|Ask the Experts|
November 9, 2020
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.
Technical Sales Manager
BLT Circuit Services Ltd
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!
S T and S Testing and Analysis
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.
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.
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
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.
Technical Support Engineer
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).
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.
Senior Manufacturing Engineer
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:
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.
Senior Applications Chemist
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.
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.
Vice President, Technical Director
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.
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.Jonathan Doan, Nordson MARCH
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.
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