Integrated circuits or integrated circuit assemblies are employed in a wide variety of electronic applications. Increasing demand for high performance yet reliable electronic products that are ever smaller, lighter, and cost effective have led to corresponding demands on the manufacturers of integrated circuits.
Such circuits have traditionally employed a circuit board and a die connected by a plurality of connectors, wires and/or solder bumps. It has been known that differences between the coefficients of thermal expansion (CTE) of the circuit board and die contribute to early fatigue failure of solder interconnects, especially during thermal cycling of the circuit assembly. Differences in CTE are especially problematic for integrated circuits used in environments subjected to high temperatures, such as applications in close proximity to internal combustion engines, i.e., on board motor vehicle applications.
Epoxy resins have been used in the manufacture of integrated circuits. In some cases, such resins have functioned to anchor or adhere various components of the circuit and/or to mitigate the differences between the CTE's of the components of the circuit.
For example, epoxy resins have been utilized as under fill materials in the manufacture of integrated circuits having a flip-chip construction, i.e., ‘flip chips’. Under fill materials are intended to support and protect the electrical connections of the flip chip while simultaneously reducing the thermo-mechanical stress on the flip chip connections.
However, prior art epoxy resins have generally been unable to provide cured under fill materials having a desirably low CTE. Epoxy resins having a CTE of 60 ppm/° C. are especially advantageous in mitigating the differences between the CTEs of the die and circuit board. Suitable epoxy resins have often been achieved only with the use of significant amounts of CTE reducing fillers.
Unfortunately, the use of such fillers has traditionally resulted in increased manufacturing challenges and problems.
For example, circuit-manufacturing processes using capillary flow under fill technology typically require the injection of an epoxy resin based under fill composition into the interstitial spaces of an integrated circuit assembly. The presence of CTE reducing fillers in such compositions can result in an increased viscosity that impedes the flow and distribution of the under fill composition. Such processes are also often characterized as unacceptably long and/or costly.
In no flow under fill processes the epoxy based under fill material is typically applied to the surface of an integrated circuit substrate. To join a die to the substrate, the die's flip chip bumps are pushed through the under fill material until the flip chip bumps make contact with corresponding substrate bumps. In this case, filler particles can become undesirably trapped between the corresponding flip chip bumps and substrate bumps.
Thus, epoxy resin based compositions having low amounts of CTE reducing fillers are advantageous as compared to those having greater amounts of CTE reducing filler.
In addition, epoxy resins useful in the construction of integrated circuits must also have a reaction profile that accommodates the reflow profile of the solder used therein. In particular, it would advantageous if the solder reflowed before substantial crosslinking of the epoxy resin occurs. However, crosslinking must progress quickly once solder reflow has occurred.
Accordingly, it can be seen that prior art epoxy resins have failed to resolve challenges associated with the design and manufacture of integrated circuits, especially flip chips.