Ultrasonic Cleaning

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Use a vapor degreaser.

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The first step would be to check with your solder / flux manufacturer for the recommended cleaning chemistry, then explore what kind of system is suitable. Ultrasonics are effective but some people avoid them because of potential damage to wirebonds inside ICs. Others say that newer systems don't have that problem. I guess you have to decide if that $120 benchtop unit from Amazon will cause you problems down the road.

Cleaning can be a challenge. Sometimes its better to leave the flux on (no clean or RMA) than to risk doing an incomplete job of cleaning and leaving a nastier residue or activator behind. Some plastics/insulation or will not tolerate some of the newer chemistries that are effective on no-clean fluxes. Some components shouldn't be immersed, period.

If you are using water soluble (organic) flux there is no question - you must clean it off (hot DI water is best) or it will cause problems. For the average hobbyist, RMA flux and brushing/rinsing with isopropyl alcohol is cheap and easy.
 
I've been using the ultrasound tank at the engine shop a block away for years to clean up circuit boards. They moved away now so I'm back to manual cleaning. I use Kester 44. On bigger solder joints I find the only way to remove the flux is to chip it away which leads to scratchs in solder mask and access problems between components. And isopropyl stinks. I've never heard of problems with with ics being damaged. I haven't had any problems but I'll keep that in mind. I've never seen a vapour degreaser. What does it use for a solvent?
 
Ultrasound cleaning devices can indeed damage integrated circuits, particularly older ones. My advice for removing flux and general cleaning of grease, smoke and dust deposits on PCB is 3-step manual method:

1. Use isoproyl or 96% ethanol alcohol and scrub both sides of PCB with soft and narrow painting brush rather than with toothbrush and apply alcohol generously; this will dissolve virtually everything which is not your PCB and components
2. Flush the still wet PCB with cold tap water, then repeat step 1 with dishwashing detergent and brush
3. Flush well with tap water and do the final rinse with distilled or demineralized water to prevent stains and residues
Allow to dry for 24 hours in a warm place. Your PCBs will look mint.
 
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Basically if you are using flux of basic chemistry? cleaning is easy with just Isopropyl Alcohol.

But modern and more complex in chemistry flux, that is specialized for rework has sticky residue and needs a more strong cleaning agent.

 

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Vapor degreasers used Freon. That why they don't exist any more. They were great. . .

The dishwasher trick was shown to me by the director of NASA's quality assurance group. The key's to success are 1) good water to start with. Check for residual conductivity of your tap water. 2) use only a very small amount of dishwasher detergent. 3) check the process for ions on the surface- essentially check for leakage on the surface. A post wash di rinse is not really necessary if everything is OK. Most of the stuff from his shop is still in service 30 years later. Do not use this (or any solvents) on non-sealed relays or non-hermetic polystyrene caps. Those should be a hand process after everything else.
 
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