# 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
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Property** & **Unit** & **Coating A** & **Coating B** \\ \hline Tensile strength & MPa & 5.54 & 1.40 \\ Tensile strength standard deviation & MPa & 0.08 & 0.26 \\ Elongation & \% & 104.4 & 142.0 \\ Elongation standard deviation & \% & 1.3 & 8.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Tensile data of conformal coatings. Figure 13: Conformal coating B tensile profile. Note that conformal coating A has a much higher (4.0 times) tensile strength and slightly lower (36.0%) elongation at break than conformal coating B. In a uniaxial traction experiment, the engineering tensile stress provided by the testing equipment is calculated dividing the force by the x-section of the test piece before the traction force is applied. However, when pulling progresses, the x-section reduces to conserve volume. For this reason, to convert the tensile stress (engineering stress) into real stress, the x-section change when elongating the material is calculated:  
\[\frac{S^{\prime}}{S}=\left(1+\varepsilon\right)^{-2v} \tag{8}\]  
With \(S\), the cross-section at rest, and \(S^{\prime}\), the cross-section at the elongation \(\varepsilon\); \(v\) is the Poisson ratio. The real stress associated with failure is calculated from measured engineering stress at break:  
\[\sigma_{local}=\sigma_{tensiometer}*\frac{S}{S^{\prime}} \tag{9}\]  
The stress at rupture measured on dog bones is compared to the lateral tension stress away from the edges calculated using FEA code and Equation (5). Shear stress values very close to the edge calculated using either equation 7 or FEA models in paragraph 3.2 are much larger stress (16 MPa) than the stress corresponding to coating failure. Therefore, delamination should be observed when increasing temperature from the temperature at which the conformal coating was applied (25 \({}^{\circ}\)C) to the temperature of 125 \({}^{\circ}\)C at which shear stress values are calculated. However, the area of shear stress value being very localized near the edge and followed by a sharp decrease, we may assume that localized delamination resulting from large shear stress allows stress relaxation, preventing crack propagation over long distances from the edge. We did not observe any delamination near the edges of the conformal coatings A or B exposed to the temperature cycle. Therefore, we concluded that shear stress near the strip edge is not an appropriate criterion for silicone conformal coating evaluation and selection. Therefore, we focus below on the sole lateral tensile stress component calculated using either Equation (5) or FEA to select the conformal coating best suitable for both applications. Conformal coating tensile performance compared with calculated and FEA modeling of lateral tensile stress is summarized in Table 5. For conformal coating A, the lateral tensile stress calculated at \(-40\)\({}^{\circ}\)C is nearly equal to 51% of the stress measured at rupture, showing a rather low safety margin compared to coating failure. On the contrary, the safety margin concerning rupture is large at 125 \({}^{\circ}\)C (calculated stress is less than 1% of the stress leading to failure). This result is explained by Young's very large modulus value measured at low temperatures. For conformal coating B, the conclusion is different because [PERSON]'s modulus value is more stable with temperature. Therefore, stress values show less difference between cold and hot temperatures, having larger stress observed at 125 \({}^{\circ}\)C. We also observe a lower tensile strength value of coating B recorded on dog bones, having the stress calculated at 125 \({}^{\circ}\)C corresponding to \(\sim\)5% of the stress at break. For a bi-layer sheet, we identified two conditions potentially critical for the two coatings being tested, the low-temperature part of the temperature cycle (\(-40\)\({}^{\circ}\)C) for conformal coating A and the high-temperature part of the cycle for conformal coating B (corresponding to 125 \({}^{\circ}\)C).