Rework of Lead-Free Area Array Packages Assembled on Ultrathin Flexible Substrates

The introduction of stacking technology has enabled the memory industry to cope with the continuous demand for higher performance and greater capacity. Among other variants of stacking, module-level stacking is gaining steady popularity and acceptance as an option to fulfill this need. One of the approaches for module-level stacking is to fold a double-sided flexible substrate-based assembly around a metal core. Carrying out rework on a flexible substrate poses several unique challenges. Moreover, the tightly spaced assembly of lead-free area array devices in module stacking further increases rework complexity. As a result, the rework process of area array devices assembled on flexible substrate needs to be carefully controlled. This paper describes the attempts to develop a process for reworking area array devices on ultrathin flexible printed circuit boards (PCBs) with a tight component-to-component spacing of 0.38 mm. The research addresses several different aspects related to the rework of area array components on an ultrathin flexible PCB. This paper has been divided into two sections. The first section deals with initial screening experiments which include designing a rework fixture, development of thermal profiles, and the selection of a site-redressing technique. The second section of this paper evaluates different reflow methods, namely a rework station and a convection oven. Locations of the rework devices as well as the options of component attachment methods are incorporated into the experimental design. Based on the experiments performed and results obtained, a recipe for successful rework is presented. The key concerns and observations related to the issues and challenges associated with the rework process are also presented.