# Method to Predict Performances of PCB Silicone Conformal Coating under Thermal Aging
###### Abstract  
The stability of print circuit board (PCB) conformal coating is critical to guarantee the long-term performance of electronic components on PCB boards. Coating exposure to thermal shock or temperature cycles may initiate cracks, a common failure mechanism of conformal coatings. Different simplified approaches are compared to help identify desired mechanical profiles for coatings to be used in a harsh environment, focusing on silicone characterized by low rigidity and high deformability compared to alternative chemistries. Evaluation of the bi-material strip bending test method appears not to be effective in the conformal coating selection. The large difference between the coating's elastic modulus of silicones compared to substrate modulus allows the use of a simplified formula to calculate the stress associated with the coefficient of thermal expansion (CTE) mismatch, the silicone accommodating displacement imposed by thermal changes. Both lateral tensile stress and local shear stress near the edge are estimated, with local shear stress decreasing quickly and moving apart from the edge with the stress relaxation preventing coating delamination. Predictions of simplified models agree with both results of grid-independent finite element analysis (FEA) models and observations of test pieces submitted to temperature cycles. This demonstrates the ability to use simplified models to predict coating's performances under thermal aging and help in product selection depending on the working environment. PCB conformal coating; crack; finite element stress analysis 202212126820221212682022121268202212121268202212121268202212121221268202212  
joints. [2; 3; 9; 10; 11]. Most PCBs have a CTE lower than 20 ppm/\({}^{\circ}\)C [3; 9; 10; 11], and most solder joints have a CTE lower than 25 ppm/\({}^{\circ}\)C [10; 11], while silicone conformal coatings have CTE values above 200 ppm/\({}^{\circ}\)C [3]. This large CTE mismatch generates stress on the PCB, solder joints, and conformal coating layer, especially when the electronic device is subject to temperature variation during its service life, experiencing many electrical power on/off cycles. The accumulation of thermal and mechanical stresses can initiate surface fractures that may expand to deep section cracks [9; 12; 13]. The risk of device failure can result from crack initiation and propagation because creating an entry door for contaminants and moisture to go through the protective layer degrades the components and electrical connections. [PERSON] et al. [3; 9] have studied conformal coatings of electronics under thermal shock test conditions. They used polyurethane or silicone as a casting material and silicone as a conformal coating material to apply on PCB to observe cycles to failure under thermal shock. It was found that conformal coating materials have a critical impact on the reliability of the electronic device. A study by [PERSON] et al. [2] found there is a strong relation between conformal coating thickness and electronic module failures under thermal cycling aging. Stress in polymer coating has created enormous interest for many years [14; 15]. One common method to simulate coating stress is to measure the curvature or deflection of a coated substrate through a bi-material strip bending (BMSB) experiment [16; 17]. The BMSB method presents the advantage of not requiring a precise determination of the mechanical properties of polymeric coating, parameters that are complex to accurately measure on very rigid material. Finite element analysis (FEA) has been widely used to study the spatial distribution of local stress in polymer coatings resulting from external constraints [10; 18; 19]. For example, [PERSON] et al. [10] take an FEA study to predict that thermal stress may cause cracks in acrylic conformal coating at the edge of components attached to the PCB. Cracks appear when local stress calculated by FEA exceeds the threshold of material failure [14; 20; 21].