Hi Mr. Aamir, it would be a great help if you could provide a reference showing that adding 0.4% wt La is coarsening the solder microstructure. Generally, the purpose of adding a fourth element is SAC based solder is to change the mechanical/wetting properties to increase the reliability. However, coarsening of solder microstructure does not help the cause. In terms of wettability, I have not seen anywhere that La helps increase wettability.

The following paper contains info regarding SAC based solder with La doping. Thought, it may help: http://iweb.tms.org/PbF/JOM-0606-57.pdf

I think it would be great if you could provide some reference on your statement to help us understand better. Thank you.

Happy Researching!!

Hi Mr Aamir. Can you tell us in which composition you have introduced 0.4%La? I agree with sound choudhury that basic concept of alloying with 3rd and 4th alloying element is to improve the mechanical, thermal and wetting properties. Usually refinement and coarsening occurs when the alloys are subjected to thermal aging. 

0.4wt% La is added to SAC305, Instead of refinement it coarsen the microstructure which was confirmed by IMCs average size and the mechanical properties including Yield and tensile strength reduces. Thermal aging was also done but on each temperature, doped alloy have minimum strength than the un-doped.

Hi, Aamir, Most of the time the RE materials greatly enhances the mechanical properties of the Lead free solder alloys. It works like vitamins for the metals. In most cases, it refines the grain size. Sometimes, it develops a characteristic like faceted geometry or so-called Chinese scripts looks like. Better to confirm the micro-structure or grain size under SEM.

Thanks. But sometimes more concentration of RE elements coarsen the microstructure. so the optimized point is necessary. It is cleared that the refinement was done due the differences of the atomic radii between Sn, Ag, etc and RE elements which can suppress the growth of IMCs but if the higher addition of Re elements is added it gives a coarsen microstructure so i just need the reason of this coarsening.

Did you check the cooling rate? micro structure also has a deep relation with cooling rate. Otherwise, "La" will never coarsen the structure.

There must be an optimized point, beyond which the properties will degrade...