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 20-3a 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. New fluxes that crosslink during reflow have been developed. They can be filled with ceramic or glass powders to achieve the required CTE and modulus. When these polymer fluxes are used in flip chips, a robust assembly is created without the need to underfill. We will report on a multilayer approach to flip chip assembly that eliminates the need for post-reflow underfilling