The reliability of protective coated electronic assemblies with regard to climate and harmful gas safety essentially depends on the freedom from interference and the closed nature of the coating. In particular, an edge loss at connection contacts and pore channels in coat pool areas have a damaging effect. Premiered at the SMT Hybrid Packaging show, the Zestron Coating Layer Test reveals defects in protective coatings or non-closed layers in so-called μ-coatings due to blackening. Thus, the test complements the standardized methods for coating thickness measurement by enabling quick and non-destructive proof of closed and dense coatings. The test can also be used in production for cost-effective randomized testing.