Original Date: 01/23/1995
Revision Date: 01/18/2007
Information : Low-Residue, No-Clean Soldering Process
In July 1991, Sandia National Laboratories entered into a joint program with Motorola's Government Electronics Group and Los Alamos National Laboratory to evaluate a no-clean soldering process acceptable for DOD applications. The no-clean process studied incorporates a dilute adipic acid flux which is ultrasonically sprayed on the PWBs to remove oxidation before soldering. The adipic acid evaporates or is consumed in the soldering process. To inhibit oxidation of the PWBs during the soldering process, the wave solder chamber is filled with a nitrogen cover blanket. A formic acid vapor is bubbled into the nitrogen as an oxygen "getter" to further reduce oxide formation in the soldering zone. The formic acid then decomposes in the heat of the soldering process to carbon dioxide and water. The net result is a clean, soldered PWB when it leaves the wave soldering machine.
Since military specifications and standards did not allow the use of adipic acid as a fluxing material and mandated that all PWBs be cleaned after soldering, the second step of the program was to qualify the new process for military applications. Sample PWBs were manufactured and tested for visual defects, ionic cleanliness, surface insulation resistance, temperature cycle, temperature-humidity, long-term storage, joint (electrical), joint (mechanical strength), and surface chemical analysis. The tests concluded that the visual solder joint quality, PWB reliability, and product shelf life were unchanged by the new process. Minimal residues were detected on the PWBs, but had no impact on performance or reliability. As a result of the testing, two major defense contractors have sought and were granted 69 deviations to military standards to use the process for their military products. One long-term goal of the program is to change the military standards and specifications to accept this process.
Low-residue soldering technology is proving to be less expensive than the conventional rosin or soluble flux process. Motorola has noticed an order of magnitude decrease in wave soldering machine preventive maintenance. IBM Canada has reported a savings of $752K per year over rosin flux/solvent cleaning processes and $590K per year savings over water soluble flux and aqueous cleaning processes.
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