Low Temperature Curing of Epoxies with Microwaves

There is increasing interest in using lower temperature processing in microelectronics packaging today. Some new technology devices are intolerant of temperatures above 200°C or even 100°C. There are also devices that require a low thermal budget (time-temperature) to maintain performance tolerances. Finally there are package processes that are simplified by having the ability to maintain low temperatures at critical steps.

The abundance of uses for adhesives and encapsulants based on epoxy formulation has produced a wide range of materials from many suppliers. In general, however, there are physical properties that are sacrificed by curing these materials at temperatures lower than that recommended by the manufacturers. In other cases, a lower temperature may be used but the cure times must extend into many hours or days. Incomplete cure can produce unwanted brittleness, low Tg, or solvent permeability. Flip-chip devices with low-k dielectrics are very sensitive to degraded material properties of improperly cured underfills.

It has been well established that variable frequency microwave (VFM) technology can cure epoxy materials in a wide variety of applications at much faster times then that of convection or IR heating. Increasingly, there has been interest in using VFM to cure these materials at lower temperatures while retaining the full cure characteristics of the higher temperature cure at longer times.

This work describes the mechanism for microwave curing of various epoxy-based adhesives and the recent results found in curing them below even the "onset temperature" determined by DSC. The effect of different curing agents is discussed as well as the effect on uniformity of variables such as ramp rate, fixturing and material volume. Specific results and applications will be given. The stress-strain relationships will also be compared for a set of flip-chip underfills and die encapsulants that are cured with convection and also cured with VFM. The resultant elongation, modulus, and yield differences will be described with respect to the physical and chemical structures of epoxies cured with the two systems.