Reflow-curable polymer fluxes for flip chip encapsulation

Generally, in the process of flip chip assembly on polymer-glass printed circuits, flip-chips are underfilled with a glass-powder-filled epoxy encapsulant and cured. The underfilling is required to produce reliable solder joints. The reliability of the final assembly hinges on the coefficient of thermal expansion (CTE) and modulus of the encapsulant. A CTE of 20-30 ppm/°C and a modulus exceeding 6 GPa are necessary to achieve the required lifetime of 2000-4000 thermal cycles at 25 to 125°C, yet the underfill process during assembly is expensive and time consuming. Manufacturers have been seeking an alternative approach involving fluxing polymers that can be applied to the chip prior to placement, eliminating completely the need for underfilling operations. To achieve this, new fluxes comprising synthetic organic acids that crosslink during reflow have been developed. When these polymer fluxes are used in flip chips, a robust assembly is possible