Stencils: Not As Simple As They Seem

Estimated reading time: 20 minutes

Hardin: Usually, making changes to the stencil is the quickest, easiest, least expensive way to affect a positive change in production. Sometimes, in extreme cases, it comes back to having to go get the boards revised. There could be factors that a stencil can’t really address, such as masking or silkscreen legend that’s holding the stencil off contact.

Johnson: We’ve been learning that legend and solder mask planarity are becoming precise enough that even that matters now.

Hardin: That’s why it’s a tough question to answer. My instinct to say it would be impossible for them to figure that out upfront, but impossible is not a word we like in our industry. However, it is extremely difficult for sure. To have the expectation that a board designer is going to be able to have a single component layout in their library, the stencil cut one to one with the data, and everything’s going to go great is simply not realistic.

Barry Matties: How often do you have to rework stencils? Is this a pretty common occurrence, or is it a rare occurrence?

Hardin: Bringing already-cut stencils back in-house for rework is pretty rare. If the customer goes into production after the first article goes through, and had some issues they want to address to improve yields, then we’d go through specific changes, but that’s not as common. We try and spend that time upfront, in the initial design, to mitigate rework for everyone.

Matties: Why don’t more contract manufacturers or assembly houses have their own stencil cutting capabilities?

Hardin: That’s a good question too. I think the cost of the equipment sitting on the floor for the amount of time that they’d be utilizing it would be a big negative to doing that. And then you’d have to have someone in the facility with all of that design knowledge on top of everything else you have going on to do that. Primarily, it would be having a piece of equipment sitting on the floor underutilized. With Lean manufacturing being our focus, floor space is valuable. That would be a big dead space right in the middle of your floor that did not generate regular revenue.

Matties: It seems like you're putting a lot of your experience into making the stencil right the first time, and your only focus is the stencils. I'm surprised that more and more people don't have it. What you're saying makes sense; there has to be a crossover point where they're producing so many stencils that it makes sense for them to have it in-house. And yet some of those people don't.

Hardin: If you take into consideration your average stencil house is putting out somewhere in the neighborhood of a hundred stencils a day, to fully utilize that equipment and how fast it is, it’s going to be a rare manufacturer that needs a hundred stencils a day.

Happy Holden: Pad definitions must be a challenge.

Hardin: Yes. It’s inconsistent. You would be surprised at how many different pad layouts there are for an 0402 capacitor. Different part manufacturers and board manufacturers are never the same. Even if it says, “I want to have a standardized layout for an 0402 aperture,” it’s only going to work if every board you get in has the same kind of layout for that 0402 aperture, and that never happens.

Holden: How quick can you build a stencil if everything goes right and how long does it take if everything doesn't go right?

Hardin: If everything in the data looks good, and granted I’ve been doing this a pretty long time, I can get through a simple design in five minutes and have it in production. If it gets a lot more involved, with a lot more custom designing required, it could take me an hour to do the design work on it.

Holden: Does the turnaround time to the customer vary that much?

Hardin: I have a job now that’s going to take me probably two hours to do because of some custom fencing that they want to place solder paste on the board. There are a lot of things to get around, components up near the edge of the fence and through-holes that they want to avoid. Things like that can take more time to draw out. But in general, the stencil will still turn the same day even if we’re making quite a few changes to it.

Holden: That’s pretty good. The problem would become if one after another, they were all a headache; then, it would start to back up.

Hardin: That would be a little more of a problem.

Johnson: There has been a lot of work in solder paste lately, including a number of new products delivering new capabilities. How much work is involved for you in figuring out what to do with the stencil to accommodate a particular kind of paste?

Hardin: That doesn’t factor in as much to what we’re doing, believe it or not. There was a huge change made when people were switching from Type 3 to Type 4 paste because the spheres were smaller, and the volume was a little bit more volume. That and the switch from leaded to lead-free paste have been the most significant changes that I’ve had to design around.

For the most part, the paste type doesn’t get relayed to me unless to specify that it’s leaded or lead-free paste, and that’s because lead-free paste is not as forgiving in its travel. It doesn’t like to move. Where you put it is where it stays, and you can’t get too out there with your design. It also has a tendency to solder ball when it’s printed on the mask, so you have to be pretty careful about that too when you’re looking at moving pad geometries around.

Matties: You must be able to look at a stencil and read it as if it was a printed language.

