# Method to Predict Performances of PCB Silicone Conformal Coating under Thermal Aging
## 4 Conclusions  
Silicone conformal coatings have low elastic modulus (elastic modulus multiple orders of magnitude lower than substrate modulus) compared to alternative chemistries used in conformal coating applications for the electronic industry. Soft coatings make established test methods (for example, the bi-material strip bending) no more appropriate for coating evaluation but, in counterpart, offer opportunities to establish new test methods building on this specificity. To achieve this goal, a study was initiated combining simplified solutions of solid mechanics equations, 2D FEA models, and experimental observations. Figure 17: PCB coated with Conformal Coating A, showing cracks after 517 cycles of thermal shocks and 350 cycles of thermal exposure, as described in the text. Figure 18: PCB coated with Conformal Coating B, passing 3 rounds of 517 thermal shocks followed by 517 cycles of thermal cycling without cracking, as described in the text. \begin{table}
\begin{tabular}{c c} \hline \hline
**Conformal Coating** & **Coating Crack Observation** \\ \hline A & No Crack after TS 517 shocks. Crack after TS 517 shocks + TC 350 cycles \\ B & No crack even after 3 rounds of (TS 517 shocks + TC 517 cycles) \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Conformal coating reliability test. The bi-material strip bending, a well-known and easy-to-carry-out technique for evaluating the ability of bi-layer composites to sustain imposed deformations, is not appropriate for selecting soft conformal coatings used for PCB protection. The method imposes a bending of the composite that does not take place during exposure to large temperature variations, the soft coating following the deformation imposed by the differential thermal dilation between components. For this reason, different routes were explored and tested to identify a consistent method to evaluate the performances of soft coating submitted to thermal stress. Soft coatings of Young's modulus at least 3 orders of magnitudes lower than other components of the PCB substrate:  
* Allows simplified analytical solution of solid mechanics equations to calculate both the high shear stress dominant close to the edges and the lateral tension stress dominant far from the edges where shear stress becomes \(\sim\)0;
* Creates a very localized high shear stress close to the edges, confirmed by the analysis of the analytical shear stress solution that demonstrates that high shear stress decreases very abruptly over a very short distance when the elastic modulus of the coating is much smaller than the elastic modulus of the substrate;
* Makes FEA models of shear stress near the edge complex, requiring mesh spatial density and cell dimensions differing by order of magnitude, the extent of the nonlinear shear stress decrease being \(\sim\)10\({}^{-3}\) mm. Therefore, a simplified solution offers benefits for checking the outcome of FEA models and guiding the build-up of the meshing scheme;
* The modeling and stress calculation are combined with an experimental part;
* Measurement of the stress threshold that leads to rupture carried out using dog bone test pieces;
* PCB specimens with electronic components are submitted to temperature cycles with extreme low and high temperatures equal to the temperatures used in both simplified and FEA models. Optical microscope observation of potential cracks appearance is carried out, and their correspondence with the regions of modeled high stress is studied. The high shear stress that builds up close to the edges predicts the failure of both coatings A and B when exposed to low and high-temperature cycles. However, no defects or delamination are observed at composite edges after thermal cycling. The reason is likely the very narrow extent of the high shear region and the steep drop from 16 MPa to 0.3 MPa over a 10\({}^{-3}\) mm distance, which leads to stress relaxation.