Ionograph Sensitivity to Chemical Residues from ‘No Clean’ Soldering Fluxes: Comparison of Solvent Extract Conductivity and Surface Conductivity

Abstract

This study examines solvent extract conductivity (SEC) testing, e.g., Ionograph or Omega Meter testing, which measures ionic cleanliness of printed wiring boards (PWBs). SEC has been a quality control (QC) monitor to assure product electrical reliability. Typical SEC measurements occur after wave soldered products have been solvent‐cleaned. This study concerns SEC testing on new wave soldering processes that involve no solvent cleaning, i.e., inert gas soldering with ‘no clean’ fluxes. Results show ionic residues from ‘no clean’ fluxes may have other characteristics that make QC testing for ionic cleanliness inappropriate. However, SEC may be appropriate as a process control monitor after soldering with these fluxes. An Ionograph measured SEC response for the following chemicals: NaCl, NaF, NaBr, KCl, MgCl2, CaCl2, HCl, succinic acid, malic acid, glutaric acid, adipic acid and ethylene glycol. The list includes inorganic salts, strong electrolytes, which may arise from manufacturing or PWB materials. The list also includes weak organic acids (WOAs) common to ‘no clean’ fluxes. One non‐ionic hygroscopic chemical, ethylene glycol, was studied. Ionograph response was measured via (i) direct injection of aqueous solutions and (ii) immersion of PWBs with individual chemicals as surface deposits. All ionisable compounds, including all WOAs, produced substantial SEC response. Surface conductivity was measured at 35°C/90% relative humidity (RH) with controlled amounts of the above chemicals deposited on clean PWB test circuits. Surface loadings corresponded to the molar‐ionic equivalent of 2.0 ?g/cm2 NaCl. In addition, NaCl, adipic acid and polyethylene glycol (PEG 400) were examined as a function of concentration. Several ionisable chemicals including all WOAs produced no measurable effect, i.e., surface conductivities were indistinguishable on clean and deposited specimens. Surface conductivity increased for ionic contaminants with critical RH below ∼80% and for the non‐ionic hygroscopic glycol. SEC measurements and surface conductivities were compared. The latter is more directly related to electrical reliability. Although all ionic compounds including the WOAs showed a SEC response, not all enhanced surface conductivity. Achievement of critical RH appears to be the important factor. Adipic acid required the presence of hygroscopic glycol to enhance surface conductivity. Therefore, SEC can be a misleading QC test for electrical reliability when WOA flux residues are present.

Citation

Adams, K.M., Anderson, J.E. and Graves, Y.B. (1994), "Ionograph Sensitivity to Chemical Residues from ‘No Clean’ Soldering Fluxes: Comparison of Solvent Extract Conductivity and Surface Conductivity", Circuit World, Vol. 20 No. 2, pp. 41-44. https://doi.org/10.1108/eb046251