Effect of 9 wt.% in addition to Sn3.5Ag0.5Cu solder on the interfacial reaction with the Au/NiP metallization on Cu pads

Abstract

Sn-based lead-free solders have a high melting temperature, which often cause excessive interfacial reactions at the interface. A small amount of In is added to reduce the melting temperature and to change the intermetallic compound (IMC) phases. Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu9In lead-free solder alloys have been used to identify its interfacial reactions with 2-metal layer flexible substrates. In this paper we investigate the effect of 9 at.% In addition to Sn3.5Ag0.5Cu solder during extended reflow. During reflow, Au diffuses rapidly in the molten Sn Ag Cu solder and forms AuSn 4 IMC but in the case of In-containing solder, In Sn Au IMCs form and are uniformly distributed in the solder. Some In Sn Au IMCs have been entrapped in the Sn Cu Ni In quaternary intermetallic compounds (QIMCs) due to lower diffusion rate of Au in the In-containing solder. Initially Sn Cu Ni ternary intermetallic compounds (TIMCs) and Sn Cu Ni In QIMCs form at the interface, which have higher growth rate and consume more of the NiP layer. Low-Cu QIMCs are found in the In-containing solder after 30 min reflow which are more stable in the P-rich Ni layer and significantly reduce the dissolution rate of the NiP layer. The spalling of Sn Cu Ni TIMCs in the Sn Ag Cu solder increases the diffusion rate of Sn atoms and as a consequence both the TIMCs growth rate and dissolution rate of the NiP layer also increases. In-containing solder have lower growth rate of the QIMCs and lower dissolution rate of the NiP layer than the Sn Ag Cu solder. Consumption of the NiP layer can be reduced by adding In, because of the formation of QIMCs at the interface, QIMCs are stable and are well adhering to the P-rich Ni layer during reflow.