A Repeatable and Reliable Rework Process for Lead-Free Fine Pitch BGA’s

Abstract

Fine pitch leadless components, such as Ball Grid Arrays (BGAs) and Chip-Scale Packages (CSPs), are increasingly used in modern day circuitry to aid miniaturization. Assembling these surface mount components using lead-free solder pastes has been a subject of interest for the past several years. Reworking a BGA is complicated as the solder joints are hidden underneath the component. The process window available for the rework process is very narrow and there are number of other critical factors, which complicate and affect the repeatability of the rework process. Consequently, the primary objective of this research endeavor is to develop a reliable and a 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 experimental study evaluates a number of alternatives for several rework process steps during the course of developing a reliable and repeatable rework process. Two alternatives for site redressing namely, (i) copper wick with soldering iron, and (ii) vacuum de-soldering methods are evaluated. Similarly the application of solder paste versus gel flux is compared. A localized reflow method for replacing the component at the SRT machine is developed and it is compared with forced convection in reflow oven. The pros and cons of using the two reflow methods and the effect of multiple reflows on solder joint reliability is discussed in the paper. A reliability study was conducted on the samples and the results are presented to compare the various alternatives.