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60 SMT Magazine • January 2017 SOLDER BALL JOINT RELIABILITY WITH ELECTROLESS NI/PD/AU PLATING less Ni/Au film. In the case of 0.005–0.015 µm thick Pd film (Figure 10b), the thin Pd plating points were attacked by the immersion gold plating, and there was local corrosion in the electroless Ni layer. The joints with 0.005–0.015 µm thick Pd film were better than those with electroless Ni/Au film. With a Pd film thickness of 0.02 µm or more (Figure 10c), the absence of corrosive pits in the Ni layers might have con- tributed to excellent solder joint reliability. Growth of the IMCs at the Solder Ball Joint Interface (1) Influence of Reflow Cycles The SEM observations of the cross sections of the IMCs for various Pd thicknesses and re- flow cycles are shown in Figure 11. When the Pd thickness was 0.015–0.3 µm, dendrite layers of IMCs were formed at the interface of SAC305 and the Ni plating film. On the other hand, when the Pd thickness was 0.5 µm, flat layers of IMCs were formed. To investigate the growth of the IMCs in greater detail, the cross section and surface morphology of the IMCs after the reflow cycles were observed on 0.1 µm and 0.5 µm thick Pd films. The results are shown in Figure 12. When the thickness was 0.1 µm, thin dendrite layers of (Cu, Ni, Pd)6Sn5 IMCs were formed after one reflow cycle. The size of the dendrite layers of the IMCs increased with increasing number of reflow cycles. After seven reflow cycles, thick dendrite layers of (Cu, Ni, Pd)6Sn5 IMCs [Fig- ure 12 I (Cu : Ni : Pd : Sn=23.5 : 18.1 : 0.2 : 58.2 wt%)] were formed. In the case of 0.5 µm thick Pd film, two types of IMC compositions were ob- served: one is (Cu, Ni, Pd)6Sn5 [Figure 12 II (Cu:Ni:Pd:Sn=23.4:16.5:1.2:59.0 wt%)], and the other is Cu/Ni/Pd/Sn [Figure 12 III (Cu:Ni:Pd:Sn=1.4:3.4:11.1:84.1 wt%)]. The shape of (Cu, Ni, Pd)6Sn5 IMCs was granu- lar. These results suggested that the Cu/Ni/ Pd/Sn IMCs, which included a high concen- tration of Pd, affected the shape of (Cu, Ni, Pd)6Sn5 IMCs. After seven reflow cycles, (Cu, Ni, Pd)6Sn5 IMCs increased in size and showed a more planar morphology [Figure 12 IV (Cu:Ni:Pd:Sn=23.3:16.5:0.8:59.4 wt%)]. Figure 10: Cross-sectional models of plating film and solder joint interface depending on Pd thickness.