Ask the Experts
March 17, 2022 - Updated
July 20, 2017 - Originally Posted

Cleaning Under Low Clearance Components

We have a power-control board designed for Pb-Free processing that has many surface-mount components, with parts mounted only on the top side. Several ICs and transistors are leadless (very low-profile) parts, some with thermal solder pads on the package bottom.

I am requesting a no-clean solder-paste process because of the inability to adequately clean a water-clean flux from under these low-profile parts. These boards will be placed in an outdoor environment with uncontrolled temperature and humidity.

Are my fears about properly cleaning water-clean flux from under low-profile parts justified?


Expert Panel Responses

Mostly, they are correct although there are newer Water Soluble pastes available which give a much longer cleaning window and far less activity if uncleaned compared to conventional OA Pastes.

These are almost as clean as no cleans in its uncleaned state so could be a good option for you to explore. Have a look into WS488 and WS483 from AIM Solder, hope it may help.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Your concerns are valid however the ability to clean will depend on the surface tension of the water solution you are using. Clean DI water is typically in the 72 dyne/cm range which is too high to ensure solution flow under these parts. Addition of surfactants can reduce this value to below 30 dyne/cm or less which allows for solution flow beneath devices and therefore the ability to solubilize the residues.

You still need to ensure solution exchange and removal of the soils and to ensure that any residues left from the surfactants are benign. Dwell time in the chemical chamber, solution maintenance, temperature and water quality are process parameters that you will need to control tightly to ensure cleanliness and testing regularly using Ion chromatography. It might be easier to use a suitable no clean!

Gerard O'Brien
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.

Yes, more than likely, you are jeopardizing the reliability level of your product by not removing residues from a water-based solder flux. This is why more companies are returning to Low-Rosin based "no clean" manufacturing processes and then cleaning those boards with a vapor cleaning process.

Vapor cleaning is "molecular" in that the vapor molecules can penetrate around and under your "on-board" surface mounted components with the lowest possible "surface tension" ... lower than "surfactant-plagued" water cleaning processes (which often leave surfactant residues). The future is "molecular hydrocarbon vapor cleaning" and condensing vapor molecules are the answer.

You can see electronic cleaning equipment manufactured by Baron Blakeslee, Reliance, Branson, and others, and vapor cleaning chemistry from Nugentec, MicroCare, Reliance, Petroferm, Kyzen and others. Plenty of options are available and many companies have test labs where you can send some boards for test cleaning.

Take caution in the selection of your "no-clean" low-solids solder paste as some of the advanced "no-cleans" exhibit properties similar to adhesives. Go backwards in the "no-clean" solder paste selection about 5 to 10 years to get a good solder paste, at a reasonable price, that can actually be cleaned. Feel free to contact me for assistance or for more info on vapor processes.

Rick Perkins
Chem Logic
Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.

We place thousands of the leadless components with center thermal pads that would be considered low clearance. All done with water-clean flux. We have been doing this for many years and I have yet to see an assembly come back for reliability or early failure caused by trapped or remaining flux residue.

We use an Aquastorm 100. We do have certain customers that consider their product to be of a critical nature, e.g. mission critical or space rated, that prefer in order to minimize risk we use a LO or MO flux. However, these are end use situations where replacing the assembly in the field is impossible or failure is life critical.

Tod Cummins
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
Tod has been working in the Aerospace Electronics Industry for 25 years, beginning with 4+ years working for PCB fabricator ending as the Quality Manager and 20 years with Delta Group Electronics Inc. an AS9100 registered electronics contract manufacture. Currently position is Director of Corporate Quality Assurance.

Yes. Your fears are justified. Water washable/soluble solder pastes are inherently more active/corrosive than rosin based no-cleans. If any residues are left uncleaned they can cause reliability issues such as corrosion and leakage current.

Kay Parker
Technical Support Engineer
Indium Corporation
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

Yes, your fears are absolutely justified. Residual water-wash flux is very corrosive, and it will be near impossible to ensure adequate cleaning under components such as QFNs. Your environment dictates that a good no-clean chemistry, properly process to ensure it is reacted, is used. Y

ou may also want to consider conformal coating as an additional mitigation (depending on the service conditions, level of protection from the housing and amount of self-heating, you may or may not require this).

