Flip-chip on flex for 3D packaging

In order to respond to the need for extreme miniaturization, a flip-chip process has been developed. Valtronic is using nonconductive adhesive to attach stud bumped dice without additional underfill. This original process can be applied on rigid and flexible PWBs and is described in this paper. Using a test vehicle, a qualification test program was conducted in order to quantify the limits of the process and verify all specifications required by specific markets such as industrial or medical. The flip-chip process adapts well to flexible polyimide substrates. This enables the flip-chip to be applicable to a wide range of applications, particularly for 3D packaging. First, dice are connected using a flip-chip process on a flat substrate on the flexible circuit. Next, the third dimension is obtained by bending and/or rolling the substrate on itself, accomplishing a 3D package. Finally, the package is completed by soldering on dedicated traces and filling with epoxy if required. This innovative package looks like CSP, where miniature solder balls allow the package to be attached to another substrate (motherboard) through the reflow process. Furthermore, the process offers the possibility of adding SMD components on the back side of the flip-chip areas. To develop this bending/folding method, a full scale module was assembled with daisy chain die. This module was used to evaluate the flip-chip interconnect reliability before and after bending. Finally, a cubical hearing aid module was developed and industrialized. Its final dimensions are 4.5×4.0×3.0 mm. The module contains 3 ICs and 18 SMDs