Issue link: https://iconnect007.uberflip.com/i/1537054
44 SMT007 MAGAZINE I JULY 2025 T H E K N OW L E D G E BAS E tion, particularly residues from flux activators, even from no-clean fluxes, can be hygroscopic, attract- ing moisture from the air, creating a localized, con- ductive path across the surface of the assem- bly. This phenomenon, electrochemical migra- tion (ECM), can lead to parasitic leakage, dendritic growth, corrosion, and ultimately, failure. If you've ever inspected a failed board and seen tree-like metal structures or white, fuzzy res- idues growing between leads, you've seen ECM in action in the form of dendritic growth. It doesn't take a flood to get there. In many cases, conden- sation from temperature swings or trapped humid- ity under low-standoff components is all it takes to trigger the process. It's important to note: No-clean doesn't mean don't clean. Flux residues from no-clean formula- tions are still there; they're just harder to see. With today's high-density designs, tighter spacing, and bottom-terminated components, there's even less tolerance for residue than ever before. Cleaning removes those residues. But more importantly, it removes the potential for reliability defects that don't always show up right away. They wait—and they show up later, in the field, under load, when failure is most expensive. What Coating Is—and Isn't Conformal coating has one job: to protect the board from external environmental threats like moisture, dust, chemicals, and mechanical abra- sion. It acts as a barrier, sealing the assembly and extending its service life in harsh conditions. But here's the truth: Coating is not a fix for con- tamination. If you coat a dirty board, you're not protect- ing it; you're sealing in the problem. Ionic residues trapped beneath the coating can continue to attract moisture, corrode metal surfaces, and/or trigger ECM. Since many conformal coatings are moisture permeable, they won't stop water vapor from get- ting through. That moisture just gets stuck inside, next to the contaminants you tried to ignore. Worse yet, residues interfere with coating adhe- sion. That can lead to delamination, cracking, or bubbling. In other words, coatings only work when applied to clean surfaces. If you skip or shortcut cleaning, you've just wasted your coating, and your time. Cleaning Best Practices: What Good Looks Like Good cleaning isn't about blasting boards with a cleaning solution and hoping for the best. It's a defined, measurable process with inputs and out- puts that can, and should, be monitored. Here's what matters: Know your contaminants. What residues are you dealing with? Flux type, board and component fabrication residues, handling residues, and environmental particles all need to be considered. You can't solve what you haven't defined. Choose the right cleaning chemistry. While aqueous-based cleaning technology is the most popular cleaning method, semi-aque- ous, and solvent-based systems each have their place. The chemistry must be compatible with the residue type, assembly materials, and your envi- ronmental and safety constraints. Focus on under-component cleaning. Low standoff components like QFNs and BGAs trap contamination underneath. Cleaning systems must produce the right fluid dynamics— wide diffusion, high impingement, and shadow-mit- igating spray patterns—to get under those compo- nents and flush contaminants out. Rinse thoroughly. It's not just about cleaning, it's about rinsing away both residues and cleaning chemistry. Deion- ized (DI) water rinsing, with real-time resistivity monitoring, helps confirm that the job is done. Dry completely. Moisture left behind is a recipe for trouble. Use a combination of convection and radiant drying 1 2 3 4 5 " " But here's the truth: Coating is not a fix for contamination.