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Clean before conformal coating?

Cory

#2081

Clean before conformal coating? | 4 January, 2001

We're going to begin conformal coating a few of our assemblies as required by our customer. We're using Kester No clean paste and will use cotton gloves to keep the boards clean while handling. My question is: Do we need to clean the boards prior to applying conformal coat (Humiseal TS300)? Humiseal Corp. gave us mixed answers on the subject. I'm also curious about the requirement for cleaning after wave solder using no-clean flux. Do I need to clean those boards prior to spraying conformal coat? Thanks for any help you can give.

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Mike Konrad

#2082

Re: Clean before conformal coating? | 4 January, 2001

Hi Cory,

As a manufacturer of aqueous cleaning systems, my advise is somewhat biased, however I believe it to be accurate.

We have hundreds of customers who have implemented no-clean programs. The programs have been successful when measuring the reduction of the quantity of boards requiring cleaning. Very rarely however, has a contract manufacturer been 100% successful in the complete elimination of a cleaning process. It seems true that at least some boards require cleaning. The most popular reasons as stated by our customers�?

1. Customer requirements dictate cleaning. 2. Elimination of slight visual residue (even if it is considered �harmless�) 3. Bed-of-nails tester probes get �gooey�. 4. Superior adhesion of conformal coating.

Handling a board carefully after reflow will not address preexisting contamination from fabrication.

Anecdotally, we have a customer that manufactures flight data recorders. Everything is conformal coated. The customer switched from one type of cleaning process to another and experienced a marked improvement in adhesion.

I am aware that there are companies that conformal coat no-clean boards wihtout cleaning. They have great ovens or waves with very good spray fluxers and very tight process control. Profiles are very tight.

Conclusion� Clean a board a little, get fairly good adhesion results. Clean a board thoroughly, get excellent adhesion. Skip the cleaning process, iffy.

Good Luck Mike

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CAL

#2083

Re: Clean before conformal coating? | 5 January, 2001

While there may not be any specific requirement to cleaning prior to coating, cleaning an assembly prior to coating will only serve to increase the reliability and integrity of the coating. Some residues from low-solids fluxes can interact with conformal coatings. This interaction can lead to poor adhesion or cure inhibition. Even beyond flux residues there can be other materials present on an assembly that can lead to decreased coating reliability. These include items such as: plating chemistries from bare board fab, marking inks and mold release agents from components, greases, oils, lubricants, etc. from poor handling and assembly equipment. Some cotton gloves may also leave halides on boards surfaces. These halides (chlorine primarily) are present on the gloves due to the bleaching process. Proper cleaning will always increase the reliability and robustness of a conformal coating process. Cleaning will remove most of the "factors" that can occur during assembly that affect conformal coating. If you have any questions please feel free to call me at 610-362-1200 x210. Brian Toleno- ACI/EMPF

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Jim M

#2084

Re: Clean before conformal coating? | 5 January, 2001

I've been involved with no-clean flux and conformal coat in the past.It has been my experience that the amount of no-clean flux used is proportionate to how well the conformal coat adheres to the boards.No clean paste did not affect adhesion.

The company i previous worked for used UV cure urethane conformal coating. When i first started there, i noticed the conformal coating bubbling especially along the edges of the boards. It seems during the wave process, no clean flux was pooling along the edges of the board creating adhesion problems down the road with conformal coat. I reduced the amount of flux to a minimum and the solid used in the flux to a minimum without creating more defects in the solder joints. This seem to work as our boards went right from wave solder, second assy. to confromal coat without being cleaned.

If the boards were cleaned with IPA ( which just distibutes the no-clean around to other parts of the board) adhesion problems namely bubbles were still prevalent.

No-clean solder paste was never a problem for cleanliness and adhesion.We built well over a 100,000 boards with AIM noclean solder paste. Many of these boards went direct from SMT assy right through to conformal coat. Many of our boards were in jungle enviroments (Hightemp and high hummidity). There were no RMA's that i was aware of caused by no-clean solder paste used on boards.

I was involved with this for 5 years and if i can be of any help, give me a call at 613-271-0469 direct.

Good luck

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#2085

Re: Clean before conformal coating? | 6 January, 2001

Some NC fluxes work well with conformal coatings others do not. Your coating materials supplier should be able to help. If they can't you're talking to the incorrect person.

Try our friend Graham.Naisbitt@concoat.co.uk www.concoat.co.uk CONCOAT Ltd Alasan House, Albany Park CAMBERLEY GU15 2PL UK Tel: +44 (0) 1276 691100 Fax: +44 (0) 1276 691227

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Greenman

#2086

Re: Clean before conformal coating? | 6 January, 2001

Cory, I've got a lot of experience of testing of this type. The physical (not chemical) nature of the flux residues is the crucial issue in compatibility. If you have a softer residue (either caused by a thick initial solder paste deposit, or a low peak reflow temperature - which retains solvent), you are in for trouble. You may find that some of the newer ATE-compatible pastes will cause you more problems than the more traditional Kester R244NC or Alpha Metals RMA390-DH4 no-clean types.

Also, the cure temperature is important - I've no idea if the Humiseal is a thermal-cure type, but in my experience with some Dow silicone materials, a high cure temperature (over 85�C) can be fatal, as resins start to soften around this temp.

Basically: keep your reflow profiles high and your cures low, and you will have fewer problems.

Greenman

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