Board-level reliability of lead-free SnAgCu solder joint

Due to environmental concerns, the need for lead-free solutions in electronic components and systems is receiving increasing attention within the semiconductor and electronics industries. Sn-Ag-Cu systems are some of the more promising materials to replace the eutectic Sn-Pb solder. In this paper, a chip scale package assembled on board test and a notebook motherboard surface mount assembly were used to investigate the characteristics of Sn-3.0 Ag-0.5 Cu solder alloy from three different companies. One eutectic tin-lead solder was included for comparison. For the chip scale package, the PCB´s surface finish included OSP (organic solderability preservatives) and electroless Ni/immersion Au. OSP and immersion Ag PCB surface finish was used in the notebook motherboard surface mount assembly. In-circuit test and function test were used to evaluate the properties. Thermal cycling test was used to evaluate the reliability of the solder joints. Failure modes of Sn-Ag-Cu solder and Sn-Pb solder alloys were also investigated. Scanning electron microscope (SEM) and optical microscope (OM) were utilized to observe the fracture morphology; energy dispersive spectrometer (EDS) was employed to analyze the distribution of the elements and the intermetallic compounds (IMCs) formed at the interface. All the chip scale package samples passed the electrical test after 2500 thermal cycles. All Sn-Pb and Sn-Ag-Cu solder joints of CSP possess good board-level reliability. But the TCT results of notebook motherboard assembled by Sn-Ag-Cu were not so good. About one half of samples didn´t pass the function test after 500 thermal cycles. By failure analyses, we found some cracks in the joints and all the failed components were made by ceramic-based chip.