role of flux cleaning in circuit board assembly
Keeping circuit boards clean after soldering is essential for maintaining quality and reliability of the finished products. Whether cleaning with isopropyl alcohol or alternative solvents like acetone, following proper cleaning procedures can help prevent the potential for short circuits and other problems that may arise from contaminants left on the surface of the board. This article explores the role of flux cleaning in PCB assembly, covering the attributes and risks of no-clean fluxes, as well as how specialized cleaning solutions like VeriClean can improve cleanliness for optimum performance.
In the early days of electronic circuits, assemblers used charcoal, lime, phosphorus-containing minerals and baking soda to clear oxides from copper tracks and components, helping to make sure molten solder would wet them more effectively. Later, a mixture of rosin and an activator was created to remove more contaminants, improving solder wetting for better connection quality. Today, electronics manufacturers use a variety of rosin and no-clean solder pastes to ensure strong and durable connections.
While the primary purpose of a soldering flux is to promote strong connection formation between metal surfaces, it can also have other benefits during circuit board assembly, including reducing surface insulation resistance (SIR), inhibiting electrochemical migration between metallization layers and eliminating corrosion. As PCBs have become more complex, however, there is a greater need to clean them thoroughly to eliminate contaminants and prevent problems like parasitic leakage currents, electrochemical migration, shorting and dendritic growth that can interfere with signal transmission and damage components.
What is the role of flux cleaning in circuit board assembly?
As the demand for high-quality, reliable electronic devices has increased, assemblers have looked to reduce manufacturing costs by simplifying their processes and removing steps that aren’t necessary to product quality. In throwaway electronics, this might not seem like a problem, but it is unacceptable for high-reliability PCBs that will be subject to rigorous environmental testing and other types of scrutiny.
To meet these needs, manufacturers have turned to no-clean soldering techniques that provide production efficiencies by obviating the need for post-soldering residue removal. Compared to higher activity clean fluxes, which create more reactive residues that need to be fully washed away, no-clean fluxes contain less active organic acid that is much less likely to remain reactive after cooling.
These lower activity residues are still susceptible to damage, however, especially when exposed to moisture. Ionic residues from no-clean fluxes can absorb moisture and cause conductive growths known as dendrites to form along the edges of a metallization layer or track, creating unintended connections that lead to intermittent current leakage and ultimately short circuits.
Solvent cleaning involves using chemical solvents to dissolve the residual activator and wash it away, leaving the base flux untouched. This process is efficient and relatively inexpensive, but it is limited by the strength of the cleaner and its ability to safely remove all residues. Water washing is an alternative that provides more flexibility, but it can leave residues on pads and pins that could short out connections with electrochemical reactions that aren’t prevented by a stronger cleaning agent.
