Technological innovations and the higher production of electronic devices have led
to an increased demand for cleaning chemicals in the electronics industry.
By Potshangbam July
The cleaning of PCB assemblies is essential to ensure trouble-free performance and the longevity of the circuit boards. PCBs accumulate dust easily and when exposed to harmful environmental agents, the components on them can deteriorate significantly. After soldering, a considerable amount of flux residue gets left around the solder joints. This can corrode the solder joints gradually and prevent heat loss, which causes overheating. In order to achieve good insulation resistance and improve the life span of the circuits, cleaning chemicals should be used.
There is a wide variety of PCB chemicals available in the market, like wet cleaners, solder pastes, solvents, conformal coatings, anti-static protection products, stencils, the safe-wash range, and low VOC (volatile organic compounds) lubricants and resins, to name a few.
The purpose of cleaning chemicals
The rapid growth in the electronics industry has occurred on the back of constant innovations. Currently, the size of the components has shrunk significantly, which in turn has increased the importance of cleaning chemicals in electronics manufacturing. If not cleaned properly, contaminants such as flux, solder paste, adhesive residue, dust and debris that are invisible to the naked eye can lead to the breakdown of the PCBs. However, manufacturers prefer to use no-clean solder flux to avoid the cleaning process. But there is no guarantee that no-clean flux is free from residue. Theoretically, such flux, which is composed of organic adipic and citric acid, readily breaks down when in contact with heat during soldering. Though there is minimal solid content in no-clean flux, there is still some corrosive residue left on the board surface after soldering. This could adversely affect the equipment over time. To avoid these pitfalls, the cleaning process is in fact mandatory within the electronics industry.
Jade Bridges, manager of the global technical support team at Electrolube, says, “In order to achieve good insulation resistance and ensure adequate adhesion of conformal coatings and encapsulation resins, the cleanliness of the electronic assemblies is essential. Cleaning is therefore an essential process and helps to remove harmful contaminants during PCB manufacture.”
Types of cleaning chemicals
The chemicals used to clean PCBs should be in a pure form. Currently, there are two types of cleaners that are trending in the market – solvent-based and water-based cleaners.
Solvent-based cleaners: These contain polar and non-polar components which enable users to cleanse various flux residues from lead and lead-free co-clean solder pastes. They are formulated with surfactant-free materials and they dry fast. They can be easily distilled, which makes it possible to use them in cleaning equipment with a vapour rinse. Manufacturers mainly use solvents with non-halogenated hydrocarbons, modified alcohols or hydrofluorethers (HFEs). Considering that chlorofluorocarbons (CFCs) deplete the ozone layer, HFEs were used as an alternative. Isopropyl alcohol is one of the preferred solvent-based cleaners due to its high evaporation rate, non-corrosive nature and low toxicity. When isopropyl alcohol is blended well with compressed air, removal of surface dirt on the board is effective.
As sources at Electrolube put it, “Solvent based systems are very efficient, allowing a convenient single-stage process. They are often flammable and therefore the health and safety of the operator as well as solvent emission levels must be considered when using such chemicals.”
Cleaning methodologies used: Ultrasonic (semi-aqueous), spray-under-immersion (semi-aqueous), chamber vacuum with vapour rinse, co-solvent with vapour rinse, manual cleaning, etc.
Area of specific application: All around the product, water and pH-sensitive parts, for rework/repair, etc.
Water-based cleaners: These are used for the removal of flux residues and solder pastes from electronic assemblies. They are also preferred when cleaning other electronic materials, such as coatings, masks, adhesives, etc. Water-based chemicals have several advantages including their non- flammable properties, low odour, low/non-VOC and very low toxicity.
Cleaning methodologies used: Spray-under-immersion, ultrasonic, spray-in-air (inline and batch), spray-in-air (batch), etc.
Area of specific application: For sensitive metal surfaces, for components with low standoff, etc.
Differences between water and solvent-based cleaners
- The surface tension in water-based cleaners is higher when compared to solvents.
- However, this issue can be overcome by increasing the temperature, and using different emulsifiers and surfactants. Using spray and drying systems also helps.
- Water-based cleaners have a narrow range of evaporation rates while the evaporation rate of solvent cleaners can be instantaneous or slow.
- There is no specific cleaning temperature for a water-based cleaner as it depends on the formulation and cleaner. It can be used from room temperature up to about 80°C. A solvent cleaner, has a much wider temperature cleaning range – below 0°C to above 200°C.
- Hazardous air pollutants (HAPs), also known as toxic air pollutants, cause cancer and various other health related problems. Generally, water-based cleaners do not contain HAPs but solvent cleaners do. Some solvents release toxic pollutants.
Selecting the right cleaning chemicals
Selecting the right chemicals is important in order to avoid any mishaps. The risks are high when you choose the wrong chemical as the sensitive connections can be destroyed and materials compromised, with components deteriorating. The selection criteria should include examining the nature and quantity of the contaminants to be cleaned from the PCBs.
Choosing suitable cleaning chemicals can remove the residue and contaminants in a more efficient way. The performance of these chemicals should be checked before purchasing by conducting a test on discarded components. This will help to gauge the effectiveness of the chemicals in the cleaning operation. Bridges adds, “When the choice is based on solvency power and the lowest surface tension, it can improve the level of cleanliness and efficiency. And the chemical blend used in the formulation should not be flammable, toxic nor should it cause any corrosion on the surface of the electronic components.”
Safe handling of cleaning chemicals
Before purchasing any cleaning agents, one should ensure that the products meet all health and safety standards. Some of the chemical compositions are very harsh and can ruin the circuits as well as all the associated components. The risks associated with these hazardous products must be assessed carefully. One should always check the characteristics of the cleaning materials. The products should possess very low VOCs (volatile organic compounds), be free from ozone depleting compounds, etc. Besides, the cleaning materials should be biodegradable and not contain materials banned by RoHS rules. Neither should they have any SVHC listed substances, or halides.
Bridges explains, “When assessing cleaning materials, what is hazardous or non-hazardous can be easily identified by an analysis of the technical and safety data sheets. Careful consideration of these should always be done before selecting a cleaning solution for a particular application. Consideration should also be given to the level of cleanliness required and this must be defined. The correct cleaning method should be able to achieve the ‘clean’ standard as specified by the electronics engineer.”