Abstract:

The overall effectiveness of 20 circuit board assembly processes, made up of 4 solder/flux combinations and 6 cleaning processes was investigated by using in-situ surface insulation resistance measurement, resistivity of solvent extract test, ion chromatography test, and optical inspection. The cleanliness of each process was represented by one board produced according to the IPC B-52 layout and design. Together with 4 unpopulated control boards, 24 boards in total were investigated. Surface insulation resistance test evaluated the propensity of a printed circuit board to develop leakage currents and undergo metal migration when subjected to temperature-humidity-bias conditions of 40°C and 90 ± 3% relative humidity (consistent with IPC TM 650-2.6.3.7) and 5 V. The resistivity of solvent extract test inspected the cleanliness of a printed circuit board by extracting the ionizable surface contaminants and quantifying them in terms of an equivalent amount of sodium chloride. The ion chromatography test identified the specific types and amounts of ions present on the surface of a printed circuit board. Optical inspection was a visual check of the cleanliness and possible defects associated with manufacturing processes. By taking into account all the results of the 4 methods, this study clearly shows a relative ranking of the 20 samples provided, and a pass-fail assessment of the 20 processes. A good correspondence between surface insulation resistance and surface contamination levels was observed. The conductivity of the extract was consistent with the presence of ion types and concentrations, especially inorganic anions. This study also indicates that a good solder/flux combination must be paired with an appropriate cleaning process in order to be successful.

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