause Failure Mechanism for Solder Joint Integrity of Electroless Nickel/Immersion Gold Surface Finishes," Proc. IPC Printed Circuits Expo '99, IPC, Northbrook, IL, 1999, pp. S18-5-1~5-8. 5. Z. Mei, P. Johnson, M. Kaufman and A. Eslambol- Figure 14: Cross-section and surface morphology of IMCs depending on Pd thickness and thermal aging time. Figure 15: Cross-sectional models of the IMCs depending on Pd thickness and thermal aging. SOLDER BALL JOINT RELIABILITY WITH ELECTROLESS NI/PD/AU PLATING