Effect of 0.5 wt % Cu addition in Sn–3.5% Ag solder on the dissolution rate of Cu metallization

The dissolution of thin film under-bump-metallization (UBM) by molten solder has been one of the most serious processing problems in electronic packaging technology. Due to a higher melting temperature and a greater Sn content, a molten lead-free solder such as eutectic SnAg has a faster dissolution rate of thin film UBM than the eutectic SnPb. The work presented in this paper focuses on the role of 0.5 wt % Cu in the base Sn–3.5%Ag solder to reduce the dissolution of the Cu bond pad in ball grid array applications. We found that after 0.5 wt % Cu addition, the rate of dissolution of Cu in the molten Sn–3.5%Ag solder slows down dramatically. Systematic experimental work was carried out to understand the dissolution behavior of Cu by the molten Sn–3.5%Ag and Sn–3.5%Ag–0.5%Cu solders at 230–250 °C, for different time periods ranging from 1 to 10 min. From the curves of consumed Cu thickness, it was concluded that 0.5 wt % Cu addition actually reduces the concentration gradient at the Cu metallization/molten solder interface which reduces the driving force of dissolution. During the dissolution, excess Cu was found to precipitate out due to heterogeneous nucleation and growth of Cu6Sn5 at the solder melt/oxide interface. In turn, more Cu can be dissolved again. This process continues with time and leads to more dissolution of Cu from the bond pad than the amount expected from the solubility limit, but it occurs at a slower rate for the molten Sn–3.5%Ag–0.5%Cu solder. © 2003 American Institute of Physics.

Effect of reflow profile and thermal cycle ageing on the intermetallic formation and growth in lead-free soldering

The formation and growth of intermetallic compound layer thickness is one of the important issues in search for reliable electronic and electrical connections. Intermetallic compounds (IMCs) are an essential part of solder joints. At low levels, they have a strengthening effect on the joint; but at higher levels, they tend to make solder joints more brittle. If the solder joint is subjected to long-standing exposure of high temperature, this could result in continuous growth of intermetallic compound layer. The brittle intermetallic compound layer formed in this way is very much prone to fracture and cold therefore lead to mechanical and electrical failure of the joint. Therefore, the primary aim of this study is to investigate the growth of intermetallic compound layer thickness subjected to five different reflow profiles. The study also looks at the effect of three different temperature cycles (with maximum cycle temperature of 25 0C, 40 0C and 60 0C) on intermetallic compound formation and their growth behaviour.. Two different Sn-Ag-Cu solder pastes (namely paste P1 and paste P2) which were different in flux medium, were used for the study. The result showed that the growth of intermetallic compound layer thickness was a function of ageing temperature. It was found that the rate of growth of intermetallic compound layer thickness of paste P1 was higher than paste P2 at the same temperature condition. This behaviour could be related to the differences in flux mediums of solder paste samples used.