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Printing Speeds for Fine Pitch

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#62582

Printing Speeds for Fine Pitch | 1 September, 2010

We are having solder bridges on a fine pitch QFP and was wondering whether printing speed directly affects the bridging and is there a typical formula to work out print speed?

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#62586

Printing Speeds for Fine Pitch | 1 September, 2010

Dear Leeg, Generally the faster your print speed the more down pressure you need on the blade to roll the paste and wipe the stencil surface clean. Pastes with higher viscosity need more pressure at slow and high print speeds. Too much pressure causes the paste to bleed under the stencil and causes defects like bridging. Correct printer set up of speed and pressure is critical. Rule of thumb for speed and blade pressure is what ever the speed, the pressure has to be the minimum to wipe the stencil surface clean. Paste viscosity and rheology are the important factors determining print speed and pressure process windows...the paste manufacturer will advise on the best print speed for their paste for different pitches, and then you should be able to set the minimum pressure at that speed and hopefully eliminate any bridges due to print quality.

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#62598

Printing Speeds for Fine Pitch | 1 September, 2010

Printing solder paste * Use single stroke, on-contact print cycle * Ensure the substrate is well supported, especially under areas of fine pitch print. * Select pressures of about 1- 2 lbs per linear inch of squeegee blade [Set squeegee pressure high enough to wipe the stencil clean] * Use 75 - 100 thou of down-stop works well with metal blades * Print at 1-2 inches per second [Appropriate squeegee speed ensures paste rolling action]

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#62599

Printing Speeds for Fine Pitch | 1 September, 2010

A good paste will print fine up to 5 in/sec or so. They key, as mentioned, is to get the pressure right for a given squeegee speed. If the squeegees aren't giving a clean wipe then you'll see huge variation in print volume which can lead to shorting. Another thing that can have a large effect on print variation is snap off speed. We have one paste in particular we use that absolutely requires fast snap off to keep volume in control on fine apertures.

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