Evolution of Intermetallic Compounds in PBGA Sn-Ag-Cu Solder Joints during Thermal Cycling Testing

The present work was mainly focused on the evolution of interfacial intermetallic compounds (IMCs) between solder and Au/Ni/Cu metallization in PBGA Sn-Ag-Cu solder balls during thermal cycling test. The PBGA package was assembled on the FR-4 board using a typical surface mounting assembly process, and the thermal cycling was carried out in a systematic manner for two different temperature cycling profiles in a single chamber Heraeus climate cabinet. The first temperature profile ranged between 0degC and 100degC and the second between -55degC and 100degC. It was found that the type of IMCs existing in the solder matrix and on the PCB metallization was not affected by the thermal cycling. The growth rate of the interfacial IMC (Cu,Ni)6Sn5 was very low, and the thickness of the Sn-Ni-Cu ternary IMCs layer on the PCB metallization did not show a significant change during thermal cycling testing. For the temperature profile ranging between 0degC and 100degC and up to 6000 cycles, no fatigue cracks were found in the PBGA solder balls, while fatigue cracks could be observed after 3000 cycles, both at the solder/package side and solder/PCB side for the temperature profile ranging between -55degC and 100degC. The common failure mode of the solder joints analyzed in this work were cracks originating at the solder joint corners, firstly at the component solder mask defined (SMD) pad side and propagating within the solder along the PBGA metallization interface; no fatigue cracks were found to propagate through the interfacial intermetallic layer for all the cases (both on the package and PCB side). The cracks were found to propagate near and almost parallel to the interface of the bulk solder and the intermetallic layer. The ternary (Cu,Ni)6Sn5 IMCs layer attached very well to the substrate and no spalling could be observed