Super Thin Flip Chip Assemblies on Flex Substrates - Adhesive Bonding and Soldering Technology – Reliability Investigations and Applications

Thinned silicon chips with very thin bumps (5-7μm) mounted on flexible substrates open up new dimensions in packaging technologies. The use of flexible substrates enables a large variety of geometric possibilities including folding and bending. Conventional flip chip technology using pick&place and standard reflow processes is not suitable for the assembly of ultra thin components. This is based upon different technological reasons, as e.g. excessive bending of components (critical coplanarity) as well as wetting concerns and intermetallic phase formation of small solder volumes. According assemblies may be achieved using thermode bonding processes that allow for different solder and adhesive bonding technologies. If necessary, both types may fulfill lead free requirements. The work is focused on the comparison of the reliability between flip chip modules of less than 150 μm overall thickness bonded with solder and adhesive flip chip technologies. Assemblies with thin electroplated CuSn bumps bonded on flexible substrates with Cu metallization are compared with identical chip components with thin Ni bumps bonded with ACA (Anisotropic Conductive Adhesive). For the investigations in this study a test chip layout with 100μm pitch was used. Samples have been subjected to thermal cycling conditions, high temperature storage and temperature humidity test. Electrical measurements of Four-Point-Kelvin (4PK) resistances gave results on degradation of ultra thin flip chip contacts. SEM and EDX analysis have been performed to inspect intermetallic phase formation in ultra thin solder layers. As a successful application for ultra thin flip chip assemblies on flex, functional modules, assembled within a nationally funded project (funded by BMBF) are demonstrated.