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gas to extend allowed print-to-place time?

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#75575

gas to extend allowed print-to-place time? | 21 April, 2016

Hi, is there some sort of gas (certainly refrigerated, possibly pressurized?) in which printed boards can be stored to extend the allowed time between SMT paste printing and placement? Or other technique for achieving this?

Background: we're a small research shop with an unusual job that requires a very special printing setup and very thick stencils with area-ratio-violating apertures. Reconfiguring the printer for this job and cleaning those superthick stencils afterwards is a huge undertaking, and it's really killing productivity to have to do a full printer setup/teardown each day.

We'd like to be able to print a week (or more) worth of boards even if that requires investing in specialized storage. Is there some way to do this?

The boards in question have a very large number of LGA landings with very tall paste deposits (8mil stencil). There is a huge amount of paste on each board.

Paste is Kester EP256 (leaded). Previously we were using Loctite GC10, which can actually survive a whole week on the board with excellent tack+reflow... pretty amazing stuff. But GC10 is too sticky to use on stencils thicker than 4mil (jams up the apertures, incredibly hard to clean) and for this particular job we need a low reflow temperature. Since we only buy Kester paste for this one job we don't really have a contact at Kester and the reseller we buy it through is not terribly attentive since we buy so little of it. So "ask the paste vendor" does not work here.

Experiments show that the EP256 can last 24 hours print-to-place easily, but at 48 hours print-to-place we start getting small numbers of defects (the place-to-reflow time is very short, just a few minutes at most).

Thanks,

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#75583

gas to extend allowed print-to-place time? | 25 April, 2016

I'm going to ask the obvious question first. Why can't you place the entire job the same day you paste it? Getting a sensible workflow is a far better method of improving productivity than experimenting with crazy solutions.

I can see a gas atmosphere helping with regard reducing oxidisation but whether that is going to stop the paste simply drying out is another matter.

Given that GC10 (which is no longer unique, others make pastes with these properties too although how much work is going into leaded pastes these days is another question) seemingly has the board time you require would it not be worth looking at your stencils again, perhaps the aperture design is wrong for this thickness. How are your stencils made, are the sides trapedzoidal...?

And last but not least, get on the 2nd user market and buy a printer that you then setup and use for this awkaward job alone. Yes you'll still have to clean it every day, but that should not be a complex task.

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#75584

gas to extend allowed print-to-place time? | 25 April, 2016

One word: Dispense your paste

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#75587

gas to extend allowed print-to-place time? | 25 April, 2016

Or try : http://www.sipad.com/

I have had some experience with it. Seemed to work OK.

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#75588

gas to extend allowed print-to-place time? | 26 April, 2016

Hah I meant to say that too, depending on the exact requirements you may even be able to do this with something cheaper than A MY600/Scorpion. Plenty of printers these days can dispense paste as can pick and place machines and small dedicated units. Some of them may well be painfully slow but they may be all you need.

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#75627

gas to extend allowed print-to-place time? | 2 May, 2016

> One word: Dispense your paste

That's three words!

Unfortunately the board has nine LGA components. Stenciling is the only way to get the required consistency in deposit shape and volume.

If the board were all passives+QFN I would definitely use an automated paste dispenser.

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#75628

gas to extend allowed print-to-place time? | 2 May, 2016

> Why can't you place the entire job the same day you paste it?

I can. But setting up and properly cleaning the stenciling machine takes 2-3 hours. We only run one shift. If we do that every day we're spending most the time on setup+teardown.

Much better to print a week's worth or a month's worth of boards in one day, if there were a way to do it. That's 2-3 hours per month of setup/teardown instead of 40-60 hours per month.

> Getting a sensible workflow is a far better method of improving productivity than experimenting with crazy solutions.

Actually I think having to set up and clean up the stenciling machine every day just to do a dozen boards is what's not sensible and crazy. And it leaves so many more opportunities for inconsistency to creep in. Of course we try to control everything including temperature and humidity as well as we can, but doing everything in a single run is always going to be more consistent.

