Effects of thermomechanical cycling on lead and lead-free (SnPb and SnAgCu) surface mount solder joints

Abstract

Accelerated reliability tests have been performed on leadless and leaded lead-free and lead containing SMT component assemblies. Results so far have shown that lead-free reflow soldering is a viable alternative for conventional lead based reflow soldering. The selected ternary eutectic solder alloy SnAg3.8Cu0.7 requires higher processing temperatures which could restrict the use of certain board and component types, but other than that no major modifications seem necessary. Although better SnAg3.8Cu0.7 bulk mechanical properties were obtained compared to the near eutectic lead bearing bulk solder properties, reflowed solder joints did not reflect this difference. In general, quite similar reliability results were obtained as found for the lead based solders. Dependent on board and component metallisations and use environment, the reliability of the lead-free solders could perform better or worse than the lead based solders. Temperature dependent aspects such as solderability and mechanical behaviour of the lead-free assemblies could play a role in this. Although microstructural differences can be seen between the lead-free and lead bearing solder joints, similar joint failure mechanisms occur. Resistor solder joint cracks propagate from underneath the component through either transgranular (lead-free) grains or along intergranular (lead) grain boundaries between lead-rich and tin-rich areas and into coarsened regions near the component terminations. Gullwing lead cracks were seen propagating from the heel fillet along the lead/solder interfacial intermetallic mostly (some cracks started in the heel fillet and propagated through the solder body dispersed with coarsened spherical Ag 3Sn intermetallic particles). Package design and leadframe material seem to play a more important role in the fatigue mechanism than the change in microstructure of the solder joint.