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Poor Paste Release

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#38890

Poor Paste Release | 9 January, 2006

Hi all, I want to know the possible causes of poor paste release in stencil printing process if the same brand and part number of solder paste that has been used successfully is being used with a set of process parameters which worked before. Other than the Temperature and Humidity, what else can be the causes

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#38895

Poor Paste Release | 9 January, 2006

Squeegee pressure or height, snap off, the age and/or condition of the paste and the condition/cleanliness of the stencil are about all I can come up with, unless you got a new stencil and you changed the design parameters or they made it wrong.

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#38896

Poor Paste Release | 9 January, 2006

Hi,

If the stencil was not cleaned well from the previous run, dried paste residue can cause the paste to grab in the stencil. If you have a microscope, check the small apertures for contamination.

Regards,

Grant

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abhinavajmera

#38901

Poor Paste Release | 9 January, 2006

Dear Grant, Thank you for the reply. It is really a valid reason and I did check for it but nothing turned out that way. Can you think of some other reasons if the process parameters are the same used earlier when there was no problem at all.

Regards, Abhi

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#38904

Poor Paste Release | 9 January, 2006

I assume you're using a no clean Pb/Sn paste, with a metal squeegee. Here are some generic procedures for good printing results for all pastes. Clean and dry the stencil well using alcohol and vigorously rubbing it with lint free towels, blow it off with clean compressed air, put a new batch of paste from a jar or syringe with a different lot number on the label. Put enough paste to make a roll about the diameter of a nickel. Make sure that the paste was warmed to room temp by sitting outside the refrigerator for at least 24 hours. Do not heat the paste to warm it to room temp! Some people like to accelerate the warming process by setting the paste on the oven prior to use. Stir it for a few minutes by hand if it's in a jar. Run the stencil on contact with the pcb (no snap off), use 1b-2lb force per inch of stencil contact with the pcb. Set squeegee speed to 2-4 inches/second. You want to see the paste roll actually roll in front of the squeege, sometimes varying the speed has an effect. If you still get sticking, I'd consider another career path...just kidding...try another of the leading supplier's paste, Indium, Alpha, or Kester. They will happily send you a free sample to try.

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#38906

Poor Paste Release | 9 January, 2006

Take a look at the boards, too. Could the HASL be different (in particular a domed or convex cross section) than what you're used to seeing?

Is your printer actually working properly?

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#38920

Poor Paste Release | 9 January, 2006

"Poor release" (paste hanging-up in the stencil holes) could have three main (or combination of) causes: 1 Equipment * Proper aperture area ratio provide better release * Laser cut release better than chemical etched stencil 2 Process * Snap-off distance is too small. * Temperature in your screen printer needs to stable. Paste suppliers say that 1*C change can affect paste viscosity by about 10%. * Paste needs to be worked * Paste needs to be at room temperature * Stencil needs to be clean. * Fast separtion speed is best [you have good vacuum tooling to hold the PCB down, otherwise it will stick to the stencil] * Decrease print head speed to get the paste to stick to gold pads 3 Material * Solder paste viscosity is too high * Solder paste is too tacky * Solder paste particle size is too large * Some pastes stick more than others * Some flux formulations are very sensitive to humidity

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