A Comparative Study of Failure Mechanisms of Sn-Ag-Cu Assemblies under Temperature Cycling and Board Level Drop Test

The reliability performance and failure mechanisms of lead-free electronic assemblies under temperature cycling and board level drop test was investigated. TSOP (thin small outline package) devices with FeNi leads were reflow soldered on FR4 PCB (printed circuit board) with Sn3.0Ag0.5Cu (wt%) solder paste. The effects of different PCB finishes (organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG)) on the reliability performance were studied. The results show that the assemblies with OSP finishes outperform its ENIG counterparts under temperature cycling, however, the ENIG assemblies reveal better reliability performance than those with OSP finishes under drop test. The failure mechanism is different under these two test conditions: the solder joints fracture into the intermetallic compounds (IMCs) layer under drop test, and cracks initiate in the bulk solder near to the interface under temperature cycling. The surface finishes also have an effect on the failure mode under temperature cycling. The propagation of crack in the ENIG case is along the device/solder interface, while in the case of OSP, the crack extends parallel to the solder/PCB interface.