> Given that GC10 (which is no longer unique, others make pastes with these properties too

Such as? If there's a Type-3 (not Type-4) PbSn (not Pbfree) paste I am definitely very interested.

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#75629

gas to extend allowed print-to-place time? | 2 May, 2016

> Or try : http://www.sipad.com/

I think sipad is incredibly cool and would be the perfect solution, but is way too expensive.

We're paying $0.80/each for four-layer ENIG boards, 50cm^2 with 0.125mm trace/space and 0.8mm-pitch LGA landings in lots of 1000 (and yes, the boards are working flawlessly at 800mbit/sec data rate / 400mhz toggle rate, please don't turn this thread into a PCB manufacturing quality religious war, I am tired of that happening).

Sipad would **quadruple** the price per board.

Personally I think their technology is great but their pricing is just stupid and unreasonable. All they do is run the board through an ordinary stencil printer, an ordinary oven, and then a special cold-press. I have a hard time believing that their special cold-press is that expensive to operate. Also our 0.8mm-pitch landings are slightly outside of their standard spec, so we'd be paying even more than the baseline pricing.

Also they don't seem to be too interested in new business. I requested a demo board from them three months ago and still haven't heard anything. That sort of thing is a Very Bad Sign from a vendor you are thinking of relying upon in a critical way.

The other problem is that we have LGAs on both sides of the board. This is not negotiable -- it keeps the critical traces short which matters more than anything else.

I have, believe it or not, begun experimenting with developing our own print-reflow-place-reflow process using a glass plate to planarize the solder deposits. The odds of it working are not great, but if it does work it will be a better fit than outsourced processing since we can do the following four steps in this order (each step is done for an entire lot of 1000 boards before the next step):

1. print+reflow the paste on the first side with planarization

2. place components on first side, re-reflow first side

3. print+reflow the paste on the second side with planarization

4. place components on first side, re-reflow second side

Step 1 and step 3 involve wet paste, so they have to be finished within one shift. That's not hard, they're the fastest and simplest steps. Steps 2+4 can be spread out over as many days as necessary.

Doing this with an outsourced provider would require shipping the boards twice and them being able to handle our boards with one side populated, which is both unlikely and risky in my opinion.

It might also be possible to do the steps in the order 1,3,2,4. In other words print+reflow the paste deposits on both sides before doing any placement. However this would require planarizing (or re-planarizing) the backside deposits during the reflow of the frontside components.

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#75641

gas to extend allowed print-to-place time? | 3 May, 2016

I've successfully refrigerated a pasted board, and placed/reflowed it the next day with minimal issues.

However, this is highly not recommended by anyone, anywhere.

You might want to invest in a stencil cleaner...might be the same time investment, but, at least it'll be automated, and can run in parallel with your assembly process.

Cheers, ..rob

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#75651

gas to extend allowed print-to-place time? | 3 May, 2016

> I've successfully refrigerated a pasted board, and placed/reflowed it the next day with minimal issues.

Me too. I'm looking for more than one day.

> You might want to invest in a stencil cleaner

Tried that. They do not work on the high-aspect ratio apertures used for this job.

And, of course, cleaning the stencils is just one of the many operations involved in starting up, shutting down, and cleaning up a stencil printing station. Rather than trying to make all of these happen 100 times faster, it would be simpler to just do them 100 times less often.

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#75652

gas to extend allowed print-to-place time? | 4 May, 2016

Loctite's new GC 10 paste allows for some pretty extended print cycles. I was skeptical, but the stuff doesn't seem to ever dry out. I've had overnight print to place cycles work at room temperature, and they spec a stencil life of three days. Might go a week in the fridge.

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#75653

gas to extend allowed print-to-place time? | 4 May, 2016

Yeah, this has been my experience too, but unfortunately GC10 is unusable with thick stencils. It jams up the apertures like nothing I've ever seen.

The cost of its miraculous properties is that it is as sticky as GLUE.

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