Residue on Printed Wiring Assemblies: An Overview and a Case History

Residues associated with soldering operations are common throughout the electronics manufacturing industry. Prevention is necessitated by MIL specs which require soldered assemblies to be free of all residues. Because of the large number of variables which contribute to residue formation, residues frequently appear under seemingly constant conditions, and detailed understanding of the materials and processes involved is needed for corrective action implementation. This paper presents the history of an investigation of white residue on solder masked printed wiring assemblies. The residue was determined to be flux related by experimentation and spectroscopic chemical analysis. Process modification testing was performed to evaluate the roles of flux identity, preheater temperatures and several cleaning process variables. Assemblies were subjected to humidity and temperature cycling, including periodic examination and electrical testing. Additionally, information learned during this investigation about a number of residue causes and cures is summarised.