The Quest for Perfect Design Data Packages

Estimated reading time: 13 minutes

There’s an ongoing problem in the PCB industry: fabrication shops are receiving incomplete or inadequate design data packages, leaving manufacturers scrambling to fill in the blanks. For a quick-turn prototype shop like Washington-based Prototron, with over 5,000 customers and up to 60% of orders coming from new customers each month, that can add up to a lot of wasted time and effort just in the quoting stage. Dave Ryder, Prototron president, and Mark Thompson, engineering support, delve into this continuing issue and more.

Barry Matties: Dave, can you give us a little information about Prototron?

Dave Ryder: Prototron was founded in 1987. We saw a need in the Northwest for a quick-turn prototype shop. The area had needs that just weren't being met by most of the traditional production houses. We were welcomed into the community right out of the gate, which was a pleasant surprise. From there, we've continued to grow over the years. We acquired a local competitor here in 1995, which greatly added to our list of tricks in our bag. We picked up the shop in Tucson, Arizona, in 1999, which has certainly been a successful addition to the Prototron family. We're in year 32 here, and we're motoring down the highway.

Matties: A lot has changed in 32 years, hasn't it? Yet we still use hot air leveling [laughs].

Ryder: So true. Technically, a big challenge for us was finding a fax machine back in the day.

Matties: Mark, what's your role here at Prototron?

Mark Thompson: I work in a pre-engineering and signal integrity capacity. As a customer liaison, if there are any CAM concerns or issues after the data packages have been released, then I'm the guy that gets a hold of the customers and bugs them about what needs to be done to get the data packages fixed and make the boards more manufacturable.

Matties: And Mark, you are the author of the eBook The Printed Circuit Designer's Guide to… Producing the Perfect Data Package. Why is this topic so important that you wrote a book about it?

Thompson: I’ve spent the last 25 years reviewing incoming data sets, even from the largest customers who you would think have excellent programs to review these data sets before output, but the data is still seriously flawed. It’s very simple things: missing drill files, drill counts that don't match the drawings, referencing impedance tray sizes that don't exist, layer counts that don't exist and providing stackups with dielectrics that aren't possible—any number of things like that. We spend a fair amount of time in the front end with the design data making the part producible.

Matties: Is there a general attitude, “It doesn't cost me anything more to send incorrect data. I send you the data file, and you'll sort it out, right?”

Thompson: Yes, but by continuing to educate our customers, at some point, they will realize that this is a benefit to them; if they could send us a cleaner data package, they would get things faster. They're going to get their quotes back faster, and a more accurate board in the end. They're going to have better functionality with their board if the board manufacturer understands what their end result is supposed to be.

Matties: One of the things that I hear from many people is that 99% of every order that comes into a PCB shop needs some sort of design data adjustment. Even though you tick off all of those benefits, it's not enough for people to change.

Thompson: There needs to be an impetus for folks to change. And as you said, if you give them everything right out of the gate, then what's their impetus to change? I hearken back to the fact that we're attempting to educate our local customers. We have over 5,000 customers, and a lot of them are local, especially in the space industry. Many new space companies startups have opened around this area, and they have brand new folks. They have fantastic scientists and engineers. But as far as practical knowledge of the ins and outs of building a PCB and then having a tangible functioning piece of hardware when all is said and done, that's hugely lacking in this industry right now.

Matties: Do they need to know that to provide you the data you need to build the board?

Thompson: I think so. We spend a fair amount of time doing presentations for our customers to do exactly that. You don't just throw me out a dielectric drawing on a material that smacks of clock traces and differential pairs and is just rife with that stuff. They're throwing it to us as a PCB manufacturer with the assumption that, "These people are dumb. They don't even know what they're looking at." However, most of the folks in our CAM department have well over 25 years of experience. If they see differential pairs and clock traces, they know that the board is controlled impedance. If they see a dielectric control or stackup, even if there are no notes regarding any controlled impedance whatsoever, we know what it's supposed to be doing.

And if we see a huge mismatch, what do I do, Barry? I might think, "What if they don't call it out? Does that imply that they don't care?" We don't make that assumption. I go through it and do a calculation. Even if I don't know what the threshold is, if it's an odd number like 107 ohms for a 90-ohm threshold, I usually can communicate with the customer and say, "Is that really what you were shooting for?" And 99% of the time, they come back and say, "No, I wasn't. And why is there this mismatch between what I'm calculating and what you're calculating?"

The big answer is this: We use a field solver. Most of our customers just use an impedance package that's tied to their CAD package. If it's embedded in their CAD package, it's not intuitive enough. It's not like a field solver, such as the Polar Instruments tools that we use. I spend a lot of time daily defending that software package. Our customers will say, "That doesn't match my calculations. Can you tell me why?" Fortunately, what they do most of the time is send me a screenshot of what system they're using, and I'll look at it. Right away, I’ll say, "Here are a couple of problems right out of the gate."

First, they're modeling it as a rectangular shape. Etchant doesn't work at perfect 90° angles. It's a trapezoid. Ultimately, there's a crest and a foot to every trace, and that's a big part of it. Second, those systems usually can’t model things like solder mask over the top of the traces. When everything was 8/8 spaces and traces 10 years ago, an additional one or two mils of polymer plastic over the top of the trace didn't mean anything. Today, we're talking about 0.1-millimeter lines and spaces. I guarantee an extra two mils of plastic over the top of the trace is going to affect the impedances.

Matties: So, the designers just may not have the right tools available.

Thompson: I think the tools are adequate; it's the knowledge of the tools that is lacking in many cases for folks who are brand new in the industry. Even if you have a mentor who brings you in and says, "This is how this software works," they’re only going to tell you what they know about that software. They aren’t giving you the definitive use of everything inside that software.

A great example that we get all the time is a netlist mismatch, which is a huge deal. They'll send us a data set, and we come back almost immediately because it's one of the very first things we do in the process—organize your files, set up the drill strings, and run your IPC netlist. If we come back with a mismatch, the very first thing I do is I communicate it back with you. We try really hard not to try to interpret them.

Some things are obvious. If it's a net zero or one or an A-ground to D-ground short, it's most likely intentional, and we approach it that way. If we get 16 brokens and they're 16 castellated pads around an outside periphery, you've heard me say it before; we make that a foregone conclusion. When can I not do something like that? What if I come up with over 200 broken nets or open nets? At that point, I'm looking at their data for whole sections of missing copper pour or plane because that's a lot of connections not being made.

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