Reliability, fatigue and mechanical characterization of lead-free solders for electronic packaging applications

Doctoral thesis, 2007

The ever increasing amount of electronic waste, most of which ends up in landfills, has become a serious worldwide concern. In addition to this, the fact that Pb is highly toxic and harmful to humans has lead to the ban of this element from the electronics industry. The main objective of this thesis is to give a better insight into some important issues in the field of lead-free solders reliability. Particularly, to understand how some of the most common lead-free solders behave in different conditions, both thermal and isothermal, in tensile, fatigue and shear modes, and both as bulk materials and solder joints. Moreover, facts like microstructure, crack initiation and propagation, and the effect of IMCs are also investigated in the context of this work. Different test methodologies and fatigue prognostic methods are also evaluated. A new lower cost alloy composition where the expensive silver was replaced by cobalt was also developed and mechanically characterized. The results show that all the lead-free solders tested performed somewhat better in LCF compared to the eutectic Sn-37Pb, except for the Bi and Zn containing alloys, which proved to have a very poor LCF behavior, mainly due to their higher strength and brittle failure mode behavior. Corrosion affects negatively the fatigue behavior of SAC solder joints and is more detrimental than both high temperature and humidity. Concerning the comparison between test methods, the data acquired from bulk fatigue tests can not be directly applied to solder joints as a result of the fatigue behavior being strain dependent and solder joints depicting lower fatigue lives at higher strains and higher fatigue lives at lower strains compared to bulk solders. Regarding the investigated fatigue prognostic models, is more accurate to use the energy based Morrow’s model since it accounts for both stress and strain and therefore can also account for the cross sectional area decrease resulting from crack propagation during fatigue testing. Moreover, since both the plastic strain range and the energy density change as a function of number of cycles, it is more accurate to take in to account all the cycles in the test when calculating the average value of these units. Subjecting lead-free SAC 388PBGA packages to thermal cycling results in random crack initiation and propagation. Moreover, the crack pattern in the solder ball array did not follow the DNP theory. The effect of two thermal fatigue profiles was also analyzed. The harsher profile resulted in earlier crack initiation and faster crack propagation rate. Finite element modeling corroborated the experimental results as the highest creep strain was measured in the solder balls situated in the middle of the PBGA package. Microstructural coarsening during thermal cycling was observed for both SAC and Sn-Ag alloys, however, only the SAC alloy showed an increase of IMC layer thickness. Furthermore, it was found that the shear strength of solder joints, subjected to thermal cycling, is a result of both microstructural changes and crack initiation and propagation. Shear strength measurements were found not to be a good measurement tool to indicate when failure takes place. The new developed alloy has a eutectic composition of Sn-0.4Co-0.7Cu and a melting temperature of 224 °C. The tensile strength of this bulk alloy was lower compared to both SAC and Sn-37Pb alloys. The elastic modulus was lower than the SAC, however, higher than the Sn-37Pb. This alloy showed, however, a stable microstructure, composed of large Sn-rich dendrites with dispersed CoSn2 and Cu6Sn5 IMCs.