Assembly and reliability issues associated with leadless chip scale packages

This paper addresses the assembly and reliability of 0.5 mm pitch leadless Chip Scale Packages (CSP) on .062″ immersion Ag plated printed circuit boards (PCB) using Pb-free solder paste. Four different leadless CSP designs were studied and each was evaluated using multiple PCB attachment pad designs. Assembly was performed by stencil printing solder paste over the PCB pads, placing components with a high-speed placement machine, and reflowing in a forced convection oven. Vision issues were encountered during the placement process and a "video model" was required to place certain leadless packages. Post assembly characterization included electrical measurements, visual inspection, X-ray inspection, and representative cross-sectioning to examine the 2 ^nd level solder interconnects. Assemblies were subjected to 20-minute 0-100°C air-to-air thermal cycles. Cycle to failure (CTF) data was found to be quite sensitive to optimization of the solder paste printing process and, possibly, to the board pad dimensions.