Probing lead-free solders in electronics

Materials World magazine
,
1 Aug 2009

An engineer at the University of Leicester, UK, claims to have revealed, for the first time, the formation of crystallograhically-facetted voids in the ß-Sn matrix of lead-free solders during soldering and ageing.

The discovery occurred during research to improve the reliability of lead-free solders in electronics. The targets are SAC (Sn-Ag-Cu) alloys of tin, silver and copper.

Sergey Belyakov says, ‘This phenomenon has not been described yet [but] the crystallographically-facetted voids are an undesirable effect that need to be terminated. We are developing a theory that could explain [it], based on which, we could [eventually] give recommendations on soldering conditions and materials to be used. So far I can state that thin substrates (metallisation pads of about <50µm) can retard this voiding’.

Due to the toxicity of lead, efforts have been made to replace traditional soldering alloys with new compositions. The alloys explored in the present study are SAC405, SAC305 and CASTIN (the latter also contains antimony), as well as a less established alloy – SN100C (silver, copper, nickel and germanium).

‘The microstructure of a traditional tin-lead solder is represented by a mixture of tin-rich and lead-rich phases, [while] a vast amount of new environmentally-friendly solder materials have ternary or quaternary compositions, which result in intermetallic phases whose physical and mechanical properties differ from the ß-Sn matrix,’ explains Belyakov. This reduces microstructural homogeneity and leads to a deterioration of solder joints and, in turn, the reliability of the electronic equipment.

Belyakov’s research, which he acknowledges is in the same vein as other experimental work in the area, has focused on investigating approaches to suppress intermetallic phase formation. Discovery of the crystallographically-facetted voids was therefore a ‘new’revelation, he says.

Research into the influence of ultrasonic vibration on solder microstructure has, for example, demonstrated that high intensity ultrasound does promote fine and equiaxed structure formation, which results in solder microstructure anisotropy decrease and zonal segregation inhibition. Ultrasound also reduces eutectic phase formation during crystallisation and promotes more uniform distribution of intermetallic particles.

However, Belyakov notes some of these positive effects of ultrasound ‘vanish’ when considering the complete solder-substrate system (solder joint). This is ‘due to intensification of the diffusional processes at the substrate interface’.

Further work will now examine the crystallographically-facetted voids phenomenon and the SN100C system. Inventor of this alloy, Nihon Superior Co Ltd, based in Osaka, Japan, is sponsoring the work. The SN100C ‘will probably replace the most commonly used SAC alloy family due to features such as solder shrinkage reduction and ductility increase’, proposes Belyakov. ‘Extensive research needs to be done to develop an understanding of solidification paths for different compositions – the SN100C has a larger amount of intermetallic phases and some of those are metastable.’