Developing a Repeatable and Reliable Rework Process for Lead-Free Fine-Pitch BGAs

Abstract

Area array components are increasingly used to miniaturize modern day circuitry. Assembling these surface-mount components using lead-free solder pastes has been the subject of interest for several years. Reworking a ball grid array (BGA) is complicated as the solder joints are hidden underneath the component. The process window to rework a BGA is narrow. There are several critical factors which complicate and affect the repeatability of the rework process. The objective of this research is to develop a reliable and repeatable process to rework lead-free fine-pitch BGAs. The process steps to rework a BGA are component removal, site redressing, solder paste/flux deposition, component replacement, and reflow. This study applied designed experiments to evaluate multiple alternatives for several rework process steps. Two alternatives for site redressing, namely copper wick with soldering iron and vacuum desoldering are evaluated. Similarly, the application of solder paste versus flux is compared. A localized reflow method at the rework machine is compared with solder reflow using a reflow oven. The pros and cons of using the two reflow methods and the effect of multiple reflow cycles on solder joint reliability is discussed. Nozzles with and without thermal shields were evaluated. A nozzle with deflectors on the outer side was found to work satisfactorily. Localized reflow in a semiautomated rework machine resulted in partial reflow of components adjacent to the reworked BGA. Thermal repeatability was also a concern. A 15degC drop in temperature was observed with the use of CoolCaps. The soldering iron with wick redressing method was less time consuming. Applying paste using a ministencil posed challenges due to lack of real-estate around the reworked BGA. A reliability study consisting of cross-sectional analysis, scanning electron microscopy (SEM), thermal shock, and thermal cycling were conducted to determine the reliability of the reworked solder joints. The copper wick with soldering iron method was favorable in terms of cycle time. The reflow oven is preferred as it avoids partial reflow of adjacent components. Localized reflow is effective when CoolCaps are used. The flux attachment process was easier as the real-estate around the reworked BGA is very limited. All the assemblies subjected to thermal shock and cycling passed the functional test.