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The Chemistry of Low Stress Epoxy Curing
Keywords: stress, epoxy, curing
The properties of thermoset polymers are largely determined by the profiles that are used to cure them. Final adhesion, Tg, CTE, moduli, chemical resistance, and thermomechanical stability are primarily constrained by the extent of cure in the polymer network. Adhesion, for example, is exactly proportional to the extent of cure. At gelation, an infinite network chain is produced that becomes rigid and further crosslinking requires increasing heat to continue. For this reason, continued extent of cure requires a temperature above the cure Tg. If the temperature is not increased then the network becomes a solid glass (vitrification). This glass is thermally reversible, so subsequent higher temperatures can continue the reaction towards full cure and the ultimate Tg (Tg∞). There are many epoxy-based thermosets in microelectronics assembly processes. It is often the case that practical manufacturing issues limit the resin cure temperatures, which in turn reduces the extent of cure, which substantially affects the properties listed above. At the same time, there are increasing pressures to reduce stress in the assembly of increasingly CTE mis-matched materials by lowering adhesive and structural thermoset cure temperatures. If a partially cured epoxy is later heated to higher temperatures (like solder reflow) the cure continues and the above properties are modified. This has been the cause of many reported assembly failures in adhesion, cracking, and voids. The amount of stress in epoxies can be reduced by lowering the cure temperature as long as it is still above Tg∞. The use of variable frequency microwave (VFM) curing of epoxy resins has been reported to produce the full extent of cure at temperatures low enough to substantially reduce stress and warpage of microelectronic assemblies. It is now reported that VFM can further reduce the cure temperatures of epoxies to well below the Tg∞ with full extent of cure. This capability is not possible with standard oven curing because the lack of polymer chain mobility below Tg∞ creates vitrification of the polymer network. The simple combination of two popular epoxy components, BFDGE and MDA, which normally requires a 170C cure for full extent of cure, was cured with VFM at temperatures between 80C and 140C. Extent of cure was measured by Tg (DMA), tan maximum, and FTIR and compared to literature values. At all cure temperatures down to 100C, the extent of cure was complete and thermomechanical properties were identical with high temperature oven curing. It is likely that the inherent heating mode of microwave rotation of polarizable dipoles is the primary reason for high chain mobility even after gelation. There was only found evidence of some vitrification at the 80C cure temperature with VFM. To investigate the differences between the reaction mechanisms of thermal and microwave curing in the early reaction phases of the sol region, the pre-gel reaction products were directly followed with 13C-NMR spectroscopy. It was found that the VFM cure produces predominately chain extension with smaller amounts of crosslinking at the low cure temperatures compared to random crosslinking with standard oven heating.
R. L. Hubbard, Director, Technology Development
Lambda Technologies
Morrisville, North Carolina

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