On the extension of CP models to address cathodic protection under a delaminated coating
Cathodic protection models for pipeline and other metallic structures developed to address the presence of coating flaws or holidays do not account explicitly for the presence of a delaminated region surrounding a holiday. The potential drop in the delaminated region was obtained by solving the coupled, non-linear, partial differential, governing equations that describe transport and electrochemical reactions within the delaminated region. The results were used to develop an analytic expression for the potential drop across the delaminated region as a function of delaminated region length and the resistivity of the bulk environment exterior to the holiday. Expressions developed using this approach can be utilized by existing cathodic protection models to deliver improved potential distributions along protected metallic surfaces. The simulations were used to test the commonly employed assumption that transport by diffusion can be neglected in the delaminated region. The influence of diffusion on current was shown to be significant.
Simulation of the Relaxation Potential Profile of an ac-dc-ac Test
The relaxation period of the accelerated ac-dc-ac test for coatings is associated with the transient electrochemistry that occurs when the immersed coated system is allowed to return to a stable open-circuit condition after being subjected to a cathodic potential. A mathematical model of the transient electrochemistry that occurs during this relaxation period is presented for coated aluminum. Expressions for the corrosion potential and corrosion current as functions of the local pH at the metal-coating interface were developed using reported experimental results. These expressions enabled the simulation of the transient electrochemistry under the constraint of balanced anodic and cathodic current densities. Regression of the transient relaxation potential profiles to exponential decay functions provided time-constant characterization of the profiles. Simulated results are presented that demonstrate the influences of the coating's porosity and thickness, the applied dc potential and the metal-coating interface condition on the time-constants associated with the relaxation profile. Interpretation of experimentally reported relaxation potential profiles supported the analysis of the simulated results.
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.