Underfill encapsulant technology for flip chip assembly

摘要

Direct chip attach (DCA) flip chip assembly offers many advantagesin terms of signal speed and performance, compactness, weight and longterm cost savings. The growth of portable electronics has focusedattention on the use of flip chip attach, where all of these virtues areattractive. Selection of uniquely compatible materials for advancedassembly and packaging is rarely possible and a compromise is nearlyalways needed. This is particularly the case with DCA of complex Sidevices using bumped technology for interconnection and mechanicalattachment to the substrate. While there is considerable interest in theuse of specialised conductive polymer adhesives, the implementation ofDCA mainly relies on metallurgical connection between chip and substratepad, whether it be the C4 solder bump process or the recent Au studtechnique. The inevitable CTE mismatch between chip and compositesubstrate must be managed for long term joint reliability. Early work inDCA/flip chip development has shown clearly the possibility of jointdislocation and failure when the device is thermally stressed,particularly under extended rapid temperature cycling. Polymer-basedunderfill encapsulant materials have been developed and formulated tominimise the effect of ΔCTE induced stresses which manage theproblem effectively by acting as a mechanical compensator layer betweenchip and substrate. This paper outlines the basic requirements of theunderfill material, the effects of CTE mismatch and methods ofapplication. An overview of typical problems with flip chip/underfillencapsulants is also presented