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

Accelerated reliability tests have been performed on leadless and leaded le ad-free and lead containing SMT component assemblies. Results so far have s hown that lead-free reflow soldering is a viable alternative for convention al lead based reflow soldering. The selected ternary eutectic solder alloy SnAg3.8Cu0.7 requires higher processing temperatures which could restrict t he use of certain board and component types, but other than that no major m odifications seem necessary. Although better SnAg3.8Cu0.7 bulk mechanical p roperties were obtained compared to the near eutectic lead bearing bulk sol der properties, reflowed solder joints did not reflect this difference. In general, quite similar reliability results were obtained as found for the l ead 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 p lay a role in this. Although microstructural differences can be seen betwee n the lead-free and lead bearing solder joints, similar joint failure mecha nisms occur, Resistor solder joint cracks propagate from underneath the com ponent through either transgranular (lead-free) grains or along inter.-ranu lar (lead) grain boundaries between lead-rich and tin-rich areas and into c oarsened regions near the component terminations. Gullwing lead cracks were seen propagating from the heel fillet along the lead/solder interfacial in termetallic mostly (some cracks started in the heel fillet and propagated t hrough the solder body dispersed with coarsened spherical Ag3Sn intermetall ic particles). Package design and leadframe material seem to play a more im portant role in the fatigue mechanism than the change in microstructure of the solder joint. (C) 2001 Elsevier Science Ltd. All rights reserved.