Electronics Assembly Knowledge, Vision & Wisdom
The Dynamics of Low Stress Epoxy Curing
The Dynamics of Low Stress Epoxy Curing
This study compares uniform variable frequency microwave (VFM) field cure to standard oven curing at temperatures below Tg (transition temperature).
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Authored By:
Robert L. Hubbard, Ph.D.
Lambda Technologies, Inc.
Morrisville, NC, USA

Iftikhar Ahmad, Ph.D.
Lambda Technologies, Inc.
Morrisville, NC, USA

David R. Tyler, Ph.D.
Department of Chemistry and Biochemistry, University of Oregon
Eugene, OR, USA

Summary
It is now possible to reduce the high stresses built into electronic assemblies by reducing the cure temperatures of epoxy adhesives and encapsulants. For the first time, an epoxy thermoset resin can be fully cured at temperatures well below its Tg (transition temperature). Even though the cure temperatures are low and the cure times are not long, the extent of cure is complete; the stress advantages of low temperature cure are obtained; and several improvements in thermomechanical properties result.

This study compares the use of a uniform variable frequency microwave (VFM) field cure to standard oven curing at temperatures below Tg (transition temperature). Since commercial flip-chip under-fills have already been shown to be cured with VFM at low temperatures with lowered stress and improved reliability, it has now been found that the levels of silica filler can be reduced and elastomeric additives eliminated. Thinned, molded, epoxy-embedded silicon wafers were also shown to have lower warpage and much lower shrinkage stress even when just the post-mold cure was done with VFM at lower temperatures.

Conclusions
The low temperature processing capability of microwaves allows the users of commercial epoxy composites to obtain desirable high Tg (transition temperature) values with low modulus at reduced stress levels. The low modulus from lowered crosslink density is also produced at lowered silica loadings and without the need for elastomer additives in a commercial resin. Low warpage and reduced shrinkage stress was also demonstrated in post-mold cures of molded wafer level constructions.

Initially Published in the SMTA Proceedings

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