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Stencil deformation patterns

Views: 3685

#80441

Stencil deformation patterns | 21 May, 2018

Hi experts, I would like to ask to the forum if you could describe what could be the causes for stencil deformations. We mainly see a wave like pattern.

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#80446

Stencil deformation patterns | 21 May, 2018

How old is the stencil and how many cycles does it have

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#80447

Stencil deformation patterns | 21 May, 2018

Honestly, I wonder the same. Is it Age the only variable to this? I can not reduce ageing.

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#80449

Stencil deformation patterns | 22 May, 2018

there are numerous factors:

-stencil age -stencil finish and thickness -stencil-board vertical alignment -board support -squeegee pressure -squeegee blade material and condition

question here is did stencil changed its dimension over time? are there any positions misprinted because of this? on this images, it looks like there are warps at squeegee blade start, that could be caused by raising board too much, pressing it against stencil and warping it. i can clearly see blade start and board start. do step cycle and see is is there space between board and stencil after alignment. your problem is most likely caused by bad board thickness setting, or raising table calibration.

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dwl

#80451

Stencil deformation patterns | 22 May, 2018

metal blades scraping against a metal foil; do it enough times and any stencil will look like that.

It's possible the squeegee pressure is set too high, exacerbating the condition. Try reducing squeegee pressure a little at a time, making sure you are still getting a good print. you want enough pressure so that there is no film of paste left on the stencil, but no more.

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#80453

Stencil deformation patterns | 22 May, 2018

This is caused by usage. Excessive squeegee pressure can make it worse over time.It will become more pronounced on thinner stencils.

Unless it is causing printing defects, don't worry about it in my opinion.

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#80461

Stencil deformation patterns | 23 May, 2018

Those patterns are either: a) the stencil has recieved some streching stress - either being pushed on and dented by some obstruction, or b) has been worn thin and therefore streched as a result. This type of change to your foil may cause a slight shift of the image, but probably not much (under 1mil). However, it may make it harder for the stencil to lie flat on the board, so it could lead to bleeding problems during printing.

To reduce the occurance of this issues: a) Always have your operators check and insure the "setup" of the tooling/stencil prior to running - be sure the pcboard is flat, there are no protrusions, such as edge rails too high. Be sure setncil is brought right down and maybe 5-10mils over shoot the surface of the pcb. and b) Always tweak your squeegee (assuming metal squeegee) always set the pressure low - and do test printing - until you get a clean wipe, then increase pressure 10percent (to allow for variations ) - this will give good printing with minimum pressure / wear.

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#80465

Stencil deformation patterns | 24 May, 2018

Bottom line....if you have an SPI machine, that's the judge and jury.

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#80486

Stencil deformation patterns | 30 May, 2018

Thank you all guys. I appreciate all your help. I am running a new series of tests to reducy this in new projects.

There is an equipment to messure stencils deformation pressure, but we happen to not have the software. Do you know if there is any standard for it?

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#80487

Stencil deformation patterns | 31 May, 2018

Diego,

In my opinion you are wasting your time testing. Set the squeegee pressure until its just cleaning the stencil off. That is the correct pressure.

Unless you are getting failures at SPI (if you have SPI) or getting solder defects don't worry about what the stencil looks like.

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