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A PCB Cleaner to Remove Soldering Flux

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Soldering is an integral step in PCB manufacturing.  Residual soldering flux must be removed.  An ultrasonic PCB cleaner is widely used as a final step in removing flux contamination.  Its removal must be undertaken with care because flux residues in the ultrasonic cleaning solution must be disposed in a safe manner.  This post discusses ultrasonic PCB cleaner equipment and cleaning solution formulations.

A point of clarification:  We are discussing PCB manufacturing, not cleaning PCBs in PDAs due to exposure to dirt, water and other contaminants during use. 

7 Reasons to Specify an Ultrasonic PCB Cleaner

  1. Removes flux, solder paste, and other residues
  2. Zero damage to delicate electronics
  3. Lowers QC rejects, increases product reliability
  4. Uses eco-friendly chemicals
  5. Penetrates into tight spaces with complex geometries
  6. Superior to spray washing
  7. Maintenance cleaning of dispensing nozzles and print heads

Ultrasonic PCB Cleaner Equipment Selection

Aside from cleaning solution capacity and tank dimensions, ultrasonic cleaner frequency and ultrasonic power are important considerations for a PCB cleaner.   These and other specification criteria are described in our post on selecting an ultrasonic cleaner.

Here's an example of cleaning PCBs with ultrasonic cavitation as developed by IDC Technologies.

While it relates to cleaning hand-assembled circuit boards using a water-soluble flux its value lies in the fact that the customer developed the optimum PCB cleaning process through experimentation.

IDC selected the 5-gallon capacity Elmasonic P180H model ultrasonic cleaner .

The dual-frequency Elma P series offers adjustable ultrasonic power that can be set from 30% to 100% and the option of using either a 37 kHz or 80 kHz frequencies.

As another feature the microprocessor-controlled generator senses and adjusts for light or heavy loads in the tank. Digital operation ensures reproducible performance from run to run.

It also provides the critical “Sweep” function to prevent harmonic vibrations that could damage delicate PCB electronics, and a degas mode to drive off cavitation-inhibiting entrapped air from freshly prepared cleansing solutions.

IDC’s PCB Cleaning Steps

Boards are carefully positioned in the cleaning basket so they do not contact each other.

The ultrasonic cleaner is readied by turning on the unit, activating the sweep mode, setting the thermostat to 30⁰C (86⁰F) and the power to 30%. Set and actual values are displayed on the control panel.

When all is ready the basket is immersed in the solution and the timer set for 7 minutes.  Further details are presented below.

An advantage of the Elmasonic P units is that selecting the correct power level can be achieved by experimentation during operational tests.

The company looked for gentle but through cleaning and this is optimized by keeping the level below 10 watts per quart. And, since ultrasonic cavitation produces heat 30⁰C was specified so the boards do not get too warm and subject to thermal shock during the rinsing operation.

The typical 7-minute cleaning cycle may vary depending on the number of boards cleaned and their condition.

At the end of the PCB cleaner cycle the basket is removed and the boards are rinsed using deionized water then air-dried under a fan.  A caution is that while the process in general is excellent for PCB cleaning it cannot be used when they contain sealed board-mounted relays.  

Ultrasonic Cleaning Solution Selection

Most ultrasonic PCB cleaning formulations are water based and biodegradable.  Fluxes and solder pastes, however, may or may not be soluble in water.  If not, they require special solutions based on the flux formulation.

Two cleaning solution concentrates that can be used to remove water-soluble fluxes are the mildly alkaline elma tec clean A1  formulated for PCBs, mother boards, electronic parts, fine optics and plastics.

Another candidate is neutral elma clean 260 d&s which may be effective in removing some fluxes that are not water soluble.

Rather than our presenting a shopping list of solutions to remove fluxes please contact our scientists for recommendations based on your specific requirements.

Ultrasonic Cleaning Solution Selection at IDC

In IDC's case mildly alkaline concentrate elma tech clean A1 proved the best choice for cleaning electronics such as printed circuit boards. It can be reduced to 3 to 10% with water.

A 5.5% dilution was prepared by combining 13 quarts of water and 0.75 quart of A1 in the cleaning tank.

The unit is started and the degas mode activated to thoroughly mix and degas the solution. This step applies to each time fresh cleaning solution is prepared.

An Operating Summary for Ultrasonic PCB Cleaners

A number of ultrasonic cleaners on the market can be used by PCB fabricators, the model described in this post being a good example. 

In general, the process is rather straightforward and can be summarized thusly for water-based solutions:*

  • Select an ultrasonic cleaner with a “Sweep” function to provide uniform cleaning action and prevent potentially damaging harmonic vibrations.
  • Follow cleaning solution manufacturers’ instructions concerning diluting the concentrate, the correct cleaning temperature and estimates of cleaning time.
  • Fill the ultrasonic cleaning tank with water then add the correct amount of cleaning solution formulation. 
  • Turn on the unit without a load to mix and degas the solution.  Degassing removes air trapped in the liquid, which slows down the cleaning process.  Some units are equipped with a degassing mode to speed the process.
  • Place the boards in a basket or rack in such a way that they do not contact each other.  When ready lower them into the solution..  If the unit is not equipped with auto-sweep activate the Sweep function.
  • At the end of the PCB cleaner's cycle remove and inspect.  When satisfied rinse the boards with de-ionized water to thoroughly remove cleaning solution residues and allow them to dry.  Large capacity PCB fabricators may elect to employ electric dryers for this process.

PCB Cleaning Solution and Solder Flux Disposal

As suggested above, many cleaning solution formulations are biodegradable and can be drained into sanitary sewers.  But always check your local regulations. 

Flux residues are another matter.  These must be isolated from the cleaning bath and disposed of in a proper manner. 

Suitably equipped ultrasonic cleaners are essential in this aspect. Equipment design also helps you to extend the life of the cleaning solution by removing contaminants that build up in the solution during the PCB cleaning process. 

Contaminants that rise to the surface can be manually skimmed off or, for larger units, by the use of  weirs that direct them to a collection container for storage and later disposal. 

Industrial ultrasonic cleaner tanks can be equipped with diverter valves allowing operators to direct floating contaminants or those that settle to the bottom of the tank to filtration systems, returning treated solution to the cleaning bath. 

Tanks fabricated with sloping bottoms facilitate collection of settled fluxes for routing to the filtration system.  

Eventually, however, PCB cleaning time will increase as the solution loses its effectiveness in removing solder flux.  It must be replaced. 

Drain the tank and dispose of the spent solution according to local regulations.  Take this time to thoroughly rinse the tank with fresh water to remove contaminants that remain on the bottom.  Fill the tank with fresh solution and proceed as above.

*A Note on Solvent-Based PCB Cleaning Solutions

Solvent-based cleaning solutions for water insoluble fluxes may require special precautions.  For example, non-flammable solvent-based solutions can be used very effectively on water-insoluble fluxes.  BUT: if you use these in filter-equipped tanks be certain that the filtering system is compatible with the formulation.  AND: if you use flammable solvents you must use an explosion-proof ultrasonic cleaner.

Need More Info?

The ultrasonic cleaner experts at Tovatech are ready to help you select an ultrasonic PCB cleaner sized and equipped to meet your requirements and recommend the proper cleaning solution chemistry.  Give us a call.

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