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Conformal Coating Testing in Various Test Environments
Analysis Lab |
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Authored By:P. Singh, L. Palmer IBM Corp, NY, USA C. Xu, J. Kaufman Nokia, NJ, USA H. Fu iNEMI, Shanghai, China S. Strixner, H. Schweigart, M. R. Meier Zestron Europe, Germany C. Wang Zestron China, Shanghai, China M. Pudas Picosun Oy, Espoo, Finland M Smith, J. Payne 3M, MN, USA M. Zhai, S. Calvelli, H. Shi, A. Locquet, D. S. Citrin Georgia Tech, Atlanta, USA D. Hampannavar HP, Bangaluru, India M. M. Khaw, K-L Tan Keysight, Penang, Malaysia H. Akbari Schlumberger, TX, USA SummaryConformal coatings have traditionally been tested by determining the mean time to failure of conformally coated hardware exposed to corrosive test environments. This test approach has serious shortcomings: The test temperatures are most often too high. At these high temperatures, the conformal coating properties may be quite different from those at the application temperatures. In addition, the times to failure are unacceptably long extending into many months. Overcoming these shortcomings is an iNEMI championed test that involves exposing conformally coated thin films of copper and silver to sulfur vapors at 40-50 oC in flowers of sulfur (FoS) chamber and using the corrosion rates of the coated metal thin films as a measure of the corrosion protection capabilities of the conformal coatings. The test temperatures are similar to the application temperatures, the test durations are no more than a week and can be conducted under various temperature and humidity conditions. The purpose of this paper was to determine if testing in the industry-standard mixed-flowing gas corrosion chamber would give similar results as those using the FoS chamber. Acrylic, fluorinated acrylate, and atomic layer deposition conformal coatings were tested in three environments: (a) flowers of sulfur (FoS), (b) mixed-flowing gas (MFG), and (c) iodine vapor. The performance of the coatings tested in the FoS and the MFG corrosion chambers were quantitatively similar. The iodine vapor test results were in qualitative agreement with the FoS and MFG test results. In addition, we present early results pointing to the utility of terahertz-frequency imaging as a technique for measuring conformal-coating thickness nondestructively. ConclusionsThe flowers of sulfur (FoS) and the mixed-flowing gas (MFG) test results were in quantitative agreement in their characterization of the corrosion protection provided by the acrylic, the fluorinated acrylate, and the atomic layer deposited coatings. The coatings tested provided much better corrosion protection to the underlying copper than to silver, probably because of the better adhesion of the coatings to copper. The FoS chamber is of simple construction, and easy to operate [8]. It is a couple of orders of magnitude less expensive to own and operate compared to the MFG chamber. This study demonstrated that the FoS chamber is as effective an environment as the industry standard MFG chamber in testing conformal coatings. The iodine vapor test can be done in an hour. Its characterization of conformal coatings is in qualitative agreement with the results from the FoS and the MFG tests. The non-destructive THz thickness measurements of the acrylic conformal coating were carried out. Film thickness measured was in reasonable agreement with the thickness obtained by cross sectioning means; however, the small discrepancy might be due to aging effects of the film. The THz measurements suggest the acrylic coating is somewhat thicker on the Cu serpentine films than in the space between them., an observation yet to be independently verified. Possible future research effort may focus on detecting incipient sub-coating corrosion and chemical degradation of the conformal coating itself. Initially Published in the SMTA Proceedings |
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