It is known to directly attach solder bumps on integrated circuit die to bond pads on printed circuit boards to form solder bump interconnections. A gap is formed between the integrated circuit die and the printed circuit board in areas that are not covered by solder bump interconnections. Solder bump interconnections, however, are brittle and can fracture in use. In order to strengthen the solder joints without affecting the electrical connection, the gap is filled with an encapsulant material, which is typically formed of a filled polymer.
One common method of underfilling integrated circuit dies utilizes encapsulant materials to increase the reliability of solder interconnections by dispensing a polymeric precursor on a printed circuit board adjacent to an integrated circuit die. The polymeric precursor is heated to a first temperature and drawn into the gap by capillary action. The precursor is then heated to a second temperature to cure the precursor to form an encapsulant. This method is an improvement over unfilled assemblies, but has limitations. One limitation is the difficulty in getting the polymeric precursor, which is typically very viscous, to flow evenly under the integrated circuit die. The uneven flow leads to uneven underfilling of the integrated circuit die, which leads to solder connections that are susceptible to fractures. Wetting agents and flow modifiers have been used to enhance the flow of the precursor. These materials, however, dilute the precursor material and consequently degrade the properties of the subsequent encapsulant.
Therefore, a need exists for an assembly that is effective in reliably underfilling the gap between an integrated circuit die and a printed circuit board. Further, a need exists to underfill an integrated circuit die with an encapsulant not diluted by wetting agents or flow modifiers.