# Method to Predict Performances of PCB Silicone Conformal Coating under Thermal Aging
## 3 Results
### Modelling Stress Build-up Associated with CTE Mismatch during Thermal Cycle Aging
The constant stress value at the center of the strip corresponds to the lateral tension stress calculated using Equation (5), lateral tension stress being the dominant contribution to the Von Mises stress far from the edges. The parameters used to build the material's model are the ones used in paragraph 3.3 and given in Figure 6:1 Not studying the sharp stress profile close to the edges allows the use of a looser mesh in the FEA model build-up without impacting the calculated stress value. An example of the FEA result is shown in Figure 12 for coating A exposed to \(-40\)\({}^{\circ}\)C. Similar stress spatial profiles are observed for coatings A and B at low and high temperatures that will not be shown here. Von Mises stress values in the center of the strip (equal to lateral tension stress) are compared to the stress calculated using equation 5 for all explored conditions (coating type and temperature exposure). Results summarized in Table 3 show agreement between the FEA and the simplified approach. Comparing Stresses Calculated Either by Simplified Model or FEA to the Local Stress Threshold Leading to Silicone Failure  
Lateral tensile stress and local shear stress near strip edges induced by temperature changes have been calculated using simplified and 2D FEA models of the bi-layer strip. The next step is to evaluate if those stresses are large enough to initiate a local failure  
Figure 6:2 FEA modeling of conformal coating A stress distribution at \(-40\)\({}^{\circ}\)C; x, y units are in mm. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Property** & **Unit** & **Coating A** & **Coating B** \\ \hline Tensile stress in x direction (\(-40\)\({}^{\circ}\)C, 2D Equation (5)) & MPa & 5.77 & 0.08 \\ Tensile stress in x direction (\(-40\)\({}^{\circ}\)C, 2D FEA model) & MPa & 5.98 & 0.08 \\ Tensile stress in x direction (\(125\)\({}^{\circ}\)C, 2D Equation (5)) & MPa & 0.07 & 0.18 \\ Tensile stress in x direction (\(125\)\({}^{\circ}\)C, 2D FEA model) & MPa & 0.07 & 0.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Conformal coating tensile lateral stress calculation. in the coating. Therefore, the local stress threshold that creates coating failure under uniaxial tension is measured for both coating A and B using dog bone test pieces, with the engineering stress value at the break being recorded. The choice of a dog bone (or uniaxial test piece) is made because test piece geometry is designed to allow free reduction of the cross-section when pulling (to conserve the volume of silicone, a nearly incompressible material). Therefore, the local stress value is constant over all test piece volume and directed towards the pulling direction. In PCB conformal coating application, coating thickness is generally -0.1 mm. It is very difficult to prepare thick samples with those materials; therefore, dog bones 0.1 mm thick were prepared for material characterization. Three specimens are used for the traction test experiment to determine the parameters of both linear and Neo-Hookean models. Figure 13 shows the strain-strain curve measured on coating B at 125 \({}^{\circ}\)C.  
In Figure 13, the line of magenta color is the Neo-Hookean model used to fit the nonlinear behavior of the material measured at large elongation. This Noe-Hookean hyperelastic behavior is plotted using the Lame parameter identified from bog bones made of coating B and tested at 125 \({}^{\circ}\)C. The blue straight-line curve corresponds to the linear model, the two models (the linear and Neo-Hookean) superposing for small elongations. Three dog bones were used to understand reproducibility. The replication of the stress-strain testing shows curves superposing, allowing an accurate estimate of both linear and nonlinear models. Larger variations are observed for both the stress and the strain value at the break, those two quantities being more sensitive to defects created during sample preparation. Table 4 below shows the average tensile strength and elongation at break for different conformal coatings and their related standard deviations.