The Complexity of Clean: Understanding Residue Challenges in Modern PCB Assembly
For decades, electronics manufacturers have relied on standard cleanliness tests to determine if a printed circuit board assembly (PCBA) is free of contamination. However, modern assemblies present a growing challenge—residues are more complex, circuit spacing is tighter, and traditional testing methods don’t always catch potential risks.
So what makes cleanliness such a difficult issue in PCB manufacturing? Let’s explore some of the key factors.
Multiple Sources of Residue
A PCBA moves through many different steps before it’s complete, and each of these steps introduces potential contamination:
Fabrication residues – Fluxes, plating chemicals, and solder mask materials can leave behind residues that impact conductivity.
Assembly residues – No-clean fluxes, solder pastes, and reflow processes contribute additional layers of contamination.
Rework and repair – Brush cleaning with isopropyl alcohol (IPA) or other spot-cleaning methods can spread residues rather than remove them, creating hidden risks.
Environmental contamination – Even after assembly, exposure to enclosures, humidity, or airborne contaminants can further alter a board’s cleanliness profile.
These overlapping processes create a “residue cocktail” that can’t be evaluated by looking at just one type of contaminant.
Why Traditional Cleanliness Testing Falls Short
Many manufacturers still rely on broad testing methods like ROSE (Resistivity of Solvent Extraction) to assess cleanliness. While ROSE can detect conductive residues on an entire board, it doesn’t pinpoint contamination in critical areas. Localized residues—such as flux trapped under a component—might not affect the board’s overall conductivity but can still cause localized corrosion or shorting.
A more effective approach is to test at key locations on the board using localized extraction methods like C3/CI (Critical Cleanliness Control/Corrosivity Index), which identifies residues in specific areas rather than averaging results across an entire assembly.
The Future of Cleanliness Testing
As circuit board designs become more advanced, manufacturers need to rethink their approach to cleanliness. Instead of relying on outdated broad-scale testing, they must adopt more precise methods that focus on specific risk areas. By doing so, they can ensure that their assemblies perform reliably in real-world environments—without unexpected failures caused by hidden contaminants.
The complexity of cleanliness isn’t going away, but with smarter testing and process control, manufacturers can stay ahead of the challenges and build more reliable products for the future.