Underfill encapsulant technology for flip chip assembly

Direct chip attach (DCA) flip chip assembly offers many advantages in terms of signal speed and performance, compactness, weight and long term cost savings. The growth of portable electronics has focused attention on the use of flip chip attach, where all of these virtues are attractive. Selection of uniquely compatible materials for advanced assembly and packaging is rarely possible and a compromise is nearly always needed. This is particularly the case with DCA of complex Si devices using bumped technology for interconnection and mechanical attachment to the substrate. While there is considerable interest in the use of specialised conductive polymer adhesives, the implementation of DCA mainly relies on metallurgical connection between chip and substrate pad, whether it be the C4 solder bump process or the recent Au stud technique. The inevitable CTE mismatch between chip and composite substrate must be managed for long term joint reliability. Early work in DCA/flip chip development has shown clearly the possibility of joint dislocation and failure when the device is thermally stressed, particularly under extended rapid temperature cycling. Polymer-based underfill encapsulant materials have been developed and formulated to minimise the effect of ΔCTE induced stresses which manage the problem effectively by acting as a mechanical compensator layer between chip and substrate. This paper outlines the basic requirements of the underfill material, the effects of CTE mismatch and methods of application. An overview of typical problems with flip chip/underfill encapsulants is also presented