Is it safer to leave noncorrosive flux residues on the board following hand soldering?

A large OEM recently commented on IPC’s TechNet forum about topically cleaning ROLO (rosin with no halide activators) flux residues from circuit assemblies. The OEM inferred that spot-cleaning methods are inconsistent and damaging due to the tendency to spread residues over large areas of the board and under low-clearance parts. The OEM, not having access to automated cleaning once the final assembly is complete, strongly advocated leaving RMA flux residues on the assembly, since rosin is an excellent electrical insulator and often results in higher surface insulation resistance (SIR) levels than bare, fully cleaned boards.

In consideration for the OEM’s argument, topical cleaning agents are formulated with organic-based solvents designed to dissolve the residue. Common topical cleaning solvents include oxygenated solvents such as isopropyl-alcohol (IPA) and halogenated-alcohol blends. As the OEM contends, the dissolved flux residues have a tendency to spread across the board and under components (Figure 1). When the solvent evaporates, non-volatile contaminants may still be present. These non-volatile residues, whether non-ionic or ionic, may create a reliability risk.

To compound the topical cleaning issue, partially cleaned flux residue exposes organic acids present in the flux. Ionic residues are hydroscopic with the tendency to attract mono-layers of moisture. Moisture in combination with ionic residues dissolved into mono-layers of water becomes an electrolyte that can transfer trace metals across conductors. Metals, being positively charged ions, are attracted to the negative cathode. When the board is powered up, the electrical bias will slowly plate metals present in the electrolyte. Over time, the metals will form a tree-like dendritic path that will eventually short the component (Figure 2).

So, does this mean that the use of topical cleaners is bad? During assembly and joining, soldered assembly reliability is directly related to cleanliness. Topical cleaners provide an effective method for removing flux and handling residues following precleaning, hand soldering, component replacement and rework. Topical cleaning is an intermittent step for removing these harmful residues during the build process. Final board cleanliness becomes the most important metric to achieving a reliable assembly. Once the assembly process is complete, the board should be processed through automated cleaning equipment to remove any remaining residues left on the assembly.

As with other processes used to assemble boards, when best practices are employed, reliability is not compromised. For topical cleaning, IPC- AJ-820, Assembly and Joining Handbook, recommends recognizing that each step in the assembly process has the potential to leave harmful residues, and knowing the characteristics of the residue. Non-polar contaminants include compounds such as rosin, oils, and waxes, which are usually insulators. Ionizable polar contaminants are usually salts introduced to the assembly during production via flux activators, human skin, and fabrication plating chemistry. Topical cleaners should be capable of removing any electrically conductive (ionic residues) and nonconductive (non-ionic or organic) residues.

Finally, per IPC-AJ-820, topical cleaning should occur soon after soldering, and the final assembly should be cleaned using automated equipment.

The fundamental issue that started the TechNet discussion stemmed from an OEM that employs a no-clean assembly process. Since the OEM has no automated cleaning equipment, its past experience found that it was better to leave noncorrosive flux residues following hand soldering, component replacement and rework. The argument was based on the fact that topical cleaners can spread residues over the assembly and under components. In respect to the OEM assembly process, I agree it may be prudent not to use topical cleaning agents when an automated cleaning process is not available to perform a final cleaning step. For assemblers that use topical cleaning agents, a final cleaning step using automated cleaning equipment should be employed to remove non-ionic and ionic residues from the assembly.

Mike Bixenman is chief technology officer at Kyzen (kyzen.com); mikeb@kyzen.com.

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