Rework of Lead-Free Surface Mount Components

摘要

Implementation of lead-free solder into an electronics assembly process necessitates the reassessment of the individual factors involved in component attachment and rework. During rework, a component is subjected to multiple thermal cycles to remove and replace the component as well as in preparation of the rework site. With the use of lead-free components, the assemblies are subjected to multiple thermal cycles at higher temperatures than those required for eutectic tin-lead. This increase in temperature imposes additional constraints on the printed circuit board and component materials. The rework of area array devices is complicated by the fact that the solder interconnects are hidden underneath the body of the component. The assembled solder joint (of fine pitch CSPs) has a very small standoff height, adding additional process concerns for rework operations. Other concerns pertinent to rework are the temperature of the neighboring components during rework and the temperature of the component being replaced. Thermal shock to the component being replaced and the neighboring components should be avoided. In addition, the temperature of the neighboring components should be main ained below reflow temperature during rework. These issues, coupled with the limitations of rework equipment to handle lead-free reflow temperatures, make the task of reworking lead-free assemblies more challenging. Reworl processes that are developed for these assemblies are required to be robust. This implies that no damage is done to the PCB and component, and a repeatable process is developed, while preserving the reliability of the assembly and ensuring high throughput. This research, presented in this paper, is focused on rework processes for a variety of fine pitch Chip Scale Packages (CSPs) and passive surface mount devices. Tin-lead, tin-silver, and tin-silver-copper assemblies were included in this study. Comparisons between tin-lead and lead-free processes are presented. Reliability assessment of the reworked assemblies was performed utilizing air-to-air thermal cycling.