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56 SMT007 MAGAZINE I NOVEMBER 2024 Understanding Residue in Circuit Assemblies In the manufacture of printed circuit assem- blies (PCAs), the reflow soldering process is oen accompanied by the presence of residues. ese residues can arise from fluxes, solder pastes, adhesives, and various chemicals used throughout the board fabrication and assem- bly processes. Residues may be ionic or non- ionic, and their composition can vary based on the specific materials and processes employed. Ionic residues, which include salts, acids, flux activators, and other conductive materi- als, are of particular concern. Under normal conditions, these residues may not immedi- ately compromise the functionality of a circuit assembly. However, when exposed to moisture or high humidity, ionic residues can dissolve in the water layer that forms on the surface of the assembly, creating conductive pathways. is sets the stage for electrochemical migration, a phenomenon where metal ions are transported across the surface of the circuit board, leading to the formation of dendrites—tiny, tree-like structures of metal that grow over time and can ultimately cause electrical shorts between conductors. The Role of Harsh Environments Harsh environments exacerbate the risk posed by residues on circuit assemblies in sev- eral ways. High levels of humidity, extreme temperatures, and exposure to corrosive gases, pollutants, and other environmental stressors reduce the tolerance for contamination on the surface of PCAs. As the severity of environ- mental conditions increases, even minimal lev- els of residue that might have been acceptable in benign environments can become a signifi- cant reliability risk. Humidity and Moisture Humidity plays a central role in electrochem- ical migration. In environments with high rel- ative humidity, moisture readily condenses on the surface of circuit assemblies, dissolv- ing ionic residues into a conductive electro- lyte. is dissolved solution of ions provides a medium through which electrical current can flow, accelerating the migration of metal ions from one conductor to another. With suffi- cient voltage across the gap between conduc- tors, dendrite formation is initiated, creating the risk of short circuits. e presence of hygroscopic residues, which absorb moisture from the air, worsens this problem. Even in environments with moderate humidity, hygroscopic residues can attract and retain moisture, further increasing the likeli- hood of electrochemical migration. In critical applications where system failure could lead to catastrophic consequences—such as aerospace or automotive safety systems—the presence of moisture-induced ECM is intolerable. As a result, stringent cleaning processes or alterna- tive flux chemistries are oen required to mini- mize residue levels and mitigate this risk. Temperature Extremes High temperatures accelerate chemical reac- tions, and this holds true for the formation of dendrites in electrochemical migration. When exposed to elevated temperatures, residues on circuit assemblies can degrade, releasing ionic species that further promote ECM. Con- versely, low temperatures can lead to the con- densation of moisture on cold surfaces, ampli- fying the effects of high humidity. e ther-