Assembly and reliability of ultrathin flip chips on flexible substrates

Miniaturization enforcement of electronic modules in complex as well as low end applications is the driving force to integrate flip chip technology in common surface mount device processes. Thinned silicon chips with subsequent very thin bumps (5-7m) 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 assembly using reflow processes is not suitable for small solder volumes. Assemblies with total heights lower than 100m have been mounted using thermode bonding technologies on flip chip bonding equipment, concurrently the use of noflow underfiller was evaluated in case of reliability investigations. This work will give an overview on reliability results of flip chip assemblies with electroplated CuSn-bumps bonded on flexible substrates with Cu-metallization. The wafers have been thinned to 60m. The reduced bum p height offers the possibility for extreme fine pitch application as the tendency to short contacts can be lowered. No solder mask was applied. For the investigations in this study a testchip layout with 100m 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 analyses have been performed to inspect intermetallic phase formation in ultra thin solder layers. The reliability of thin solder joints is a key issue. Intermetallic phase formation has a larger influence because the intermetallics consume the majority of the solder alloy.