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July 1, 2019

Maximum Allowable Time Prior to Washing When Using Water Soluble Paste

When using a water soluble paste, after SMT reflow, what is the maximum allowable time prior to washing? Also, is the maximum allowable time any different when using leaded or lead free paste?

What should be done with assemblies if they are not washed within that maximum allowable time?

M.M.

Expert Panel Responses

Many water soluble paste fluxes are acidic.

I would not leave it on my board one second longer than absolutely necessary. However, whether it is because of its potential corrosive effects or if its effective removal is reduced if left on too long, it will depend on the chemical makeup of the paste flux.

You must consult the paste manufacture or the published specification. Myself, I would wash it immediately after solder.

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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.

This is often an issue that is driven by the "end of shift, can I wait until tomorrow to wash these?" scenario. It may also be that there is the desire to process the boards through any subsequent solder processes and clean it all in a single wash cycle.

Always refer to the recommendations of your solder paste supplier. I've seen data sheets that say it can wait anywhere from 8 to 72 hours after reflow before washing. Although OA flux residue is normally much easier to clean than rosin-based residues, note that the residues will not get any easier to remove as time passes, as they sometimes harden up over time.

I'd suggest washing earlier than later, though you may be able to complete post-SMT solder processes provided the boards don't wait around for a period that exceeds your paste supplier's recommendations.

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Kevin Mobley
Manager, Manufacturing/Process Engineering
Tektone Sound and Signal, Inc.
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.

Reader Comment
We have received only a few of these over the last two quarters. It looks as if the ones we have received have been centered around parts placement and assembly. This requires us to go to our Assembly partner for clarification/resolutions. We are ok with that, but may take longer to get responses. Just making sure that we are also included in the category of design and manufacturing of PCBs as well.
Kelly Atay, Sunstone Circuits, LLC


The maximum allowable time for washing a water soluble solder paste depends upon the solder paste itself. The manufacturer can give specific recommendations for this. Generally speaking washing within 4 hours is preferred, but 8 hours is acceptable.

Some solder paste fluxes can be washed off after 24 hours, but this is not recommended. It is also advisable to wash off the flux reside after each reflow, e.g. wash after each reflow for double sided surface mount products.

Tin-lead and lead free fluxes may have different recommended times between reflow and wash, but again this is dependent upon the suppliers recommendations.

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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.

It is difficult to provide guidance without knowing more information such as is the factory environment such as temperature and humidity controlled, is excess solder paste removed from stencils and re-used, etc. The best approach would be to contact your paste supplier and obtain their recommendation since they know the specific chemistry and working life of their solder paste

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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.