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.
Types of Fluxes
There are four major types of fluxes and each one further subdivides into six different categories. In other words, we have 24 different types of fluxes to choose from. However, the last category, IN (inorganic flux), is not used in the electronics industry since they are too aggressive for electronics products. In effect we have only 18 types of fluxes to choose from instead of a total of 24—still a daunting task. Please see Table 1 for a quick summary of all 24 different categories of fluxes.
The major four categories of fluxes are: rosin (RO), resin (RE), organic (OR), and inorganic (IN). Each flux type has three activity levels to choose from (low, medium, and high). These levels—L, M, and H—come with or without halides. When you do the math, there are a total of 24 different flux categories to choose from. Fluxes without halides have 0 at the end of their designations, while fluxes with halides have 1 at the end. For example, rosin flux without halide will be called ROL0 and rosin flux with halide will be called ROL1.
These designations repeat for RE, OR, and IN as well. Halides in L1, M1 and H1 are less than 0.5%, 0.5 to 2%, and more than 2%, respectively. The activity of the halide-free fluxes comes from naturally occurring acids. The higher the flux activity, the better the soldering results. However, more active fluxes must be cleaned properly to prevent corrosion in the field. No-clean fluxes can be RO or RE with or without halides. But OR fluxes must be without halides (ORL0) to be classified as no-clean.
Due to high activity levels, inorganic (IN) fluxes, commonly used by a plumber, are not used in the electronics industry.
Types of Cleaning Materials and Processes
It is generally thought that cleaning surface mount assemblies is very difficult because, for example, stand-off heights between the surface mount components and the board are small, creating a tight gap that may entrap flux 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 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 low-residue 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.
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-STD-001 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.
This column originally appeared in the July 2021 issue of SMT007 Magazine.