I've always used a combination of wet & dry. Many of the newer model printers allow you to choose any combination but before you choose there some questions you should answer to determine your cleaning process:
- Type of stencil - regular etch, laser etched or electro-formed.
- Does the stencil aperture walls have any special coating to help release of solder paste?
- Are there any fine pitch components on your board?
- Are there any BGA's?
- Last but not least - are you using the correct solder paste for your application?
Having the answer to these questions will help you determine the best under stencil cleaning for your process. For example, in my case, I have laser-etched stencil, 21 mil pitch components, 2 large BGA's and type 3 NC solder paste. We set the stencil at Wet, Wet, Vacuum, Dry; every 10 assemblies and have no issues.
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.
Underside stencil cleaning should be performed based on the solder paste supplier's recommendations. We recommend using a 3 part cleaning process including a wet wipe followed by 2 dry wipes with vacuum. It is important to use an appropriate solvent for the wet wipe to loosen and remove the solder paste from the bottom of the stencil.
It is likely that some of the solvent will mix with the solder paste inside the apertures, and it is important that the solvent contaminated solder paste be removed from the apertures. That removal is accomplished through two dry wipes with vacuum.
The material used for wiping will also affect this cleaning process. Some materials are better at trapping and holding solder paste than others. I suggest working with your cleaning chemical supplier and wipe supplier to get the optimal cleaning and wiping materials for the solder pastes used.
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.
I would only recommend wet clean, then vac, then Dry. Using a 50gsm material not 68gsm. Seen too many Dry wipe, Vac, Wipe just smear the Paste powder underneath the Stencil, causing all kinds of issues, even bridging in the worst cases.
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.
There are many ideas on this subject. From my experience as a onetime solder paste developer is the stencil cleaning chemical compatible with the solder paste that is being used. If it is not you may create a gooey mess that will not remove paste residue easily. This will lead to insufficient prints as the paste will stick in the aperture and the next print will be insufficient.
Assuming the chemistries are compatible, and vacuum is available, wet wipe then dry wipe then vacuum wipe. In the absence of vacuum; wet wipe thane dry wipe.
If you are still having solder paste release issues, poor print from blead out or insufficient deposits, remove the stencil and check the bottom for dried paste or coined on solder spheres. This can be removed with a razor blade if needed be.
Karl Seelig
Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.
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