Separation And Controlling Of Intermetallic Compounds In Lead-free Sn-Ag Solders

The formation and distribution of Intermetallic Compounds (IMCs) in the microstructure of lead-free solders which acts as the thermal, electronic and mechanical connections, directly affect the soldering performance during micro- electronic packaging process. In the present paper, near eutectic composition Sn-Ag alloys were selected and the separation of IMCs in the solidification processes of them was systematically explored by changing the content of ingredient and the subsequent solidification rate of the alloy melt. The formation mechanism of bulk Ag₃Sn IMCs in solidification was clarified according to the competitive growth of two eutectic phases by means of metallographic analysis, calorimetric measurements and thermodynamic calculation. The evolution of IMCs in the microstructure of the solders working under high-temperature environment was investigated by high- temperature aging treatment. As last, the governing of IMCs and the corresponding strengthening effect was discussed in view of the addition of the third component and ZrO₂ nanoparticles in the solder. More details were given as follows.Firstly, the formation mechanism of IMCs in hypoeutectic, eutectic and hypereutectic Sn-Ag alloys under different solidification rates was systematically investigated by changing the cooling medium of the solder. The results indicate that the bulk Ag₃Sn IMCs only formed in slowly-cooled hypereutectic alloy. All solidified alloys exhibit typical hypoeutectic features, that is, primaryβ-Sn dendrites and lamellar eutectic structure of (β-Sn + Ag₃Sn), which could be explained by the actual metastable pseudoeutectic reaction resulting from the kinetic undercooling in the condition of the non-equilibrium solidification. The influence of applied solidification rate on the alloy structure fits well to the prediction of classical theory of dendrite growth in eutectic system: the Secondary Dendrite Arm Spacing (d ) of theβ-Sn dendrites decreases with increasing the solidification rate and can be described by d = 3. 7t0f.43. It is also found out that those Ag₃Sn nanoparticles distributing in eutectic zone is benefit to improve the microhardness of alloy, which agrees well with the prediction of Dispersion Strengthening Principle.A new valid method, to determine the volume fractions of bulk Ag₃Sn IMCs in the solidified microstructure of the solder, was firstly developed from the formation rate of solid phase from the measured apparent heat capacity data during the slow cooling process. The thus obtained fractions of bulk Ag₃Sn IMCs in the hypereutecticalloy were compared to that from the metallographic analysis and thermodynamic calculation. During solidification, the leading Ag₃Sn phase could nucleate adhering to the primary Ag₃Sn phase due to their matching crystalline orientation relationships, leading to the formation of bulk Ag₃Sn IMCs at small melt undercooling, then the fraction of bulk Ag₃Sn IMCs increased gradually with increasing the applied cooling rate.Next, the structural stability of Sn-3.5Ag solder during serving in high- temperature environment was explored by means of high-temperature aging treatment. It is observed that the Ag₃Sn phase had broken up and spheroidized in the equilibrium specimen, while the Ag₃Sn phase combined to bulk Ag₃Sn IMCs after the boundary migration of primaryβ-Sn dendrites in non-equilibrium specimen during high-temperature aging. The evolution of the Ag₃Sn IMCs during annealing fits with the minimization of total Gibbs energy. The precise determination of heat enthalpy difference and structure analysis show that the driving force for the growth of Ag₃Sn nanoparticles is coming from the high surface energy when the alloy is in a metastable state. However, the non-equilibrium structure of the solder maintains to be stable for the Ag₃Sn nanoparticles distributing only in the eutectic zone.At last, the possible governing of the formation of bulk IMCs was proposed by adding a small amount of third component (Cu, In and Zn) or ZrO₂ nanoparticles in the solder to avoid the formation of the bulk Ag₃Sn IMCs in slowly-cooled Sn-Ag alloy which may bring serious problems in solder joint. The results indicate that only the addition of Zn can suppress the formation of bulk Ag₃Sn IMCs. This observation could be attributed to the formation and separation ofβ′-AgZn phase, which will decrease the melt undercooling significantly at small undercooling in Sn-3.5Ag-2.0Zn alloy. Hence, the powerful method in governing the formation of bulk Ag₃Sn IMCs by adding third element in Sn-Ag alloy is to select a possible element which can react with Ag element and form other IMCs before the separation of Ag₃Sn phase during solidification. As an effective surface-active material, the introduction of ZrO₂ nanoparticles can suppress the formation of bulk Ag₃Sn IMCs by reducing the surface energy of Ag₃Sn according to the Surface Adsorption Effect. Hence, they can greatly refine the microstructure and improve the mechanical property.