Cleaning PCB for Conformal Coating

[tkuney]

Greetings,

We are in the process of trying to properly clean PCB so that they will accept a conformal coating. These boards have a slight dust/oil build up on them which does not allow the conformal coating to adhere properly.

I'm looking for ideas, cleaning with denatured alcohol and an anti-static swab is not cutting it.

Thanks

-Tim

 

[melone]

Have the PCB manufacturer clean the boards before shipping. This is not an uncommon request.

 

[tkuney]

I should have added.. These PCBs are coming out of equipment that had been in service for 1 year or less and are being conformal coated before being placed into a new application.

 

[BrianG]

It is possible to obtain stronger solvents which can be applied by brush (note that Health and Safety Regulations in your country may prohibit this). The manufacturer of the conformal coating can often provide a suitable cleaner for this purpose.
Otherwise, you may need to get the boards put through a the sort of cleaning tank used in production assembly of circuit boards. This uses a strong solvent vapour to remove grease and flux after assembly. Typical solvents (e.g. "Arklone&quot are from a similar family to those used for dry cleaning clothes. Whilst these are usually safe on most plastics, they are highly volatile and must only be used in a proper tank installation with a vapour trap. If your circuit cards have any potentiometers or wound tubular polystyrene capacitors, unless they are fully encapsulated types it is normally not recommended to subject them to this treatment and they would need to be removed temporarily. (In production they would be fitted after the main cleaning operation.)

 

[tkuney]

Great feed back, thanks.

I'm a little concerned about using such strong cleaning agents since my company is not the OEM of these boards. What about using de-natured alcohol to remove the "grime" and then putting the boards through a de-ionized water bath. would that be the safest bet?

-Tim

 

[Comcokid]

Isopropyl alcohol (2-Proponal) is a common circuit board cleaning agent used for bench work and low volume defluxing. Scrub with a fine bristle brush and blow dry. This may leave some white residue which is a flux residue.

There are also spray cans defluxer for technician/benchwork use (On our bench, I find Techspray Defluxer 1676

http://www.techspray.com)

It cleans better than isopropyl.

In volume manufacturing, warm water and a supplemental mild detergent are used to clean boards, followed by blowdrying. This may be more suited to your needs if there is an unusual oil-film left on the boards from application. I don't know exactly what detergent might be used.

Be careful using pressurized air to blow dry your boards. This can sometimes force the cleaning agent past seals in electrolyitc capacitors and sealed items. Hold the air nozzle at least 12" away from the board.

There are components that don't like cleaning - usually items like potieometers, trimmers, and PC mounted transformers.

Whatever you do, dry throughly before coating.

 

[felixc]

If you have any quantity, I suggest that you go ask to a pcb assembly house for a quote, just for passing the boards through their cleaning machine. Should'nt be that expensive, and it may assure you of a better chance for your conformal coating to adhere well and to not trap contaminants under. If not, you can also try Axarel 2200, and use an air blower to dry the boards.

 

[tkuney]

Thanks for all the great info all!

 

[47gandalf]

And for a really simple, cheap-and-dirty approach that is surprisingly effective try this.

Put the boards in a dishwasher and wash them with ordinary dishwashing soap.

This will solvate many types of contaminants, but does not bring the issues that organic solvents do as others have pointed out (like dissolving capacitors).

Of course your coating manufacturer's applications information is very important.

If you do this and want to accelerate the process, after cleaning put the boards into a low temperature oven (like about 95 C) for a few minutes (15-20 ?) to dry them completely before coating. As well as drying them, this leaves the boards clean and warm and often promotes good adhesion of the conformal coating to the board and components. And in many cases it reduces the drying time for the coating.

Several companies I've worked for have used this technique successfully for years. I was pretty disgusted when I first heard of it, but it really is effective for many applications.

 

[Comcokid]

47gandalf method is one frequently used in PCB cleaning. At one company I worked at that made small circuit boards in high volume, water-soluable flux was used for the solder (both IR reflow for SMT and a small "drag" solder machine for thru-hole) and dishwashers were used for cleaning. More specifically, one Maytag and one GE dishwasher bought at the local discount store. These proved to clean better than more expensive circuit board washers.

In the old days of chassis, tubes, and point-to-point wiring, a recommended technique for cleaning a dirty business-band tranceivers call for putting it into a dishwasher followed by drying it in a oven set to 200 F. If a component didn't survive the cleaning - it needed replacement anyway. I once cleaned a very dirty radio that had been junked by a taxi cab company by spraying it with "Gunk engine degreaser" and pressure-washing it at the local car wash. The radio came out clean, and was not damaged by the wash.