In today’s electronics manufacturing, there are several unique flux chemistries utilized in the assembly process. The
decision to use a particular flux is based on the end use environment of the printed circuit board. This dictates the allowable flux residues left on the printed circuit board that may have long term reliability effects. Cleaning processes are defined based on these types of flux chemistries.
Types of Cleanable Flux Chemistries
The three common types of cleanable flux chemistries used today are Water Soluble, RMA only, and RMA/Low Solids (Hybrid).
Water Soluble – The most active of all fluxes as it
contains mostly organic acids. It is primarily used to increase solder wetting especially on boards that have been oxidized or consist of land platings that are difficult to solder. Water Soluble chemistries require cleaning in deionized (DI) water that has temperatures ranging from 130oF-150oF. A cleaning agent or saponifier can also be used for better results.
 RMA Only – Rosin Mildly Activated flux is less aggressive and consists of smaller amounts of activators than water soluble. This results in fewer residues left behind after soldering. However, because of the multiple types of rosins used, this flux requires a saponifier heated at 100oF-150oF to properly remove any residues. DI water is recommended for a final rinse. Regent grade isopropyl alcohol (IPA) can also be used for manual cleaning by agitating with an acid brush.
RMA/Low Solids (Hybrid) – This flux leaves behind low active residues that do not have to be washed. However, it is active enough to solder like an RMA with the option to be cleaned. This too can be cleaned using a saponifier heated at 100oF-150oF as well as manually with IPA.
Choosing a Cleaning Process and Equipment
The cleaning process is defined by the quantity of printed circuit boards produced, the type of flux being used, and the types of components populated. The four types of cleaning processes used on printed circuit board assemblies are Semi-Aqueous, Aqueous, Emulsion, and Vapor Degreasing.
Semi-Aqueous – This system is typically a two part batch process that consists of a solvent step in one unit and DI treatment in a second (Figure 5-1). The organic solvent is used to clean flux residues and is then rinsed under heated DI water.
Advantages:
- Can be closed looped and have zero solvent discharge
- Environmentally friendly
- Excellent cleaning ability
- Low cost solvents
- Good compatibility with most electronics materials
- Disadvantages:
• Requires DI water rinse and dry
• Water may be difficult to remove:
○ From complex shaped parts
○ Parts cleaned in large batches
• Water may corrode parts / assemblies
• Generally requires waste treatment
• Some semi-aqueous solvents have flammability issues
• Capital equipment may be costly
 Aqueous – Is typically a one part process that can be either in-line or batched. Aqueous cleaning consists of using either DI water alone (for water-soluble fluxes only), with a saponifier, or surfactant (Figure 5-2).
Advantages:
• Can be a partially closed-loop process
• Minimizes discharge
• Less costly process and chemistry than semi-aqueous
or solvent / co-solvent processes
• Aqueous solvents do not have flammability issues
• Allows flexibility - use either water soluble or
rosin based fluxes
Disadvantages:
• Requires DI water rinse and dry
• Water may be difficult to remove from complex
shaped parts or parts cleaned in large batches
• Water may corrode parts / assemblies
• Generally requires waste treatment
• Capital equipment may be costly
• Requires process monitoring when using saponifier
 Emulsion – This process consists of the suspension of water in small globules. The water is then mixed with an immiscible solvent where suspension is attained by agitation to remove manufacturing residues. DI water is used to perform the final rinse (Figure 5-3).
Advantages:
• Can be closed-loop, zero discharge
• Solvent readily separates from water
• Allowing for easy waste treatment
• Less costly
• It only uses 5% solvent with 95% water
• Less waste disposal
• Environmentally friendly
Disadvantages:
• Requires DI water rinse and dry
• Water may be difficult to remove
○ From complex shaped parts
○ Parts cleaned in large batches
• Water may corrode parts / assemblies
• Limited drying ability
 Vapor Degreasing – The solvent or rinse tank is boiled creating a vapor atmosphere that encompasses the board removing any residues. The board, traveling in a basket, is then cooled at room temperature (Figure 5-4).
Advantages:
• Recycles solvent creating less waste
• Degreasing units are self contained and do not
require recirculating systems as do in-line cleaners
• Less floor space is used compared to in-line
cleaning
• Cheaper operating costs than in-line systems
Disadvantages:
• Limited in the number of boards cleaned
simultaneously
• The solvent can be costly
• Some solvents used in vapor degreasing are
environmental hazards
• Throughput is not as fast as in-line
cleaning systems
In closing, the cleaning process used during assembly depends on factors such as flux chemistry, the type of populated components, and the quantity of boards being produced. These factors will also dictate whether to use a cleaning agent, as well as the type of equipment best suitable for the number of boards. It is important to understand and define the cleaning process being implemented, for it is one of the major concerns that effect long term reliability in today’s electronics manufacturing.
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