Journal of SMT Article

ELECTROLESS Ni/Pd/Au PLATING FOR PACKAGE SUBSTRATES WITH FINE PITCH WIRING

Authors: Yoshinori Ejiri, et al.
Company: Hitachi Chemical Co., Ltd.
Date Published: 10/31/2009   Volume: 22-4

Abstract: Electrolytic Ni/Au films are typically used for solder ball and Au-wire bonding at the Cu terminals of semiconductor package substrates. We investigated the minimum Ni film thickness required for solder ball joint reliability after thermal aging. We verified the joint reliability using a high-speed solder ball shear test which is highly correlated with drop tests. We found that the minimum Ni-P thickness of electroless Ni-P/Pd/Au that can afford the required solder ball joint reliability is 1 µm, whereas in the case of electrolytic Ni/Au, the minimum Ni thickness is 3 µm. This result indicates that electroless Ni-P/Pd/Au plating is more suitable for highdensity wiring, as compared to electrolytic Ni/Au plating.

Furthermore, we investigated the cause of the poor ball-joint reliability of the case of electroless Ni-P/Pd/Au whose Ni-P thickness was thinner than 1 µm, comparing with that of bare Cu. In the case of the 0.1-µm-thick Ni-P, Cu-Sn intermetallic compounds (IMCs) of (Cu, Ni)3Sn and (Cu, Ni)6Sn5 were formed after thermal aging. It was found that the average thicknesses of the Cu-Sn IMCs in the case of bare Cu and 0.1-µm-thick Ni-P were equal regardless of the thermal aging time. However, in the case of Cu-Sn IMCs containing microscopic amounts of Ni, the growth of (Cu, Ni)3Sn was prevented but that of (Cu, Ni)6Sn5 was promoted. It was clear from focused ion beam / secondary ion microscopic (FIB/SIM) and electron back-scatter patterns (EBSP) observations that the grain sizes of (Cu, Ni)3Sn and (Cu, Ni)6Sn5 were smaller than those of Cu3Sn and Cu6Sn5, respectively. We consider that the small grain size of (Cu, Ni)3Sn and (Cu, Ni)6Sn5 IMCs with a lot of grain boundaries might be the reason for the poor reliability of the solder ball joints. In the case of the 0.3-µm-thick Ni-P, the Ni-P film remained after the solder ball reflow process; however, voids formed and grew at the Cu/Ni-P interface with the thermal aging time. We suppose that the voids may be the cause of the poor adhesion at the interface.

Key words: Electroless Ni-P/Pd/Au, Solder joint reliability, Intermetallic compound (IMC), Thermal aging test, High-speed solder ball shear test



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