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58 SMT Magazine • January 2017 ability of the solder ball joint with a 0.05–0.2 µm thick electroless Pd film was similar to that obtained with conventional electrolytic Ni/Au 19 films. Thus, the optimum thickness of an elec- troless Pd film for a reliable solder ball joint was found to be 0.05–0.2 µm. Covering Property of Pd Plating Film on Ni Plating Film Figure 6 shows the results of SEM observa- tions with different Pd thicknesses before and after applying ultrasonic waves. In the case of 0.01–0.015 µm thick electroless Pd film, pin- holes occurred after applying ultrasonic waves. In contrast, in the case of electroless Pd films with thickness more than 0.02 µm, there were no pinholes after applying ultrasonic waves. A model depicting the covering process of Pd plat- ing film on the Ni plating film is shown in Fig- ure 7. The initial stage of Pd deposition on the electroless Ni surface involves a replacement re- action (Figure 7a). Since the electroless Pd plat- ing bath includes a reducing agent, corrosion hardly occurs, because the reduction reaction progresses immediately on the Pd film plated by the replacement reaction (Figure 7b). How- ever, the covering of the Pd film at the replace- ment reaction points was delayed compared to the other points. Thin Pd plated points existed in the case of 0.01–0.015 µm thick Pd film (Fig- ures 7c and 7d). It is presumed that the thin Pd plated points might be the cause of occurrence of the pinholes. All the samples with electroless Ni/Au film showed brittle fracture in each of the evaluat- ed conditions. The SEM images of surface mor- phology of the electroless Ni film, after the dis- solution of Au or Pd, are shown in Figure 8. In the case of electroless Ni/Pd/Au film, no corro- sive pits were formed in the Ni layers, as shown in Figure 8a. On the other hand, in the case of electroless Ni/Au film, low reliability was ob- served, because of the formation of corrosive pits in the layers of electroless Ni, caused by the dissolution of Ni during the immersion plating of Au (Figure 8b) 17 . The TEM images of the cross section of the electroless plating film are shown in Figure 9. In the cross section of the electro- less Ni/Pd/Au film with 0.02 µm thickness of Pd (Figure 9a), there were no local corrosive pits at the interface between the electroless Pd and electroless Ni. We found that 0.02 µm thick electroless Pd film can be considered as an effec- tive barrier film to prevent the Ni layer from dis- SOLDER BALL JOINT RELIABILITY WITH ELECTROLESS NI/PD/AU PLATING Figure 6: Coverage of the electroless Pd plating film on electroless Ni plating film.