Degradation Reaction
Moisture-induced chemical breakdown cleaves ester linkages in polymer chains through nucleophilic attack by water molecules. Within electronic assemblies and printed circuit board materials, ester hydrolysis degrades polyester-based conformal coatings, flexible substrates, and specific resin systems exposed to high humidity and elevated operating temperatures. This breakdown yields carboxylic acid groups and hydroxyl functional groups.
The resulting acidic fragments accelerate corrosion of adjacent copper traces and solder joints. Standard accelerated aging tests such as temperature humidity bias screening detect the onset of dielectric breakdown from this chemical degradation.
Chemical Mechanism
Water diffusion into the polymeric matrix initiates the cleavage of covalent bonds along the ester backbone. Autocatalysis frequently occurs when newly formed carboxylic acids lower the local pH, accelerating the rate of subsequent ester hydrolysis reactions under sustained bias. Elevated ambient temperatures increase the reaction velocity following the Arrhenius relationship.
Flexible printed circuits composed of polyethylene terephthalate suffer severe embrittlement as molecular weight falls. Solder mask layers containing ester linkages lose adhesion to the underlying copper foil as bond scission progresses. Fourier transform infrared spectroscopy tracks the decay of ester carbonyl absorption bands to quantify degradation extent.
Assembly Impact
Conformal coatings experience loss of electrical insulation resistance following prolonged hydrolytic breakdown. Dielectric breakdown failures emerge when ionic contaminants dissolve into the moisture laden, degraded polymer matrix. Selecting hydrolytically stable chemistries such as polyurethane or silicone prevents premature field failures in automotive environments.
