Folded stacked package development

The trend towards miniaturization of electronic devices has pushed the microelectronics packaging industry towards multi-die packaging where both the package footprint and the height are required to be as small as possible. This paper presents a folded stacked package that has many advantages including an extremely thin profile (four memory dies were successfully stacked with an overall height of only 1.2 mm), the ability to stack identical die, and a manufacturing process based on that of the familiar lead bonded μBGA® package. These stacked packages can also be made with wire bonded structure, but with some increased package height and overall dimensions. An additional required step, the substrate folding process, must be thoroughly understood in order to ensure high reliability. As the μBGA package has high reliability, the key to ensuring high reliability of the folded stacked package is to minimize any interaction due to folding between individual dies. A folding design rule has been developed for this purpose. Fast-cure adhesive materials were also evaluated to minimize the folding process time. To minimize overall package height, a 140 μm ball height was used and a compatible test socket contact mechanism was identified. Also, electrical and thermal performance of the package were determined to assess the suitability of this package for various applications.