Lead-free solder in "Green" Electronic Assembly

Development for Sn-Ag-Cu (SAC) Pb-free Solders for Broad Application and High Reliability in “Green” Electronic Assembly

Near-eutectic Pb-free solder alloys that are based on Sn-Ag-Cu (SAC) have been implemented worldwide for electronic assembly as a robust replacement for leaded (Sn-Pb) solders that can become environmental toxins.  Eventually, reliability issues appeared in aggressive operating environments and with advanced assembly methods that involve slow cooling, e.g., reflow of solder spheres for ball grid array (BGA) joining of multichip modules. 

To develop a more universal SAC solder, micro-alloying of Cu-enriched Sn-3.5Ag-0.95Cu (wt.%) solder with Al, Mn, and Zn was developed for controlled heterogeneous nucleation of solder solidification to produce consistent joint microstructures with improved reliability, even with multiple reflows. Also, globule wetting balance studies of solderability for these SAC3595+X alloys at concentrations between 0.01-0.25wt% established that normal SAC wetting was maintained for Cu substrates. 

Preference has developed for the Al and Zn additions due to their compatibility with cooling rate variations and either air or N2 reflow atmospheres.  Thus, further studies will focus on BGA testing of SAC+Al and SAC+Zn spheres, compared to a common SAC composition, Sn-3.0Ag-0.5Cu (wt.%).  After receipt of mounted spheres arrays that will be processed by Nihon-Superior (at a subcontractor), long-term BGA thermal cycling tests (-55˚C/+125˚C) will be started at Rockwell-Collins. 

Also, “drop-shock” impact testing of BGA spheres will be arranged by Rockwell-Collins with another subcontractor, as another important joint performance test. For this project, samples from these cycling and impact tests will be metallographically prepared and observed by optical and scanning electron microscopy (SEM) to observe test-induced joint microstructure development and failure pathways.  Analysis of this data, especially focused on any partially failed joints or grab samples, will help establish any benefits of the Al or Zn on the BGA joint reliability, compared to consumer electronic and avionics performance standards, building on past thermal cycling work with SAC+Zn. 

Program mentor: Iver Anderson, adjunct professor, Ames Laboratory and Materials Science and Engineering, Iowa State University