The popular conventional coatings such as silicone, epoxy, urethane, and acrylic, that are applied in a liquid state (immersion, brushing, or spraying) tend to change their volume during vulcanization or polymerization. Some of the materials that penetrate under the components in the gaps between the PCB and the component itself will cause lifting and disconnection from the PCB. On the other hand, materials that tend to shrink during polymerization will draw two adjacent components together and will thus cause breaks or shorts.
In contrast, parylene (or type XY as specified in MIL-I-46058C) that is applied during the vacuum coating process (CVD) does not apply any kind of stresses whatsoever to the electronic components and thus guarantees preservation of the electrical and mechanical characteristics of the component over the years.
It should be noted that use of conventional conformal coatings, even if it does not lead to tearing and/or lifting of components, introduces stresses into them during polymerization and is liable to cause changes to their electronic/ mechanical characteristics. This is also one of the reasons why parylene has been found to be a very effective coating in reduction of the appearance of tin whiskers.