Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Bill Haynes

#12632

Squeegee Pressure | 12 February, 1999

I am fairly new to the SMT game and I am getting conflicting answers to the following. Maybe someone can help me! When printing, one person is telling me that the stencil should be wiped completely clean of all paste because the quality of the print will be better. Another person tells me that there should always be a thin layer of paste on the stencil because too much pressure can cause paste to be pushed under the stencil and cause poor print quality. We use stainless squeegees and do an automatic wet/dry wipe every 20 prints. Thanks in advance!

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Craig Lowe

#12633

Re: Squeegee Pressure | 12 February, 1999

Bill, I would go with the second piece of advice by leaving a little film of solder paste on the screen. This is good advise if you are using a lookup/look down type of vision centering. Like on MPM and newer Fuji. On older Fuji GSP2/3 style printers. I would go with full contact on the screen and decrease the appeture size on all fine pitch parts to reduce shorts and solder balls. Hope this helps. Craig

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Jon Medernach

#12634

Re: Squeegee Pressure | 12 February, 1999

Bill, You should wipe clean. The thin layer will dry out quickly and cause other problems. Remember squeegee down stop and squeegee pressure are seperate issues. If your downstop is to low the stencil and board will flex down, after the squeegee passes it will spring back to the original position causing the print to deform and solder on the bottom of the stencil.

| I am fairly new to the SMT game and I am getting conflicting answers to the following. Maybe someone can help me! When printing, one person is telling me that the stencil should be wiped completely clean of all paste because the quality of the print will be better. Another person tells me that there should always be a thin layer of paste on the stencil because too much pressure can cause paste to be pushed under the stencil and cause poor print quality. We use stainless squeegees and do an automatic wet/dry wipe every 20 prints. | Thanks in advance! |

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Kelly

#12635

Re: Squeegee Pressure | 13 February, 1999

| I am fairly new to the SMT game and I am getting conflicting answers to the following. Maybe someone can help me! When printing, one person is telling me that the stencil should be wiped completely clean of all paste because the quality of the print will be better. Another person tells me that there should always be a thin layer of paste on the stencil because too much pressure can cause paste to be pushed under the stencil and cause poor print quality. We use stainless squeegees and do an automatic wet/dry wipe every 20 prints. | Thanks in advance! |Bill, your stencil should be wiped fairly clean, but you need to make sure that you are not using so much pressure that you get a scooping effect. We use MPM screen printers here and we reduce our apperture size for fine line components. If you are uncertain of your set up, print up some boards using different setting and take a look at the boards under a high powered scope. You'll be able to see the differences! Good Luck! Kelly

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Joe

#12636

Re: Squeegee Pressure | 16 February, 1999

| I am fairly new to the SMT game and I am getting conflicting answers to the following. Maybe someone can help me! When printing, one person is telling me that the stencil should be wiped completely clean of all paste because the quality of the print will be better. Another person tells me that there should always be a thin layer of paste on the stencil because too much pressure can cause paste to be pushed under the stencil and cause poor print quality. We use stainless squeegees and do an automatic wet/dry wipe every 20 prints. | Thanks in advance! | The paste thickness is determined by the stencil thickness. This means that by wiping the stencil clean you should be getting a paste thickness very near to what your stencil thickness is. This thickness has been determined, together with aperture reduction, to give the optimal solder deposit so you can produce the perfect solder filet. Leaving a thin film on top of the stencil will give a variable solder thickness over the whole board,and this can vary from board to board.This will eventually lead to shorts, solderballing, wrong filet shapes.....and anything else you can think of. Get your downstop and squeegee pressure in order,(everything should be parallel), the speed should match your paste,(slower speeds wipe cleaner), and you can wipe clean everytime. Joe

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Ross Berntson

#12637

Re: Squeegee Pressure | 16 February, 1999

| I am fairly new to the SMT game and I am getting conflicting answers to the following. Maybe someone can help me! When printing, one person is telling me that the stencil should be wiped completely clean of all paste because the quality of the print will be better. Another person tells me that there should always be a thin layer of paste on the stencil because too much pressure can cause paste to be pushed under the stencil and cause poor print quality. We use stainless squeegees and do an automatic wet/dry wipe every 20 prints. | Thanks in advance! |

Wipe the stencil clean - no method exists for controlling "paste-film" thickness. When I set up a printer, I usually set the downstop to default and start with presure of about 1pound/inch of blade. The pressure should be adjusted up/down to leave a very thin film on the stencil. I conclude the set up by increasing the pressure just enough to wipe the stencil clean. This process will reveal planarity and fixturing problems as well as minimizing paste bleed-out under the stencil. Good Luck!

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