Ask the Experts
March 8, 2023 - Updated
August 22, 2019 - Originally Posted

Water Wash vs. No-clean

How do we decide whether to choose a water wash process, or a no-clean process, for a contract manufacturing purchase order when neither process is mandated by the contract?

I have engineers from both camps digging in deep.


Expert Panel Responses

No-clean flux processing is more efficient - less handling, less equipment, and shorter cycle time. You would want to choose no-clean processing unless you have difficult to solder components that require more active fluxes.

Rick Kompelien
Principal Product Engineer
Benchmark Electronics, Inc.
30+ years of experience working with electronic and electro-mechanical manufacturing and design (medical, automotive, military, computer, and industrial controls). Military veteran - served as a Combat Engineer with the United States Marine Corps.

It is always safer / more reliable to wash off the flux residues along with other board contaminates. This exposes the circuit board and components to multiple water wash cycles and the components must be compatible with a water wash process. The fluxes are simply once source of "contaminate" that we add to a circuit assembly.

There are a variety of other contaminates that come from the bare circuit board, the components, and operators which can cause failures in the circuit assembly. If you plan to wash off the flux residues and other contaminates, it is always best to use water washable fluxes in the solder paste, wire solder and liquid fluxes.

Water washable fluxes are designed to be removed. No clean fluxes are designed to remain on the circuit board and are, by their nature, more difficult to wash off.

If you choose a no clean process, then I recommend not washing the no clean fluxes. They are safest when left undisturbed on the circuit assembly. No clean flux residues could cause electrical "noise" or other signal issues in sensitive devices so the circuit assembly must be designed with that in mind. With that being said, no clean fluxes, solder pastes and wire solder are used in high volume every day and can create reliable circuit assemblies.

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

From a CM point of view there would be no reason to add the extra step of cleaning to the process. In addition, there would be exposure to the potential that product gets out with corrosive agents on board.

From the OEM perspective, I would hope they have at least given a thought to the question.

Stephen Schoppe
Process Sciences, Inc.
Stephen Schoppe is President of Process Sciences, Inc., and has 19 years experience providing SMT services to electronics manufacturers. Stephen provides consulting to several Fortune 500 clients on solder and SMT processes, and is a frequent guest speaker at SMT industry events.

If the contract does not specify a cleaning process, but does specify a solder paste / flux system, then the contract should be modified to specify how to handle the situation. If a water soluble OA type (highly acidic) flux is required, then a water based cleaning process is definitely required and should be charged to the customer.

If a low solids system is specified and it is characterized by the manufacturer as “no clean”, then no cleaning should be required.

If the customer specifies a high-reliability product with limited failures and cleaning is not required, then you should clean the assemblies in a saponified water-based or semiaqueous solvent system, but renegotiate the contract based on the expected reliability level of the final product.

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.

If the assembly is NOT conformal coated and meant to be used in "reasonable" environmental conditions, a no-clean solder process without a wash is likely fine.

If conformal coated, high-rel, high impedance, or used in harsh environments, I'd clean it no matter what soldering process used.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

The chemistry you decide to use in your manufacturing process is decided by the customer requirements, most efficient process flow, type of components used and the best quality. If the customer does not require a certain process, you should fit the product

to the process that would work best for that particular product giving the customer the required end result. Some components cannot go through a water wash system. Many component manufacturers recommend a particular solder paste and wash requirements.

All of these need to be considered.

Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.
Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

I can understand that there are valid engineering arguments on both sides... on the one hand, you may not know what the customer's downstream processes and/or end-use environment is/are like, and leaving a no-clean flux in place may involve some risks.

An example of such a risk would be incompatibility with a customer's coating or potting material. On the other hand, water-soluble fluxes have their own risks, namely the challenge of effective removal from beneath very small stand-offs.

If you understand your cleaning process capability and are confident that you can effectively clean the assemblies, water soluble would seem to represent a bit lower downstream risk. It may, however, represent a bit higher risk in your own factory, since it does not normally print as well, may slump more during reflow and requires attention to minimize delay between reflow and cleaning.

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.

No-clean flux residue always has some possibility of electrochemical migration resulting in dendrite growth, although the likelihood is fairly low if the flux reaches sufficient temperature to deactivate the active elements of the flux. If the assembly is used in a non-indoor type environment moisture can result and increase the risk of no-clean further.

If it's a high reliability assembly it would be prudent to clean the assemblies. Highly active organically activated (OA) water soluble fluxes can be cleaned with just deionized water, but will rapidly cause corrosion if not cleaned properly. Many RMA or no-clean fluxes can be cleaned with water, but may require some type of aqueous cleaning chemistry additive to remove all of the residue.

Flux and cleaning methodology selection are critical. You should always work with your flux/paste supplier to ensure you have all of the information.

Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

This is a very political issue within a company.

The first step is to determine the level of reliably needed for the functioning hardware.

The second is what is the strength of the contract manufacturer. Have they been successful for the last 20 years with no clean or cleaning.

