1. The SMT Cleaning Process
PCB cleaners use either plain water or saponifier systems to clean the relevant parts. Various methods are available from spray, immersion and ultrasonic and are determined by the application required.
Various cleanliness standards are in place relating to specific product types such as military and medical products.
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2. Cleaning Applications
PCB cleaners are used for removal of harmful flux residues (defluxing) from soldered PCBs. Some flux residues can have a corrosive effect on both the components and PCB surfaces, so must be properly removed.
Although no clean solders are more common now, cleaning processes remain to ensure proper cleanliness specificatiuons are met and to help cosmetic appearance of products.
Stencil cleaners are used to remove solder paste and adhesive residues from the stencil surface and apertures prior to storage. This is important to prevent print problems occuringdue to contaminated/blocked apertures etc
Other multipurpose cleaning systems are used for cleaning of process items and equipment such as wave solder pallets, flux traps, magazine racks etc.
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3. Cleaning Machine Types
Cleaning equipment offers a number of different techniques depending on the job in hand. Spray in air systems normally employ a high pressure spray rotor system which sprays water or chemistry onto the product to be cleaned in a process similar to a dishwasher. These systems can be used for both stencil and PCB cleaning. Spray systems often have a closed loop system where DI water is regenerated within the unit by means of media tanks.
Ultrasonic cleaners make use of ultrasound vibration passed into a cleaning media, this causes agitation of contaminants and is particularly useful for products with small and difficult to access areas of contamination.
Spray/Air in water systems are similar to a jacuzzi type system where bubbles of air or agitated currents of cleaning media are used to release dirt etc. These systems are particular;y useful for larger heavily contaminated machine parts etc.
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4. Troubleshooting Cleaning Problems
Cleanliness is a subjective thing and you get what you pay for with regard to ultimate cleanliness levels achieved by cleaning equipment. However sometimes obvious process problems exist which can be overcome:
White Residues..
After cleaning there is often a white reside or spotting on the cleaned surfaces. This is due to contaminated DI rinse water, rinsing without DI or cross contamination of the rinse systems. The white residue is due to halide or salts in the rinse water or that have not completely been washed off.
propoer maintenance of the DI system and cleaning of relevant filters in the cleaner will reduce this issue, however other causes also exist that may be material of surface related.
Not Cleaning Pb Free Flux..
Due to the higher temperatures involved in lead free proessing, the flux tends to turn darker and harder to remove. This in addition to the likelyhood that more flux than normal is required means that a cleaning process that coped admirably with defuxing leaded solder residues is not performing as well after transfer to Pb free.
To overcome this you may be able to modify your cleaning program to give longer or more powerful cleaning, or in extreme cases may have to invest in alternative equipment which is more capable.
Blocked Apertures..
This is a common problem with stencil cleaning and is normally associated with too long a time between use and cleaning, thus allowing in particular SMT adhesives to cure in the apertures or for solder paste to dry out.
It is important therfore that you maintain good discipline and get the stencils cleaned as soon after use as possible.
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5. Expert Advice..
Please visit individual supplier websites for further detailed information.Browse manufacturers PCB and stencil cleaning equipment here
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