Solvent Attacking Flex Circuits

We use EnSolv and a vapor degreaser to clean our flex circuits assemblies. The process works great, but the Ensolv seems to be attacking the inner layer adhesives. Are there other vapor cleaning materials or process improvements we should consider for cleaning our flex circuit assemblies? Is vapor degreasing still common in the market?

T. G.

Expert Panel Responses

T.G. I have several customers today that routinely clean flex circuits using a vapor degreaser containing a nPb solution similar to what you are using. No harmful effects are noted. The inner adhesive layers will be compromised if the adhesive formula and cure cycle is not followed correctly. It must not waiver from the adhesive manufacturers recommendations for application and cure. I have seen the adhesive attacked even when using aqueous wash solutions. When correctly done it is almost bullet proof. Bottom line, the wash process has exposed the defect. There are some process guidelines you can adhere to that may minimize any chemical attack. Many vapor solvent users do not immerse the substrates into the boil sump. All the cleaning is done in Tank 2 with ultrasonic or spray under immersion energy. "Final rinse" is followed by a specified dwell in the vapor zone where the purest solvent exists. Vapor degreasing with brominated (nPb) or other HFE solvents has its niche in the North American electronic assembly market. Modern equipment is readily available to properly operate and safely manage these solvents. The limitations to the CM or OEM are the new solder paste flux formulas commonly used today. Unlike rosin flux these new "no clean" formulas do not match up well when exposed to these vapor degreasing solvents. It is important that any wave flux or solder paste formula be screened for cleaning effectiveness in your vapor degreaser prior to approving any flux formula change. If you would like to discuss this further do not hesitate to contact me.

Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation

Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.

You have discovered the convenience of using a super-strong solvent can sometimes bring with it unexpected consequences. In this case, the Ensolv material -- which is based on a chemical called "normal propyl bromide" or nPB -- is attacking the flex circuit. nPB is even used as an adhesive remover, so this is not totally unexpected. Nor is this a fault of Ensolv; ANY company offering nPB in this application would have pretty much the same effect. The strength of a cleaner is measured by its Kb value, which is a continuum of cleaning strengths from very mild to extremely aggressive. A mild, plastic-safe cleaner made of alcohol might have a Kb value of 10 or so. nPB has an Kb value of 130. This means nPB is so strong is definitely will attack plastics, elastomers and adhesives. Ensolv should have told you this. You can learn more at http://www.microcare.com/faqdetails.aspx?faqid=101. The answer is to back-pedal to a milder solvent. You can't stay with nPB; there's only one degree of cleaning horsepower with nPB and it's the one you have. But there are many good choices available from 3M, DuPont and MicroCare (none of them containing nPB) and they all are vapor degreaser compatible. Two possible candidates might be MicroCare CMS or MicroCare CFIPA, depending upon the contamination you are removing. You'll notice a couple of other benefits from switching from nPB. You won't have to do acid testing every week. You won't have to worry about the health effects of nPB (which is 10 ppm -- a VERY low number). You won't have to tolerate the funky aroma. And, with proper tech support, your consumption of solvent will probably plummet. Contact your local distributor for details.

Mike Jones
Vice President
Micro Care

Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.

I would be very careful using these types of cleaners when you have exposed acrylic adhesives.

James Mahoney
Applications Project Manager
Quick Turn Flex Circuits LLC

James Mahoney is a Technical Operations Manager with a 20 year track record in managing new product introduction. He is a skilled leader, motivator and problem solver with a strong background in Product Knowledge and Engineering Management.

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