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Paste drying on stencil

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Paste drying on stencil | 7 February, 2020

I am finding if I have to stop the pick and place machine during production very often (mostly due to feeders not pulling the tape cover back), I get paste drying in the apertures of the stencil.

To avoid this I am thinking of pasting all the boards first and placing them in a rack until putting into pick and place. I am hoping if I do this, the paste would not dry due to continuous swiping, would this be a reasonable method of dealing with the issue? I am using T4 paste and smallest part is 100 pin fine pitch TQFP


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Paste drying on stencil | 8 February, 2020

I work with small batches and this is what I do. However I don't do it because of the paste. Instead I do it because I am the sole operator on the line and I want one process finished so I can keep a better eye on the others. I specifically work with a paste that has low slump and a long lifetime. It is quite happy to spend hours at a time on the stencil and even longer on the board. If your paste is drying out I would suggest it's not suitable for what you are doing and you should find a different one. If small apertures are getting clogged you should periodically use a stencil wipe to clean the underside, I'm going to assume you are not using an automatic printer as they have features to handle this.

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Paste drying on stencil | 10 February, 2020

Solder paste has stencil life time - if you go over it means you should change it. If you print boards in front and keep them for too long before reflow, the flux is still evaporating, so you are not mitigating the risk. Paste also likes room temperature and humidity that are on the spec. If you don't follow these you can't trust paste performance.

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Paste drying on stencil | 10 February, 2020

As has already been suggested, make sure you are cleaning your stencil well between long periods of inactivity.

That said, it sounds to me like you have a feeder problem to solve before you get too wrapped up in what to do with your printing process.

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Paste drying on stencil | 10 February, 2020

... indeed, unless I'm very much mistaken you should be able to find refurbished or Chinese copies of your feeders in abundance on alibaba etc. I would suggest you try a few new ones as well as trying to refurbish the ones you have.

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Paste drying on stencil | 11 February, 2020

What paste do you use in this instance?

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Paste drying on stencil | 11 February, 2020

If you're asking me, I'm using BLT LFS-UFP-T4 (they have an even better paste to treat badly now if you add zq).

Those in the test of the world might find Loctite GC10 is similar, however the stencil and tack times on the BLT are pretty awesome and the price is pretty sharp too. Not Warton low but plenty less than AIM or Almit.

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Paste drying on stencil | 27 February, 2020

look at the specs of your solder and stencil life. Check the humidity, low humidity dries the paste. Slowing down your swipe speed and frequent wipe cleaning of your stencil can help.

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Paste drying on stencil | 28 February, 2020

My input are: i. To solve tape not pulling back if it is due to high per is better to use more feeders to minimize this issue. ii. To slow down printing speed. Hence, has proper line balancing which minimize board idling before pick and place process.

This message was posted via the Electronics Forum @ SMTASMTA

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