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

If you have the option to use a no-clean solder paste formulation then even better. You can still use a water-soluble paste but it will require additional testing like flux residue composition analysis and matching the correct solvent to remove those residues.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

The question "Are my fears about properly cleaning water-clean flux from under low-profile parts justified" can be answered in a most unsatisfying way: Yes, and no.

Yes, your fears are valid. If wash solution is allowed to remain on an assembly, that would be worse than leaving flux on the assembly. Cleaning is an all or nothing proposition. My advice is best surmised with the following statement: "Clean well or don't clean at all". Now for the good news. It's totally possible (and commonplace) to clean well, even under low stand-off components. Cleaning machines equipped with best-practice spray and contamination detection technology routinely provide very clean and consistent results.

One also must consider the risk of not cleaning. Keep in mind a cleaning process removes far more than flux. During the journey to become a circuit assembly, the board is exposed to a lot of contamination species. Board and component fabrication as well as human and assembly contamination (including flux) add to the totality of residues commonly found on a circuit assembly. My not removing the flux, all other residues are allowed to remain.

All assemblies have a residue tolerance (the ability to withstand contamination). The amount of residue toerance is determined, by several factors including:
  • Component density
  • Voltage
  • In-use climatic environment
  • Cost of failure
Based on your description of the assembly, I would be concerned about component density ("very low profile") and in-use climatic environment ("outdoor environment with uncontrolled temperature and humidity"). My fears would not be about the cleaning process rather the results of not cleaning.

Mike Konrad
Aqueous Technologies
Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.

Cleaning under low standoff components is particularly difficult. With appropriate equipment and process setup it is possible to clean under such components, but typically the process window is limited. Appropriate process monitoring and cleanliness testing will be need to be confirm and maintain the process performance.

Having said that there can be issues with no-clean fluxes on such components especially those with large thermal pads, the flux can fill all the available space under the component, this can result in solvent retention in the flux residue which can in turn lead to dendritic growth.

Neil Poole
Senior Applications Chemist
Henkel Electronics
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.

Your concern about hi-rel cleaning under very tight stand-offs is well-founded. OA fluxes - even no-clean OA fluxes - produce shiny solder joints but at the expense of generally using highly activated fluxes and pastes. They must be properly cleaned which, as you suspect, is a problem. Your no-clean materials probably won't get the job done, either. They still contain activators that love humid environments and thermal cycles. With all the activators left on the PCBs in a humid environment, both of these types of materials easily can cause white residues, corrosion and dendrite growth.

I would recommend using an easy-to-solder and easy-to-clean RMA flux, and then cleaning the boards properly.

Water cleaning is not a bad idea but you are right to reject it. For non-critical applications aqueous cleaning can work quite nicely. However, there is the fundamental limitation in the water molecule itself. Water cannot get into the tightest spaces or smallest apertures because it has a relatively high viscosity and high surface tension. If you can't wet a surface you can't clean it - which is exactly the problem under a tightly-fitting SMT component.

Companies offer all sorts of band-aids to fix this problem. Vendors add saponifiers and surfactants to improve the surface tension of the water. These single-use additives generally are expensive. Equipment makers add more kinetic energy in the form of longer machines, bigger pumps and more powerful sprays; these also add expense. Water does not easily traverse 2 or 3 micron filters so the only way to deliver micron-grade waste-water treatment is with big, energy-hungry pumps to force the water through the filters, which add more operating costs.

But, as my colleague John Hoffman likes to say, "You can't fool Mother Nature." There's only so much you can do with a power-washer. The water molecule itself is the limitation. This mean those hard-to-reach locations will not be wetted and they will not be cleaned. You have every reason to expect those fluxes and pastes will remain under those chips for years to come.

