Process Optimization Products

ZESTRON Coating Layer Test

Chemical test for localized detection of protective coating layer defects on PCBs

The reliability of protective PCB assembly coatings, with regard to climate and harmful gas safety, is essentially determined by the uniform application of the coating without interferences or gaps in the protective layer. In particular, common trouble areas such as solder joint edges and pore channels in coating pooling areas, when lacking a proper coating layer, have detrimental effects on the final board assembly.

The ZESTRON® Coating Layer Test utilizes a black color reaction as a visual indicator of the defects in the protective coating, even in the case of μ-coatings. The test thus adds the standardized methods for coating thickness measurement by enabling rapid detection of closed and dense coatings. The indicator liquid can also be used to check the readiness for solderability of components prior to soldering.

Therefore, the test can also be used during production for cost-effective sampling.


Key benefits of the ZESTRON® Coating Layer Test:

  • Quick, easy, and cost-effective method
  • More precise than black light/ UV inspection ⇒ thinner layers detectable
  • Test is part of the validation specifications of automotive OEMs
  • Shows coating defect and missing edge covering on the PCB
  • Avoids expensive cutting in the event of edge covering problems
  • Can be used as a test for solderability (especially for THT components)

Simple Test procedure in a few steps:

Rinse and dry
PCB cleaning before coating




  • Detection of protective coating layer defects
  • Solderability testing

Coating types:

  • Solvent coatings
  • µ-coatings
  • Parylene

Application surfaces:

  • Tin, Copper, Nickel/ Nickel containing compounds
  • Other less noble metals and alloys

Additional application: solderability test for components

The indicator liquid can also be used to check the readiness for solderability of components prior to soldering.

When the contact point is immersed in the test liquid, a black colour reaction occurs when the wetting ability is present. In this case, a clean and good quality metallization on the component can be assumed and the components can be used in production. The black colour has no negative effect on the solder joint.

If there is no visible colour reaction the intermetallic phase has grown through the surface making it more difficult to form a connection with the solder. This is especially common with chemical tin, and can result in weak solder connections, which can lead to PCB assembly field failure.


Immersing a THT connector                Black coloring = good solderability         Above: Reaction with test
in the test fluid                                                                                                             (good solderability)
                                                                                                                                       Below: no reaction with test
                                                                                                                                       (bad solderability)