Formation of Interfacial Products

The reliability of the joints in the functionality and reliability of electronic components are strongly dependent on the interface reactions that occur between solders and substrate materials. The intermetallic compounds at the interface, especially the one that forms first, is critical to the mechanical performance of the soldered joints. Here, using the TCS Solder Alloy Solutions Database (TCSLD), the calculation tries to predict the first formed interfacial compound during soldering through calculating and comparing the driving force of formation.

This scheme for predicting the first formed compound was proposed by [1997Lee], who suggested that during soldering, the phase which forms first at the interface is the phase with the highest driving force of nucleation from the solder liquid in metastable equilibrium with the substrate. The driving force is thus the Gibbs free energy difference between the set equilibria, i.e. metastable local equilibrium at the interface between the substrate and liquid solder, and the possible forming phases.

The table lists predicted first formed interface reaction product compared with experiments [2008Jao; 2010Jee] . The calculations successfully predict for all compositions except for the Sn-3 wt.%Zn alloy. It should be noted that at w(Zn)=3%, the driving forces of Ag₅Zn₈ and AgZn are very close to each other, as shown in the figure above. In such a case, other effects, e.g., interfacial energy, misfit strain energy etc., should also be considered. Nevertheless, thermodynamic calculations are a very good starting point to obtain information on possible IMC formations and to understand the interface reactions.

Sn-xZn/Ag couple, 260 °C
Alloy	Calculation	Experiment (22,23)
Sn-1 wt.%Zn	Ag3Sn	Ag3Sn
Sn-2 wt.%Zn	AgZn	AgZn
Sn-3 wt.%Zn	Ag5Zn8	AgZn
Sn-5 wt.%Zn	AgZn3	AgZn3
Sn-9 wt.%Zn	AgZn3	AgZn3
Sn-14 wt.%Zn	AgZn3	AgZn3

References

[1997Lee] B.-J. Lee, N.-M. Hwang, H.-M. Lee, Prediction of interface reaction products between Cu and various solder alloys by thermodynamic calculation. Acta Mater. 45, 1867–1874 (1997).

[2008Jao] C.-C. Jao, Y.-W. Yen, C.-Y. Lin, C. Lee, Phase equilibria of the Sn–Zn–Ag system and interfacial reactions in Sn–Zn/Ag couples. Intermetallics. 16, 463–469 (2008).

[2010Jee] Y. K. Jee, J. Yu, Interfacial Reactions and Joint Strengths of Sn-xZn Solders with Immersion Ag UBM. J. Electron. Mater. 39, 2286–2291 (2010).