The numbers don't lie. The best way to judge this for yourself this is to compare the "wetting index" of different cleaning fluids. The "wetting index" predicts the relative cleaning effectiveness of different fluids. All things being equal, cleaning is improved with lower surface tension, lower viscosity and higher density. The wetting index combines those characteristics - viscosity, density and surface tension - into a single number, allowing apples-to-apples comparisons.

The wetting index of water is very low - about 11 - and 15 the wetting index of IPA isn't much better. But there are popular, affordable, nonflammable and environmentally-acceptable solvents on the market today from a variety of vendors (MicroCare, Chemours, 3M, Honeywell, Solvay and others) which offer cleaning fluids that perform very well and have wetting indices over 100. These products can get under your tight-fitting chips and dissolve those fluxes, delivering perfect cleaning virtually 100% of the time.

These fluids are used in vapor degreasers. Vapor degreasers clean the parts in boiling solvent. The system then distills the solvent, purifies it, and re-uses the purified liquid in the next cleaning cycle. High capacity systems use very little electricity, have small footprints, are easy to use, highly reliable and deliver extremely consistent cleaning. Most importantly, the solvent never wears out and can be re-used hundreds of times.

All in all, neither "no clean" fluxes or OA fluxes with water cleaning are ever going to give you the long-term quality you need for your hi-rel PCBs in a harsh environment. If you want to sleep at night, you need a nonflammable, fast-drying, environmentally-progressive cleaning fluid and a vapor degreaser. That combination will give you the reliability you need at a cost you can afford.

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.

The use of a low-solid no clean flux when used properly, i.e., correct profile and reflow cycle, the residues will be benign to the functionality of the product. Cleaning beneath these surface mount components is almost impossible hence the major reasoning to qualify your fluxes and thermal profiles. Qualifying the fluxes to the J-STD-004 will provide information as to whether the residual material remaining on the product will be detrimental to the functionality of the product.

Because the products are to be used in a hostile environment, perhaps the consideration of using conformal coating may be an option to enhance the protection from the environment.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

I agree with your concern about cleaning a water-clean flux beneath low profile components. There can also be concern about having full activation of a no-clean flux beneath these components, as well. Either situation may be assessed using IPC-9202 and IPC-9203 as a guideline with a customized SIR area for testing if the IPC-B-52 board does not address your challenge.

Another point for low standoff components is that the incoming bare board cleanliness may need to be confirmed, as well since placement of the component can prohibit "cleaning" of the shielded area.

Renee Michalkiewicz
General Manager
Trace Laboratories
Renee has been with Trace and an IPC member for 16 years. She has managed all military and commercial PB qualification and conformance testing and training, as well as product qualification and testing in the areas of solder pastes, fluxes, solder masks, and conformal coat. She is the chairman of the IPC Testing and the IPC-J-STD-004 Flux Specification Committees and the Vice Chairman of the Assembly and Joining Committee. She has published more than a dozen papers and presented at numerous electronics conferences.

Reader Comment
This situation could use plasma treatment for a few reasons. Plasma treatment would prep the surface and board for conformal coating and underfilling. If the user intends to place the board in an environment with temperature fluctuations, then there are more concerns than just water-clean flux. Any air pocket or thermally sensitive material could expand or contract in low/high temperatures. If the underfill or flux is not void-free, then it will be problematic.

In another case, the lack of conformal coating will expose the flux and electrical components to the environment. Moisture and temperature will lead to erosion, which will shorten the life and reliability of the part. And finally, the conformal coating might not adhere uniformly if the surface is not properly prepared and treated. We have seen where residual flux causes issues with adhesion for the conformal coating. The board may still need to go through an aqueous wash to completely remove bulk amounts of flux prior to plasma treatment, which is intended to remove trace flux residues.
Jonathan Doan, Nordson MARCH

In an update to the original post five years ago, there are now available aqueous cleaning solutions designed specifically to clean water soluble / OA flux residues. Used at low percent concentrations these advanced cleaning agents are targeted at organic acid fluxes.

These cleaning solutions completely remove all water soluble residues and other possible manufacturing contaminates and effectively cleans underneath low standoff or flush mount components.

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.
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