DENDRITIC GROWTH FROM CHEMICAL CONTAMINATION AND PARTIAL CLEANING: FUNDAMENTAL TESTS AND APPLICATION STUDY

摘要

Electronic assemblies are building in more functionality using fine pitch leadless components. A reduction in the distance between conductive paths, coupled with challenging operating environments make these assemblies particularly sensitive to electrochemical migration and corrosions effects. These failure modes are strongly impacted by various sources of chemical contamination from the board, components, assembly materials and even the process. Flux residues trapped under low stand-off components with large form factors are of particular concern for their potential to promote dendritic growth and corrosion effects, as well as the difficulty to remove them quantitatively from the system. The purpose of this study is to analyze at a fundamental level the impact of various chemical residues on dendritic growth and corrosion of conductive traces and to correlate these electrochemical effects with real-life application conditions. For this purpose, a non-standard test board enables the quantification of the dendritic growth kinetics between Y-shaped traces, in function of the chemical residue characteristics and environmental conditions. This test is complemented by the application of traditional electrochemical techniques (Electrochemical Impedance Spectroscopy, Linear Polarization Resistance) to measure the fundamental corrosion parameters of each chemical package. The classification of the fluxes in terms of their corrosion and electrochemical migration (e.g dendritic growth) rate and potential will then be correlated with SIR and ECM experiments executed on populated printed circuit boards, where the fluxes are trapped under various surface mount components. These boards are assembled with standard industrial equipment and processes, and submitted to various cleaning conditions using a state of the art in-line cleaner. This study generates a useful body of knowledge for the design of robust fluxes, assembly processes and testing methods for the production of reliable assemblies operating in challenging environments.