SMTCleaning Options : Aqueous Cleaning, Solvent Cleaning, No-Clean Flux

2.1 Aqueous cleaning

Aqueous cleaning is used for cleaning of ionic contaminants left by use of organic acid fluxes. The cleaning process can be done by either of the following methods.

1. Batch aqueous cleaning system

2. In-line aqueous cleaning system

Batch aqueous cleaning is good for low volume production. A batch aqueous system is very much like a house-hold dishwasher, the assemblies are loaded vertically like dishes, detergents are added, and cleaning and drying are accomplished in different timer-controlled cycles.

In-line systems are very much like batch systems except that the board passes different modules rather than different cleaning cycles as in batch systems. In-line systems are used for high volume production. A typical sequence for in-line cleaning system is given below:

The prerinse module is used for the removal of gross contaminants. It is kept separate from the wash stage to prevent contamination. The wash stage needs saponifiers for rosin fluxes. Water is heated to 60 to 70 0 C. Generally additives are not required for Organic acid fluxes. There may be additional recirculating wash stages if detergents are used, or if higher cleanliness is necessary. In recirculating stages, water is directed towards the conveyor carrying the cards at pressures ranging from 10 psi to 30 psi.

The assembly goes through the final rinse to remove primarily the contaminated water that got accumulated during recirculating wash stage. The clean water is blown away with air knives and dried. It may be noted that the boards must be completely dry before it comes out of the cleaner to prevent dust accumulation.

2.2 Solvent cleaning

Hydrocarbon solvent or water with saponifiers is needed for cleaning of boards with rosin fluxes. Water is not a natural solvent for rosin, therefore, when aqueous cleaning is used for rosin fluxes, alkaline chemicals called saponifiers added to the water. The cleaning solvents for rosin-containing fluxes that are typically used are chlorinated and fluorinated hydrocarbons, such as 1.1.1 trichloro-ethane or trichlorotrifluro-ethane mixed with ethanol, isopropanol and methylene chloride (called azeotrope mixtures). Because halogenated hydrocarbons are found to be harmful to the environment, the use is being restricted. Given below are typical process steps involved in solvent cleaning of rosin based flux residues.

In the solvent cleaning process the boards are placed in a basket with their solder side facing downwards and the basket is placed in the vapour phase for about a minute. During this phase, the vapour condensates on the boards and dissolves the contaminants. The basket is then transferred to second stage (rinse) and immersed in the solvent. Dwell time is 1 to 2 minutes. The rinse stage is sometimes given ultrasonic vibrations for better results. In some cases, the solvent is forced onto the boards through nozzles. The above process can be repeated until desired cleanliness is achieved. The process is operator dependent because the entire process may have to be repeated a number of times for desired cleanliness.

The cards can be dried with compressed air or be allowed to dry by natural means. It is important that the cleaning operation is performed soon after the soldering process. With longer time lapse, the rosin becomes hard and it would be very difficult to remove the residues. A maximum time limit of one hour may be allowed if required.

2.3 No-clean flux:

Adopting a no-clean soldering process is preferred for not only due to the rapid phase out of cleaning with CFCs but also because no-clean can be very cost-effective process. No-clean fluxes contain less than 5% of solids after soldering process is completed which is negligible for consumer and industrial electronics products. However, if active fluxes are to be used due to any reason, then the option of using no-clean flux does not arise.