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How long can you leave paste?

Views: 2366

#48409

How long can you leave paste? | 15 March, 2007

Hello

Just wondering if anyone has done trials on how long you can leave paste (tin-lead or pb free) on a board after you have printed it before it starts going 'off'?

Thanks Chrissie

FYI - have searched and found the 'pasting ahead' thread - just wondering if anyone went away and had a good look? They paste them here at this company (just joined this week) and they leave them on the shelf (room temperature, uncontrolled room, not even a clearoom!) for x amount of time. That x seem to vary, but it's over hours. and hours.

Been out of the industry for a while, so just wondering if it was just me thinking these people are crazy or if there had been some advances.

If it helps i can find out paste type.

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#48410

How long can you leave paste? | 15 March, 2007

Chrissie,

This varies widely by paste, room temperature, and humidity. You would need to do a DOE for you exact set-up. Since you say the room is uncontrolled you need to keep in mind that time will drop as temperature goes up. Different pastes react to humidity in different ways but most will have a shorter time as humidity gets to either extreme.

Jerry

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#48412

How long can you leave paste? | 15 March, 2007

It will also depend on if the paste is OA (water soluble) or No-Clean. I believe you can leave No-Clean out longer because it absorbs moisture slower than OA.

As always check with the paste manufacturer to see what they say. It should also be on the paste spec sheet.

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Wayne

#48438

How long can you leave paste? | 16 March, 2007

Have to check with paste manufacturer. paste on the stencil should be at least 12 hours paste on the board before mounting should be at least 4 hours.

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#48439

How long can you leave paste? | 16 March, 2007

Hi Chris,

You should check your data sheet from the paste manufacturer, but the bottom line is, does this wait step cause any quality problems? If not, then it's a hard argument to say this is a bad practice.

Next you may want to find out why they are doing this. It is an odd thing to do, unless they are buffering several lines with one screen printer.

Let us know what you find.

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#48441

How long can you leave paste? | 16 March, 2007

1 Print paste on a bare board or laminate. 2 Place components in the paste. 3 Gently invert and support the board above a table. 4 At one hour intervals, record the number of components that have fallen from the board.

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#48442

How long can you leave paste? | 16 March, 2007

Thanks for that - i now have a starting point! Always the hardest bit. I'll report what i find!

Chrissie

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#48480

How long can you leave paste? | 19 March, 2007

After a little investigation (with my eyes closed and fingers crossed) how about up to 24 hours?! ?! I think i might have found one of the root causes of my oooooodles of rework.

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#48485

How long can you leave paste? | 19 March, 2007

We use a no-clean lead free paste with our manufacturing floor temp 70-75F and humidity 20%-30%. We have found that if the time from pasting a board until it goes into our reflow oven is less than 4 hours we have no problems. If it is greater than 4 hours the paste starts to dry out and rework goes up.

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#48486

How long can you leave paste? | 19 March, 2007

Really, you need to do a DOE on with your paste to figure that out.

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#48488

How long can you leave paste? | 19 March, 2007

This test nicely demonstrates the bond between parts and paste over time, but what about tack of paste on unpopulated boards?

It seems she would also need to be testing by placing parts on boards that have been printed for 15 minutes, 30, 1 hour, 2 hours, etc., THEN inverting.

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#48490

How long can you leave paste? | 19 March, 2007

I have been involved with the evaluation of close to a dozen pastes from 5 manufactures. I never found one that would rest on a board for anything close to 24 hrs and still have acceptable performance.

Jerry

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#48498

How long can you leave paste? | 20 March, 2007

Already figured i would do that if it came to it - still planning my approach, which will hopefully eliminate this whole 'waiting after pasting' nonsense anyway.

Suggestions very welcome though, as it may well get to the test and see stage!

Cheers Chrissie

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#48735

How long can you leave paste? | 29 March, 2007

Started measuring rework last week - on some of the boards we are looking at a shocking 20% right first time yield. The paste thing has to be fixed as it's being left more than four hours.

Thanks for all the comments - Will let everyone know the reasons the line operators come up with for leaving the paste in the first place.

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#48740

How long can you leave paste? | 29 March, 2007

chrissie, at my last place of employment the reason they did it was so they could print the last of a lot, then change over the printer and start printing up for the next lot and have some ready to go when they finished changing over the chip shooter. Saved them all of about 10 seconds per board per lot when all said and done, beFORE accounting for rework for missing parts.

Some things are tougher to communicate than others, even to supervisors.

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#48943

How long can you leave paste? | 10 April, 2007

Stop asking this.

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reflow oven profiler

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