PCB cleaning methods, solvents and techniques

[No.Mad]

Hi everyone,

during my time here as a veteran passive reader and fresh discussion participant I noticed that there is little information about cleaning PCB after soldering or rework, general cleaning of equipment internal assemblies and so on. I decided to write this looooong topic so everyone can contribute. Before you start reading small disclaimer - I am somewhat OCD perfectionist when it comes to my work (Prototype Electronics Technician - for now ) so pardon me for that. Let's dig in, shall we?

First of all cleanness of PCB assemblies is covered in IPC 5704 standard (IPC - Institute for Printed Circuits) and covers most of industry acceptable methods, solvents, techniques and machinery used. Unfortunately this standard is priced quite high and not everybody can afford it.

Let's discuss fluxes first. Flux is a chemical that helps with soldering process, improving property called wetting by chemically getting rid of any contaminants, i.e. oxidation. Simply speaking it's helping out with flow and proper adhesion of a solder due to capillary effect. We always choose the right flux for right solder and job we do.

More about wetting: https://www.quora.com/What-is-wetting-as-used-in-soldering-process
More about capillary effect: https://en.wikipedia.org/wiki/Capillary_action

Types of flux used in electronics
Most of a time we distinguish three major types of fluxes:

• Rosin based - standard amber liquid that smells nice. We like it much, 'amrite?
• Water soluble - tricky stuff that you can wash with water. Much WOW
• No-clean - if you are a lazy bastard and don't feel like cleaning

I won't dive into halidates, viscosity, percentage of solids. I will try to keep it simple.

Rosin based - back in a day it was mostly resin rosin (or kalaphonia in some places). Today it's more complicated, but let's leave it.
PROS: It has really good properties when it comes to flux and it's forgiving, specially for DIY, makers, hackers and hobbyst. Why? It has long life during soldering process, thoroughly cleans oxidation from component and pad/via, forms a protective film over solder joint, trap ionic contaminants and most of a time is alcohol based so it's possible to clean it with IPA. Its vapour is not as aggressive as rest, but just don't go around and inhale it.
CONS: Overheated could be electro-chemically conductive over time if not cleaned, might contaminate rest of equipment and potentially could fail in harsh environment. Due to standards needs to be always cleaned.

Water soluble - stepping up the GAME ladies and gentlemen. As solvents for rosin based are much more expensive than tap water, industry made flux soluble in water.
PROS: Even longer life time than rosin based (virtually impossible to burn it), great amount of activity, so cleans off oxidation and contaminants easily. Can be washed down the drain with tap water.
CONS: Very aggressive. Corrosive over time. Needs to be washed PERFECTLY and with small SMD components it's a difficult task. Regarding industry standards always needs to be washed. When it comes to rosin based, as a hobbyst, you could leave flux residue after soldering. If you do it with water soluble it can literally eat your PCB over time - it's called dendrite growth Even tiny quantity. Vapour is really aggressive for your respiratory system.

No-clean - some electronics wizardry for lazy people... I mean engineers. Really low quantity of solids inside.
PROS: Lower temperature required for good flow and wetting so it's really good for wave/oven reflow soldering and hot air reflow rework. Huge variety on market. Do not need washing. Easy on equipment.
CONS: More expensive. It's fairly new so properties and long residue exposure is not researched as much as rosin based and water soluble. And if you are looking for lack of critical failure you want to clean it anyway. Vapour is really aggressive for your respiratory system.

Why would you like to clean No-Clean flux: https://www.kester.com/Portals/0/Documents/Knowledge%20Base/Publications/International%20Conference%20on%20Soldering%20and%20Reliability%20ICSR%202016_paper_Why%20Clean%20A%20No-Clean%20Flux.pdf?ver=2017-01-26-102146-280

So, how should we clean our PCBs? Let's jump to solvents.

Types of solvents used in electronic assemblies

• Water - your pure tap water, free of charge
• DI water - easy to buy, better than tap water due to lack of ions in it. Used as last step in cleaning process.
• IPA - popular, cheap, alcohol based. For general flux residue cleaning and tap water absorption
• Cyclohexane - for more stubborn residue. Combine it with IPA to get better results. Example: Electrolube FLU200D
• Isohexane - for heavy stuff including No-Clean flux residue. Example: Chemtronics Flux-Off No Clean Plus
• Safewash - for submersion, mist-spray and brush application cleanings. Eats everything. Needs to be rinsed with water afterwards. Biodegradable (in this specific example) and easily disposable. Exapmple: Electrolube SWAS05L

Cleaning methods

• Application by brush
• Submersion
• Ultrasonic submersion

Let's review closely these methods.

But before we do that, throw away your toothbrush or leave it for mechanical cleaning. Get yourself a proper ESD nylon brush like that:
 

First, before any attempt on cleaning a PCB, it's good habit to test solvent on various parts, specially plastic ones.

Application by brush - spray/pour solvent on board or in dispenser/container. Apply solvent on flux residue with brush and using little to no force scrape gently until clean. What I was taught is to use on flux residue a bit of cyclohexane, wait a bit for solvent to kick in and then clean flux+cyclohexane with a lint-free cloth and ESD brush soaked in IPA. The idea is to soak residue with solvent in a cloth and then clean PCB with IPA to solve rest of it and wick it in cloth again. Dry it with hot air/pressured air. For spot application you can use lint-free swabs or cotton buds. This methods works with almost all solvents, but for safewash it's better to use brush made out of natural bristle (wierdly it's ESD Safe too). You can clean almost anything like that.

Submersion - in glass container (I recommend Pyrex) pour solvent for submerging all assembly. Put PCB in and do according to instructions - for SWAS05L it's usually 15-20 min in submersion bath. You can decrease this time by gently scrubbing surface of PCB with brush. After that rinse it with water. If you are using DI water your good to go for drying. If you are using tap water it's good idea to submerge it in IPA (cause it's hygroscopic). After that it's common to BAKE boards in 60^oC for 2 hours to take all the remaining moisture. With that method you cannot clean most of non-sealed electromechanical components (potentiometers), displays, like LCDs, already fitted and so on. It's a good practice to desolder them prior cleaning with this method.

Ultrasonic submersion - as far as my knowledge is reaching, I know that this method is used for heavy corrosion and major dirt on PCBs that could not be removed with other methods. It's similar to submersion method but we add ultrasonic resonance in contaminants by effect called cavitation. Treat it like micro scale explosions and implosions that ripps apart dirt and corrosion. With this method not only we have option of heating solvent, which increases its solving properties, but also we can vary frequency of ultrasound itself. Low frequency is producing more powerful and more destructive cleaning process on the same power setting as higher frequency. Higher frequency is therefore more gentle, produces less noise but takes more time and might not clean all contamination. This method is a no-go for all electro-mechanical parts with moving internal parts like oscillators, crystals, accelerometers, sensors and so on. Good habit is to desolder them prior cleaning.

Some more info on cavitation: https://en.wikipedia.org/wiki/Cavitation

I hope this topic gave you a handful of knowledge to apply in your future project. I might got something wrong or leave some information out so if you could be so kind please add some tips and tricks in posts below.

Regards

[Psi]

One thing i will say about IPA is it saturates with flux very quickly.

New IPA + a brush will remove flux extremely well but after cleaning maybe 2-3 pcbs there is so much flux dissolved in the IPA that when it evaporates off the PCB it leaves behind a white IPA residue. So you think you have cleaned the board but when it dries the flux stains are back.

Proper PCB flux cleaning solutions don't have this issue to anywhere near the same degree.