Then the most important issue to look at is how sensitive are the circuits to stray voltage and surface contamination?

I would suggest building hardware with both conditions and putting in to heat and humidity testing under power and functional testing to see how they compare. Then in parallel to some cleanliness testing and understand both the primary and secondary soldering effects.

Terry Munson
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

In general the decision as to whether to use a post soldering cleaning process or a no-clean process is driven by the end-use environment and life expectancy of the product. High humidity environments and longer life expectancies tend to require post soldering cleaning.

Most but not all high-reliability and mission critical products typically use a post soldering cleaning process of some type but even that is changing somewhat with the development of new flux and solder paste chemistries.

Today it is estimated that approximately 75-80% of electronic products produced worldwide are using a no-clean process. Additionally, if the assembly will be conformally coated, the protection afforded by the conformal coat typically allows for no post soldering cleaning.

Even though one particular process is not mandated by your contract, you should check with your customer to verify what level of post-soldering ionic contamination is acceptable since this will help to determine which process to use.

Carlos Bouras
General Manager
Nordson SELECT
Carlos Bouras is the General Manager of Nordson SELECT and has over 30 years of experience in the electronics manufacturing industry. Carlos's expertise is in process engineering, product development and manufacturing operations. For the past 15 years Carlos has focused specifically on automated assembly issues and is the holder of several US patents for non-contact dispensing and precision dispensing of adhesives for the packaging of microprocessor devices.

That's a great question and is quite topical.

If one reflows a circuit assembly using rosin or water soluble solder paste, then cleaning is a given. These flux types require cleaning. The use of no-clean solder paste does not necessary mean the assembly will not be cleaned. In fact, our research indicates more than half of all assemblers reflowing with no-clean flux clean the assemblies. If one chooses not to clean, then no-clean solder paste / flux is the only option.

One very important factor to consider is the purpose of a cleaning process. Historically, we described a cleaning process as a "defluxing process". This is not an completely accurate description. A cleaning process is designed to remove all contaminant species from an assembly, not just flux.

There are a host of contaminant species on all circuit assemblies, regardless of flux type. Residues from the board fabrication process, component fabrication process, the assembly process (including but not limited to flux), human contaminants, and other environmental contaminants litter the assembly with problematic residues. If one chooses to not remove the flux, one then chooses not to remove anything.

In the earlier days of no-clean fluxes, not cleaning was an accepted norm. Component densities, size, and stand-off heights were all considerably larger than in modern times. The amount of residue tolerable on a circuit assembly was historically higher than it is today.

Today, the totally of assembly residues frequently create a contamination volume totality which is frequently problematic to the reliable operation of the assembly (electrical-chemical-migration, dendritic growth, peracetic leakage, corrosion, and conformal coating delamination to name a few).

To recommend a true no-clean process (no cleaning), I would have to ask a series of specific questions including:
  • What is the component density (higher densities produce a lower residue tolerance)?
  • What is the lowest component stand-off height (low stand-off heights can trap flux activators, leading to ECM issues)?
  • What is the end-use climatic environment where the assembly will operate (higher levels of moisture/humidity increase the ECM probability)?
  • What is the cost of failure?

As these questions are considered, a suitable clean/no-clean strategy will be evident

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.

Your contract should specify which process to use. This is agreed between the two parties. It is a good practice to offer the customer with a presentation of your capabilities highlighting the pro's of each process and let them decide. The majority of customers will lean toward the no-clean process. However, you also need to consider the product. For example, the medical field tends to go with water soluble. The goal is to maximize your build efficiency and to minimize costs.

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.

If it is not specified, you can choose your own process.

The decision for the process type comes form a list of factors involved:
  • what is the boards population (component types, bottom termination components, etc)
  • is there coating, encapsulation, potting, etc involved
  • assembly application

Each process has pros and cons and that is why the team can be divided as every engineer is more comfortable and actually like a process better than the other for one reason or another

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at

Reader Comment
I agree with all of the comments The only other thing I would add as an assembler is that the process windows in terms of solderability are wider with water soluble flux chemistries due to the more aggressive nature of the fluxes. If the board is being conformally coated I would use a cleaning process for the most consistent results in CC. The painful part, if the component count and diversity is high, is that precautions to liquids need to be considered. This engineering effort may be time consuming.
Bob Wettermann, BEST Inc

Reader Comment
I have worked both on Clean and No clean chemistries. My personal preference is always on clean chemistry as long as you have the right machine to clean the PCBA and measure the effectiveness of cleaning process using a contamination tester. Cleaning No clean fluxes is always a pain and never an agreeable process as different people from various functions will have their own judgements on the acceptable cleaning levels of the boards.
Amarpreet Singh, Ideaforge Technology

Submit A Comment

Comments are reviewed prior to posting. You must include your full name to have your comments posted. We will not post your email address.

Your Name

Your Company
Your E-mail

Your Country
Your Comments

Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.

Insert Your Email Address