Effect of 9 wt.% in addition to Sn0.5Ag0.5Cu solder on the interfacial reaction with the Au/NiP metallization on Cu pads

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 Sn3.4Ag0.5Cu solder during extended reflow. During reflow, An diffuses rapidly in the molten Sn-Ag-Cu solder and forms AuSn4 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 An 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. (c) 2004 Elsevier B.V. All rights reserved.