| A power cycling apparatus, capable of simultaneously assessing the thermal performance and long-term reliability of up to four die-attachment interfaces has been developed and utilized to evaluate various types of bonding materials. A standard lead-based solder (SnPb), lead-free solder (SnAgCu), eutectic braze alloy (AuSn), a polyimide (Epo-Tek's P-1011), a conductive epoxy (Epo-Tek's H20E), and a nanoparticle silver paste are the materials chosen for comparison. In addition, a preliminary investigation into the thermal performance and reliability of a novel, lead-free "universal" solder for power electronics packaging is included. A direct comparison is made between this material, a Sn3.5Ag0.5Cu-RE (SAC-RE) solder with small additions (0.5 to 2.0 wt%) of a rare earth element, and the conventional solder, Sn3.0Ag0.5Cu (SAC-305). Depending on the form of each joining material, the bonding was either performed manually in atmosphere or inside a vacuum-pressure furnace. Scanning acoustic microscopy and/or x-ray analysis are used for void characterization of each interface. Before and after power cycling assessments and die-shear analysis are used to assess the strength of bonded joints. Failure analysis showed a correlation between a 10% increase in thermal impedance and significant void growth for SnAgCu solder. Despite a large percent voiding before power cycling, AuSn performed without failure to 20K cycles. Die cracking and debonding were experienced with H20E and nanoparticle silver paste, respectively. Improvements were not observed with SAC-RE over SAC-305 in regards to bonding strength, however, power cycling performance of the "universal" solder continued to up 38K cycles without failure at the bonding interface. |
