Dielectric properties of epoxy/silica composites used for microlectronic packaging, and their dependence on post-curing

We studied the dielectric properties (dielectric constant and loss factor) of epoxy molding compounds used for electronic packaging, as a function of frequency (100 Hz-100 kHz) and temperature (25-100 degreesC). Studies were performed for samples with different formulations (various silica and carbon black contents). At room temperature a loss peak is found at 50 kHz, whose intensity is enhanced by carbon black addition. Additional loss is detected below 1 kHz when the temperature is increased up to 100 degreesC. We also studied the influence of post-mold curing time (0-12 h at 165 degreesC) on the dielectric properties. The dielectric constant monotonically decreases with post-mold cure to level off to a minimum value for long post-cure durations. The loss factor first increases for short post-curing times, and then decreases as post-cure is continued. The origin of loss is discussed with reference to common relaxation processes observed in epoxy polymers.