RF Test Article Experiment on the Impact of Non-Hexavalent Chromium-Based Conversion Coatings on Electrical Assemblies

Abstract

Abstract Conversion coatings are treatments applied to aluminum structures to inhibit corrosion while maintaining electrical conductivity. In aerospace applications, the most common type of conversion coatings (MIL-DTL-5541 Type I) contain hexavalent chromium compounds as the corrosion-inhibiting agent. These Type I conversion coatings have a long pedigree and are highly effective in preventing corrosion; however, the hexavalent chromium compounds in these coatings are carcinogenic and water-soluble. Therefore, the use of these compounds is highly regulated in order to protect both workers and the public, leading to high cost in both use and disposal. Regulations are also beginning to restrict outright use, where new designs for the DOD are prohibited from using Type I coatings by DFARS 48 CFR Parts 223 and 252 and all use has been prohibited by Europe by REACH regulations since September 2017. In response, more environmentally friendly non-hexavalent chromium-based processes, referred to as MIL-DTL-5541 Type II conversion coatings, have become available. However, the long term reliability and performance impacts resulting from the use of these coatings are not fully understood and there is an ongoing effort in the aerospace industry organized by NASA to fully define these impacts while hardware is still in the design stage. While significant work has been performed to define the corrosion performance of various type-II conversion coatings, there has been minimal work performed to quantify the impact a type-II conversion coating would have on RF electrical assemblies. The standard methodologies defined by MIL-DTL-81706B have significant limitations and cannot fully capture the impact at microwave frequencies. For this reason, an investigation is underway at Northrop Grumman to quantify the impact these coatings may have on the quality, reliability, and performance of our electronic systems. At IMAPS 2017, Northrop Grumman introduced a RF test article designed to assess the longitudinal impact a Type II conversion coating would have on RF electrical assemblies where plated printed wiring boards (PWBs) and aluminum structures come in intimate contact. The test article features a specialized suspended stripline/microstrip stepped impedance filter designed to de-tune in the presence of galvanic corrosion. This talk follows the work presented in IMAPS 2017 by discussing an experiment where three different populations of test articles, each coated with a different type of conversion coating, are subjected to environmental testing. This talk also discusses how an initial measurement of these test articles before environmental exposure showed little difference between these populations. Finally, this talk reviews the initial results of this experiment, discussing environmental testing and the RF measurements captured during these tests.