Research Of Lead-free Solders In Microelectronic Joining

With the development of society and people's increasing awareness of environmental protection, people are paying more attention to control of lead pollution , so legislations to limit the use of solders containing lead in electronics assembly have been introduced by many countries. Therefore, the development of lead-free solders replacing traditional Sn-Pb solder has become the focus of study in material field. Right now there is no lead-free solders with self-determined property right in our country. As an important export country of home appliances, it is restricted by other nations in production and distribution. So it has important economic and social meaning to study lead-free solders with self-determined property right, good performance, low costs.In this paper, Cu, Bi and Ce are put into Sn-Zn based alloy to improve lead-free solder's wettability by alloying . Spreadability and DSC tests are performed ,as the results: Cu, Bi and Ce can improve solder alloy's wettability by putting into the based alloy; especially Bi can reduce the solder alloy's melting point as well. Study indicates: lead-free solder alloy composition of Sn+4%Zn+0.8%Cu+5%Bi+a small amount of Ce, has excellent wettability and mechanical properties ,of which the melting temperature range is from 189.6℃ to 193.8℃, tensile strength is 53.6Mpa and elongation rate is 39%.Through spreadability test and dynamic analysis of solder melting process ,it is found that: flux matched with solder is colophony containing ZnCl₂- Rate of corrosion of the flux is 0.0014 g/cm² and its non-volatile residue is 58.6%. So postwelding pieces need to be cleaned. Solder begins melting at 198℃, and totally melts at 230℃, the proper soldering temperature is at 250℃ and the holding time of soldering is 15min.Analytical instruments such as XRD, optical microscope, SEM, EDS, electron probe and universal test machine are employed to analyze microstructure, compositionand properties of the soldering joint. It is found that: microstructure of Bi -rich phase exists in terms of layer tables ,which reduces strength of base alloy and the shear strength of soldering joint; Cu put into based alloy restricts nucleation and coarsening of Cu-Sn brittle compound, which can improve mechanical properties of soldering joint. Zn put into based alloy limits diffusion and combination of Cu and Zn, which restricts growing of thickness of Cu-Sn brittle compound layers and improves the shear strength of soldering joint. Rare-earth Ce can refine microstructure of the soldering joint, then the shear strength of soldering joint can be improved. The shear strength of soldering joint increases 5.8Mpa, when percentage composition of Ce is 0.3%.In conclusion , a kind of innoxious lead-free solder with better wettability , good mechanical properties and low costs is obtained finally, by alloying method which can improve the properties of the soldering alloy.