Hardin: It’s getting there.

Matties: When it comes to defects, do they ever come back to the stencil and try to assess blame on the stencil itself? Is there any liability risk for you and your business, or is it not that level?

Hardin: We will accept some responsibility for that if we’ve gone through the design and our customer’s given us their confidence that we’re evaluating potential errors for them. If we miss narrowing up the width on a fine-pitch QFN device, and that wider aperture in the stencil results on a bridge or part floating, we will make the change and replace the stencil at no charge. That’s one of our services.

Matties: I'm thinking down to field failure, and the way traceability is going in liability is shifting. We're such a litigious society these days.

Hardin: I’ve only heard of one field failure coming back to a stencil design issue, and that was with respect to the volume of solder on a ground pad underneath a lead-less device. When those lead-less devices first came out, manufacturers were recommending about a 50% coverage, and now that they’ve been out in the marketplace, 70% coverage is what’s needed to keep those functioning. It was quite a long time ago now, but there wasn’t any product liability against the stencil manufacturer.

Matties: It sounds like yield and reliability rest on the stencil. For the stencil, manufacturers are relying on their external partners to make sure that a key step is correct and accurate.

Hardin: A lot falls into that part of the process.

Holden: Is there ever a situation in which you have to get back to them and say, "We can't figure out how to do this,” or, "You're going to have to do this with two stencils in two passes because you know what's required for this board covers such an extreme, and we don't know how to do it with one stencil or one pass."

Hardin: The only time I can think of that happening was when I had an aerospace board that came through that had 12-ounce copper buildup in places, but not across the whole thing. There were 0402 components backed right up to the edge of these big plated up areas. But with the micromachined step stencils now, you can have steps on both sides of the stencils.

Holden: One of the things that designers are slowly learning is when they're designing a multilayer, they must get in touch with their fabricator to ask about the material, and the fabricator will say, "Send us your stack-up and everything and something about your circuits so that we can determine what the as pressed thickness will be." Otherwise, you can't determine your line width for transmission line-controlled impedance. Unless this is an absolute duplicate of the way we did it before, you'll end up with the wrong impedance. It's not the fabricator's fault; your drawing is wrong, so do you have a set of checklist or pre-checks of or do’s and don'ts of best practices that if they can follow those guidelines or contact you early, it's going to be much smoother sailing and you're going to have a nice stencil that turns around quickly. That's not going to be a source of your problem with assembly.

Hardin: Especially with new clients or someone that’s new to stenciling altogether, often, they say, “I don’t know what to do with this.” I say, “Send me your file. I’ll go through it and make recommendations on what’s going to ease your manufacturing concerns.” That’s the route we usually take.

Johnson: Yet, on the surface, it seems like the stencil would be simple.

Hardin: They used to be a long time ago.

Johnson: Is there anything else that you’d like to address?

Hardin: We talked about that misconception that you could have a standardized design library that’s going to solve everybody’s problems. That is a misconception at best. Even if you had every designer and board house turning out the exact same layout for certain pad or device, we’re still going to have all these environmental factors in the manufacturing facility itself. It’s not going to be cut and dry.

Holden: You and the assembly people are trying to solve the problem without having to go back to the designer.

Hardin: That’s right. By the time they order this stencil, they have a production schedule to meet. That stencil has to go on the line in a day or maybe two. They’re getting their kits together and need to build and ship, so there’s often not time in that schedule to go back to the designer and try to work it out.

Holden: If the designer opens it up because if that phase of the project gets closed, they’re onto the next design.

Hardin: Exactly. On paper, it should work fine, but it doesn’t always work that way.

Matties: Who wants to take the cost of a re-spin as well at that point? As you said, they’re ready to put the parts on it and ship it all. They’re not interested in looking at circuit design. They need you to solve the problem.

Hardin: Right. It’s a quick turn business these days. Get it done, and keep everybody moving and making money.

Holden: Do you see anything in the future that will help you out or improve the situation?

Hardin: Not really. This has been something we’ve been dealing with quite a bit for a number of years, and I know the devices are going to continue to shrink, and we’re going to have to continue to make adjustments for that. We’ve been playing this shrinking game here for a number of years now. I remember when a 0402 was the end of the world, and everyone thought, “How are we going to print this? How are we going to place it?” Now, those devices are common.

Matties: Thank you for speaking with us.

Hardin: Thanks for your time.

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