|Ask the Experts|
September 25, 2020
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.
Principal Product Engineer
Benchmark Electronics, Inc.
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.
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.
Process Sciences, Inc.
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.
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.
Senior Project Engineer
Electronic Controls Design Inc
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.
Manufacturing Applications Specialist
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.
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.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
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.
President/Senior Technical Consultant
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.
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:
As these questions are considered, a suitable clean/no-clean strategy will be evident
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.
Senior Manufacturing Engineer
If it is not specified, you can choose your own process.
The decision for the process type comes form a list of factors involved:
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
Engineering and Operations Management
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