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Step stencil troubles


sly

#30572

Step stencil troubles | 17 September, 2004

Hi all, does anyone of you use step stencils for paste application? Did anyone experienced troubles with those kind of stencils?

thanks for answer sly

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Simon Uk

#30573

Step stencil troubles | 17 September, 2004

Yes, i have used and experienced problems with step stencils, but these were mainly to do with the communication of the requirements for the suppier. Best plan is to talk to your stencil supplier and get them to make the suggestions on what works best, it saves you time and money if you get it wrong.

I cannot tell what you require of the stencil or what device your trying to solder down that requires a step?

My company builds single board computers and we use a miriad of component types incl PIHR,BGA's and BGA sockets.

Let us know what you need. Simon UK

Simon UK

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sly

#30574

Step stencil troubles | 17 September, 2004

Hi Simon, thanks for answering so fast.

Well, our company is building several different products to monitor turbine activities (power generation and propulsion). So our boards are equipped with mixed technologies including minimelfs, 0402 chip resistors and capacities,SOP IC's, fine pitch IC's and BGA's.

What I was concerned about, is that we use single height screens to proceed paste deposit, and all literrature I could found about the process talked about step screens and different height depending on the component type.

When I asked around for my predecessor's choice and talked about step screens, it seemed that step screens is a technique wich is not commonly used in the industry, but is just a theorical concept not useable in practice. Is it really true that it's not commonly used?

sly

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RDR

#30577

Step stencil troubles | 17 September, 2004

Step stencils are used but only when necessary. One reason to use one would be a PCB with 12 mil pitch and very large coils or something of that nature. The 12 mil pitch part would require a 4 mil thick stencil to ensure release while it would cause extremely small fillets on the coil. This small deposit of paste at large component locations can also cause problems during placement from "lack of tack". Step stencils are not necessarily hard to use but certain design considerations must be followed. You need to ensure that you have adequate clearance around the step area because the squeegee blade will obviously deflect at the stepped area causing no contact to the stencil. You can get away with this on larger type pads that due to their size won't cause any release problems and the volume of solder deposited is not that critical or would have a large tolerance in fillet size formation. Sometimes it is recommended to use urethane blades to help conform to the stepped area.

I have found however, that actually needing a step stencil is very un-common from my personal experience. We use 5-6mil thick stencils for every component type and have experienced no problems. The 5 mil stencil is good down to 16mil pitch and the 6 mil will handle 20mil and larger with no issues. Obviously there are parts out there that I am sure I haven't seen and would cause issues that would require a step.

Russ

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#30580

Step stencil troubles | 17 September, 2004

We agree with Russ. * Haven't used them in 10+ years. * Size of keepout for the stepped apertures needs to be large. * Manipulating aperure size to manage paste volume for specific components is not difficult.

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sly

#30581

Step stencil troubles | 17 September, 2004

Hi russ, thanks for answers, I thought that playing with apertures was the solution if paste is able to lose from the screen with fine pitches.

But I'll take in consideration all of your remarks for the next designs to be develloped.

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Bob R.

#30584

Step stencil troubles | 17 September, 2004

I also agree with Russ - we use step stencils when we have to but generally try to avoid them because the print isn't as consistent as with a single thickness stencil. Many of our products are for harsh environments so we do a lot of thermal cycle and vibration testing. From a reliability point of view, there are only a handful of situations where paste thickness makes a practical difference. MLF/QFN is one example. We select stencil thickness based on process yield and rarely need to select it based on reliability.

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RDR

#30586

Step stencil troubles | 17 September, 2004

You are right, playing with apertures is how to ensure release. Making a stencil thinner or thicker is "playing with apertures" It all relates to aspect and area ratio calculations and what you need to do based upon the numbers you get.

Russ

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KEN

#30593

Step stencil troubles | 17 September, 2004

I have an application that uses 0201 devices coupled with ceramic column grid arrays.

The coplanarity of the balls is specified not to exceed 7 mils! 4 mil foil for 0201's with a selective buildup to 7mil for the ceramic. Bloody designers! Take your 10 GHz and shove it! (J/K)

Step stencils can work, but need design rules to allow for the defined contrast in heights. Oh and make sure you speak slowly and confirm with your stencil supplier your exact needs.

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