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.
  
  

#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,

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.

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

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,

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.