Flux and Cleaning

Flux and Cleaning— How Clean Is Clean?
Part 1
The subjects of flux and cleaning are interrelated; one cannot be discussed without the other. The
selection of fluxes and cleaning processes plays a critical role in the manufacturing yield and product
reliability of electronic assemblies. Once soldering is accomplished, any corrosive material left on the
surface must be completely removed after soldering. In this column, I discuss various types of fluxes
and why we use them, followed by various types of cleaning materials and processes. In my next
column, I will discuss cleanliness requirements to know whether the boards have been cleaned
enough to meet their functional requirements for their intended applications.
Why Do We Use Flux?
We use flux to be able to join two metallic surfaces. Joining is done by welding, brazing, or soldering.
They are essentially the same except they are done at different temperatures. In all these three
joining processes, we use flux to remove oxides so that a strong bond between the mating surfaces
can be accomplished. The key difference among these processes is that there is no need for cleaning
after welding and brazing. But when it comes to electronic assemblies, cleaning is a critical process.
You are not done after the joining processes until you remove the residues. Otherwise, field failures
are almost certain depending on what components and flux types are being used. The common
metallic surfaces for joining in the soldering process are copper and tin. Like most metals, tin and
copper have the natural tendency to oxidize. However, to accomplish intermetallic bonds between
them, you need to get rid of oxides. Flux helps provide a fresh clean surface to accomplish the
intermetallic bond between copper and tin (it is the same for both tin-lead and lead-free) to achieve
a reliable solder joint.
Fluxes do a few other things at soldering temperatures. They reduce the surface tension of solder
causing it to spread and promote wetting, which in turn makes it possible to form strong and reliable
solder joints. In addition, they protect solder from further oxidation during soldering. For additional
oxidation protection, at times we also use nitrogen. But nitrogen plays no role in the formation of
intermetallic bond while it is essential for intermetallic bond.
Consequences of Using Flux
We need flux but there is a downside to using it. As we just discussed, we need flux to accomplish
good solder joints. However, once we are done with soldering, what to do with those flux residues?
Do we leave them on the board or get rid of them? The answer is: it depends. You can leave them
alone or you must remove them depending on how harmful those residues are. The types of flux
residues or contaminants that require cleaning are determined primarily by the type of flux used.
Halides, oxides, and various other contaminants are introduced during storage and handling as
well. The use of aggressive fluxes makes soldering easier even if components and boards are slightly
oxidized and contaminated. The cleaning process to be used is selected based on type of flux, types
of contaminants, and type of assembly. For example, mixed assemblies using both SMT and through-
hole components may need one cleaning process after reflow soldering and another one after wave
soldering, but a two-sided full SMT assembly may need only one cleaning process after the second
side is reflowed. When no-clean fluxes are used, boards may not require cleaning. In no-clean fluxes,
chemicals such as carboxylic acids activate and perform their deoxidizing function, then burn off and
leave no active chemicals on the surface. But no-clean fluxes require perfect surfaces to solder.
Otherwise, solder defects will be too high. JULY 2021 I SMT007 MAGAZINE 39 and make it hard to
remove during cleaning. Indeed, if proper care is taken in selecting the cleaning processes and
equipment, and if the soldering and cleaning processes are properly controlled, cleaning surface
mount assemblies should not be an issue even when aggressive fluxes are used. It does need to be
emphasized, however, that good process control is essential when using aggressive water-soluble
fluxes. The selection of a cleaning process depends upon the type of flux being used. See Table 2 for
a quick summary of cleaning processes for different types of fluxes. Rosin and resin fluxes can be
cleaned by various types of solvents such as organic solvents or aqueous and semi-aqueous solvents.
When cleaned by aqueous solvents, additives are needed. If no-clean fluxes are to be cleaned
(sometimes they are), they can also be cleaned with these solvents, although at times, special
formulations may be required. Water soluble fluxes can be cleaned with water with and without
additives. The cleaning process selected may use solvents or deionized (DI) water or a combination
of these two processes. In the past, the commonly used solvents were CFCs (chlorofluorocarbons)
such as Freon but they were banned decades ago due to environmental concerns. The industry has
had no choice but to Table 1: Flux classification based on material composition and halide content.
(Source: Ray Prasad, Surface Mount Technology, Principles and Practice, second edition, Table 13.1)
40 SMT007 MAGAZINE I JULY 2021 use either an alternative solvent or water-soluble fluxes and
pastes for cleaning or to move on to a “no-clean” process by using low residue or no-clean fluxes and
pastes. Current technology using no-clean or lowresidue fluxes is eliminating the need for cleaning.
However, the use of no-clean flux requires a clean work environment and a culture change that not
only affects the user but their suppliers as well. In addition, the use of no-clean fluxes may require a
controlled soldering atmosphere to provide a process window compatible with their lower activity.
The use of no-clean fluxes is increasing due to the environmental concerns of using fluxes that
require cleaning and the disposal of used solvents containing lead. But we also need to keep in mind
that no-clean flux is not as active as other types of flux and hence the soldering results may be less
than desired unless adequate steps are taken not only internally at the company but also by
component and board suppliers. Conclusion No matter what fluxes, cleaning materials, or processes
are used, they all need to meet the same requirements. When using more active fluxes, cleaning
should be done with appropriate solvents to remove any contaminants that can cause field failures
due to dendritic growth and corrosion. But how do you know when it is clean enough? If you asked a
similar question about solder joints causing reliability problems in the field, it would be easier to
answer because the accept/reject criteria in J-STD001 and IPC 610 is well established. But it is not
that simple when it comes to deciding how clean is clean even though the requirements are
established in these two standards. We will discuss cleaning requirements in our next column. Stay
tuned. SMT007 Ray Prasad is the president of Ray Prasad Consultancy Group and author of the
textbook Surface Mount Technology: Principles and Practice. Prasad is also an inductee to the IPC
Hall of Fame—the highest honor in the electronics industry—and has decades of experience in all
areas of SMT, including his leadership roles implementing SMT at Boeing and Intel; helping OEM and
EMS clients across the globe set up strong, internal, self-sustaining SMT infrastructure; and teaching
on-site, in-depth SMT classes. He can be reached at smtsolver@rayprasasd.com and regularly offers
in-depth SMT classes. Details about classes can be found at rayprasad.c

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Flux and Cleaning— How Clean Is Clean?

Why Do We Use Flux?

Consequences of Using Flux

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