Theoretical Studies and Numeric Simulations

In order to correlate the electrical resistance shift as a function of humidity test time, a theoretical model has been developed based on Ref 39. It takes into account both oxidation and cracking, two primary failure mechanisms of conductive adhesive joints, and can thus explain the experimental observations quite well. 

one can obtain the relationship between the time and the resistance change  

The crack normally occurs at the interface between the adhesive and the electrode, and decreases the real contact area gradually. Here, assume that the contact area A can be expressed as  

Since polymer structures normally contain a large amount of free volume, it is reasonable to assume that the diffusion of oxygen is much faster in polymers than in metal oxides. In other words, the oxygen diffusion through the oxide layer will control the oxide growth rate and consequently the increase of the resistance in the oxide layer. 

For comparison, the experimental results are also given in Fig. 19. Before 500 test h, Eq 5 can predict the experimental observations quite well. However, the experimental results after 500 h cannot be explained by considering the oxidation of copper metal surface only, which means that fracture must have taken place during the humidity testing. In fact, cracks have already been observed after 158 h of exposure. 

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