SMT007 Magazine

SMT007-Nov2024

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60 SMT007 MAGAZINE I NOVEMBER 2024 error is exceedingly small, requiring strict con- trol over both manufacturing processes and post-reflow cleaning procedures to prevent residue-related failures like electrochemical migration. Reducing Residue Tolerance: Best Practices As the tolerance for residues diminishes in harsh environments, manufacturers must adopt rigorous practices to minimize contam- ination on circuit assemblies. ese practices include thorough cleaning processes, care- ful material selection, and enhanced testing protocols. Cleaning Processes Effective cleaning is one of the most critical steps in reducing the risk of electrochemical migration. Cleaning processes must be opti- mized to remove not only visible residues but also microscopic and ionic contaminants that can contribute to ECM under harsh environ- mental conditions. Techniques such as aque- ous cleaning, using a combination of deion- ized water and cleaning agents, have proven effective at removing ionic residues. Advanced cleaning systems that incorporate real-time monitoring of resistivity in the rinse water allow for precise control over the cleanliness of assemblies and ensure that contamination is reduced to acceptable levels. In environments with very low residue tol- erance, even "no-clean" fluxes—which are designed to leave behind benign residues— may not be acceptable, as these residues can still absorb moisture or interact with pollut- ants to create conductive pathways. In such cases, cleaning may be required despite the use of low-residue flux chemistries. Material Selection e choice of materials used in circuit assem- bly can also play a key role in minimizing resi- due-related risks. For instance, selecting fluxes and solder pastes with low ionic content can significantly reduce the amount of conductive contamination le behind aer reflow. Simi- larly, the use of conformal coatings to protect against environmental exposure can provide an additional layer of defense, preventing resi- dues from interacting with harmful pollutants. While most conformal coating materials are permeable, allowing small amounts of moisture to permeate through the membrane-like mate- rial, they can reduce, though not eliminate, the amount of moisture exposed to the surface of the assembly. However, the proverbial "fly in the ointment" is the fact that manufacturers of conformal coating materials recommend a thorough cleaning process prior to the applica- tion of conformal coating. If one embraces the rationale of conformal coating to avoid clean- ing, this creates a circular logic event. In addition, certain materials used in the manufacturing process, such as adhesives and potting compounds, must be chosen for their resistance to moisture absorption and ionic contamination. Testing and Monitoring Testing for ionic contamination has become a critical part of the quality assurance pro- cess in circuit assembly, particularly for appli- cations exposed to harsh environments. Cur- rent IPC standards now mandate an objec- tive evidence protocol to assess the cleanliness of circuit assemblies. Surface insulation resis- tance (SIR) testing u n d e r h e a t and humidity conditions is now required

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