Potting compounds are often used on printed circuit boards to improve reliability. In spite of this added protection, adhesive failures, otherwise known as delamination, can occur and lead to substantial problems from moisture ingress. Root causes of adhesive failures may be surface contaminants, inherently weak bonding between solder mask and potting material or thermal stresses that develop during temperature cycling.
In an adhesion analysis a combination of chemical and mechanical tests is often needed to determine the root cause. Chemical testing includes surface analyses such as XPS and SIMS; both reveal chemical groups available for bonding at the surface. Contaminants may be revealed during these tests. Basic tests such as FTIR, TGA and EDS provide information on the materials including fire retardants whose loadings are typically quite high, affecting adhesive bonding. Mass spectrometry can be used to identify suspected contamination areas. Ion chromatography is often used to identify weak organic acids, a byproduct from incomplete volatilization of no-clean flux. For determining an inherent compatibility of bonding surfaces, contact angle measurements using standard solvents, and surface tension measurements using the pendant drop method, can be very valuable.
Chemical testing must be done in conjunction with mechanical testing. The aim of the mechanical test is to reproduce the failure mode in the laboratory under controlled conditions. The easiest mechanical tests to perform are lap shear or peel strength tests. The lap shear test is straightforward to fixture and provides a method for comparing bond strengths among different boards or potting materials. A more sophisticated technique, the four-point bending test using pre-notched specimens, can be used to quantify adhesion energy, Gc.
For more information see:
Firas Awaja, Michael Gilbert, Georgina Kelly, Bronwyn Fox, Paul J. Pigram, Adhesion of polymers, Progress in Polymer Science 34 (2009) 948–968
Study of interfacial adhesion energy of multilayered ULSI thin film structures using four-point bending test, Zhenghao Gana, S.G. Mhaisalkara, Zhong Chena, Sam Zhangb, Zhe Chenc, K. Prasad, Surface & Coatings Technology 198 (2005) 85–89