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blocking off stencil apertures with tape

Stephen

#17852

blocking off stencil apertures with tape | 10 October, 2001

I recently started at a new company. They have a variety of bare PCB's and most have aprox. 20 variations. Maybe about 1/3 of each board on average are non-populated locations. They buy one stencil for each PCB but then use scotch tape to block off most of the unused apertures. They think if they don't, they get bridging on non-populated QFP's. It makes inspection of no-pops easier and they think it makes the board look better. Also sometimes a board will get changed after assembly and it is easier to add a part if they is no solder on the pads. I agree with the last one. Although I want to get things set up so that product will be out the door so fast the engineers won't have time to make such changes.

Does anyone have any comments. We are getting a new machine with lots more feeders and will be able to dedicate almost all parts. Taping the stencil will be the biggest amount of time for changeovers.

Stephen

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Michael Parker

#17857

blocking off stencil apertures with tape | 10 October, 2001

Why not quantify the amount of time spent taping apertures every time you want to use the stencil vs. the cost of a new stencil (about $300, usually)? Look for problems that are created by the blocked aperture method - is there rework down the line that could have been avoided? Include that rework cost into your justification. You could probably prove a return on investment rather quickly.

If that don't work, poke a hole in the stencil and force 'em to buy a new one. Don't quote me on that!!!

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Michael Parker

#17858

blocking off stencil apertures with tape | 10 October, 2001

What the heck are you doing having scotch tape in the plant? Isn't there a concern for ESD safety? Ban the scotch tape, then they won't have something to block the apertures with.

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#17859

blocking off stencil apertures with tape | 10 October, 2001

Stephen, I don't understand this at all mate. 1. If you don't place a QFP and get bridging across the bare pads, you were gonna get absolutely horrendous bridging if you did put one down. That points to poor printer set up or stencil design or both or whatever. 2. I reckon non - populated ares without solder paste would look very strange. However I can see their point if you are using non HASL finishes. The non-pop areas stand out and any area with solder and no component is a fault. This seems rife with danger to me. What happens if you mistape the stencil and go merrily on your way? Ah, we inspect anyway they say. Then what's the point of taping? What happens if a bit of tape falls off and you waste half an hour looking for non-existent faults? You get your butt kicked for increased cycle times! It would seem that they are not not confident that the machinery will be pickin' 'em up an' puttin' 'em down where it's supposed to be. 3. I think it's much easier to retro fit a component if you have the correct amount of solder already on the pad - from passives to QFPs, solder deposition is much more controlled by stencil application than by hand, ( Pace have a lot to answer for ). However this is only a matter of personal preference. 4. It sounds a bit like a hangover from masking PTH boards. 5. It's not about saving "about 1/3" of their solder paste is it? It just all seems unnecessary to me and these are some of the arguments I would use, plus the other suggestions submitted. You might be battling if the process is really entrenched but - we'll all be rootin' for ya!

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#17860

blocking off stencil apertures with tape | 10 October, 2001

We're with the Ozzy's earlier posting on this thread.

We make similar products, like: * Phantom products that are mostly complete and then are configured to order by adding or subtracting a couple of parts and some jumpers. * Version products like you do.

We just paste all board configurations using the same stencil. We don�t tape dip. Taping stencils is too painful. * Board-side taping makes you want to recalculate your paste deposit, since it increases you stencil thickness for all the non-taped pads. * Paste-side taping, if using tape that you can remove, is largely a waste. It ends-up with a lousy print, because the squeegee removes the tape during printing and the tape ends-up in the paste, and � * Paste-side taping, if using tape that doesn�t get removed by the squeegee [like Kapton], is a trial for troops to remove the tape at the end of the job. �Ooooh, I just broke my nail!!! Stevo, sweetie, you owe me a repair job over at Madam Beauveaire�s Nail Salon.� �OK, Bob. Just get the GD tape off the GD stencil.�

I have got the perfect solution for you!!!! And it doesn�t require: * Buying stencils for each board part number, regardless if you used frameless stencils or not. * Buying A single stencil with multiple board part numbers on the stencil. * Any wacky taping schemes. * Figuring-out why this just doesn't sound kosher.

Gimme a D!!! Gimme a spen!!! Gimme a zer!!!

Waazit spell? D-SPEN-ZER!!! D-SPEN-SER!!! DISPENSER!!!

You figure, the stencil printer you have now is about useless, given how slow this new P&P is gunna run. Hey, just get riddo thet dino. Stencils / printer set-up, regardless if you tape er you don�t tape, are gunna be the tall pole in the tent when yer trying to flip product. Bust a move before it�s too late.

Hey, you were an airedale, right? Ever go on a "Snipe Hunt" in your younger days?

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Brian W.

#17898

blocking off stencil apertures with tape | 15 October, 2001

Boy, does this sound familiar.

I agree with Dave, especially his point about board side taping. The kapton tape has a thickness of approximately 2mils. You artificially increase your paste thickness with this method. It also can be a cause for bridges. We now only allow taping with the engineer's approval. This means I only tape when I need to increase the paste thickness until a new stencil arrives. It's all about eliminating variation in the process. Just paste the whole board. Tasping and retaping the stencil costs beaucoup bucks over the course of a year.

Brian W.

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Leland Woodall

#17899

blocking off stencil apertures with tape | 15 October, 2001

Something you'll want to consider is what happens with bits of tape that become relocated due to manual or automatic wiping. I've seen them block fine pitch apertures and create an insufficient solder condition that, guess what, wound up in the hands of the customer.

Not good. Not good at all...

Leland W.

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Ken Bliss

#17903

blocking off stencil apertures with tape | 15 October, 2001

One thing everyone missed is what is the goal of the company. To make money!!! So how many boards per week are you shipping now and how many would you be shipping if you had seperate stencils or a DISPENSER. With quality being number one always the only thing left is through put. No you do not get to higher more people to tape apperatures either. My guess is that you are running very short runs which tells me that a dispenser is the only way you can keep production moving but if you have enough time to change the screens without holding up the line to long it wont matter which way you go as stencils will be far cheaper than buying a machine but keep in mind the cost for someone to keep taping those stencils. I highly recommend the book "The Goal" by Eli Goldratt your boss will see it your way as soon as he/she reads it. I do agree with the other tape problems mentioned above.

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