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November 1, 2017

What's Causing Sticky Stencil Blades?

We are having problems with the solder paste stencil printing application.

The problem is that the solder paste is sticking on the blades generating a curtain between the blades and the stencil, but when the next cycle starts all the solder paste is on the blades not on the stencil causing lack of solder.

We have changed the solder paste tube for a new one. We change the blades. We change the blade attack angle. But, the results have been the same.

Can anyone help us with process tips about what may be causing this condition?



R.R.

Experts Comments

Squeegee blades can make a difference as far as curtaining. Suggest you look at DuraGlide squeegee blades and see if it helps. I have attached a PDF, DuraGlide_Increase Throughput.

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Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

I'm going to guess that you are using a lead-free solder paste. This problem frequently becomes an issue with lead free solder paste, because it has very different rheology than leaded pastes. While not a 100% of the time, the squegees are very ofen the cause. When the regular blades are used (steel squegees), the aspect of the surface is very rough at the microscopic level, which is why the paste is sticking.

Now I don't want to sound like a advertisement, but I would recommend upgrading to very good quality squegees, such as Transition Automation, for exemple. Their finishes on the sqeegees seem to be smoother, and the paste doesn't stick, and I don't see this problem as often, even when the solder paste is very sticky.

Transition Automation also has a very nice system, with a wire sliding across the blade, for removing the paste from squegees.
Best regards,

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Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.

Could be that with warmer temperatures the paste is sticking to the blades. You might need to try a different viscosity.

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Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

First, ensure the paste bead size is being managed properly. Too little paste will exasperate this issue. Of course too much paste will also hurt your print quality, so be careful here. Rule of thumb is one-half to one inch of paste bead.

Depending on the Stencil printer your using, you may find paste management features available to you to address this issue. DEK Printers for example have "Dwell Speed" and "Dwell Height" parameters specific to each product file. "Dwell Speed" controls the speed that the squeegee lefts off the foil.

This parameter allows you the opportunity to match the shear of the paste, leaving the bead on the foil. "Dwell Height" controls the height the squeegee lifts off the foil after printing. By setting this to a low distance, (3-5mm) the squeegee isn't given the opportunity to lift the solder bead off the foil.

Chromium coated blades have also shown better paste release characteristics, which is why they are recommended for the tackier Lead Free processes.

Michael O'Hanlon
Applications Supervisor
DEK
Mr. O'Hanlon serves as the DEK Applications Supervisor for the Americas region. He has over 20 years of electronics manufacturing experience and has spent the last 13 years at DEK providing equipment utilization and process solutions for SMT manufacturing.

#1 The first thing to check is the squeegee blade height compared to the amount of paste your applying to the stencil. If you have too much material the paste will roll up over the blade height and attach itself to the metal fixture and will not allow the paste to drop.

# 2 If the paste is not at ambient room temperature, Recommend a minimum of 8 hours of time to bring the paste to room temperature naturally. If not the base chemistry will stick to itself and not allow the material to drop.

#3 Have the manufacturer (of the paste) check the retain sample of the base flux to be sure there are no solvent issues.

With best regards,

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Ted Marek
President
SMT Sales Associates Inc
Mr. Marek is the Founder and President of SMT Sales Associates Inc. SMT is a distributor and manufacturer's representative organization in the PCB assembly marketplace. He has over 11 years experience helping customers select soldering materials and capital equipment.

Assuming that your solder paste cannot be changed there are a few things that you can look at the help the problem;

  • Check with your solder paste vendor regarding their experience with this issue and what the suggested operating temperature is. Check to see what the temperature is within the print chamber and if it is higher than the suggested does your paste vendor suggest that lowering that temperature will help the issue.
  • Is the paste rolling in front of the squeegee during printing? Paste that rolls well during the print stroke will stick less.
  • If the paste does not roll, what is the attack angle of the blade during the print stroke, increasing print speed may increase the rolling action as well?
  • Different blade materials or finishes may also help.

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Steve Hall
President
EKRA America
Mr. Hall has spent the last 20 years in the electronics manufacturing industry. He started at Motorola specializing in the development of screen printing and reflow soldering processes. He has became known as an expert in printing technology.
Reader Comment
As stated by blade manufacturers, paste cohesion on the blades due to surface finish quality and or type of surface is a major factor to consider. Another consideration could be physical excitation of the blade when in the post UP position that would impose a "shake loose" action of the paste on the blade. Contact your stencil printer manufacturer if such an option is available.
Stephen Brodeur, Milara Incorporated
From my point of view, you have done a little bit of change overload without really knowing what the cause of this is therefore you could not get to the root cause.

One of my rules of thumb is that the paste has to roll in the application process. Should the paste not roll, you might not have enough from the beginning on your stencil (quantity).

There are way too many factors involved in this to be resolved in a feed back like this. Please e-mail me for more details and a plan to solve the problem. georgiansimion@yahoo.com
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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 georgiansimion@yahoo.com.
If everything is correct on your printer, here is one quick solution: If you have fluxophobic stencil treatment wipes, simply apply some to your stencil blades.  I am assuming that the flux in your paste is the issue.

I would start checking the humidity around your printers.  I am willing to bet it is high.
Paul Dickerson
Supply Chain Engineer
Matric Group
Mr. Dickerson is an engineer with 20 years of manufacturing experience. He has worked supporting SMT, THT, cable assembly, and box build processes. He is a Certified SMT Process Engineer.
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