Category Archives: Screaming Circuits Blog

EMS provider Screaming Circuits offers tips and thoughts on design and manufacturing

Counting Once, Counting Twice…

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Panel single scLet’s say you have two options: First, you could send in your boards for assembly as individuals. Second, you could send them in a panel. That’s all fine and dandy. For a few, send individuals. For a bunch, panels might make more sense. But, when you do go to quote and order, how do you count the parts?

Let’s take this example. As a single, this board has 32 line items on it’s bill of materials. That’s 32 unique parts. Counting all of the individual part placements, there are 56 total parts: 42 SMT and 14 through-hole. So, naturally, if you quoted the assembly of 20 of this board at Screaming Circuits, you would enter your desired board quantity as 20, 32 total unique parts, 42 SMT and 14 through-hole.

But what do you do if you send it in panel form? How do you count? It’s actually not as difficult as it seems. In this example, it’s in a panel of four. There are still only 32 BOM line items, but there are four times as many placements. That means that if you quoted this, as a panel, you would enter 32 total unique parts, 168 SMT and 56 through-hole parts. If you still need 20 of the final boards assembled, you would enter 5 as your desired board quantity.

In the end, you will have 20 assembled boards. In case you are wondering about the cost, there won’t be a difference. As long as the final number of boards (after the panel is broken apart) are the same, your cost will be exactly the same for panel vs. one up. You don’t save any money by sending in singles. However, if your board is panelized and all of your parts on on reels, full or partial, you can save money by ordering Short-Run production.

Duane Benson
50 Years ago today
Robert Rushworth flew the X-15 to Mach 5.03 at 100,400 feet altitude

http://blog.screamingcircuits.com/

Is It Your Fiduciary Duty to Use Fiducials?

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We don’t require fiducials here. It’s not mandatory. That’s because we live in a prototype world and in that world, theory doesn’t always match up with reality. That being said, there are things we can do and things we would prefer to do. Personally, I would prefer to get some ice cream, but my belt suggests otherwise.

So we do like fiducials, which begs the question: “How do we like them?” Well, the objective of a fiducial is to make machine registration of your PCB easy. That means the pattern should not be reversible. It should only have one correct orientation. Use three of them separated as far apart from each other on the board as you can. But don’t put them any closer than 4.75mm from the edge. Ideally, it would be a 1mm copper area centered in a 3mm circle with no solder mask. Oh, and all of the fiducials on the board should look the same and be the same size.

Duane Benson
Over easy, yolk not broken.

http://blog.screamingcircuits.com/

Ode to Competition

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Thanks to Theodore Roosevelt, we almost all have competition of some sort or another. I’m not a big fan of the statement made so often: “We welcome the competition, It validates the market.” or similar such sentiments. You usually hear that from a spokesperson when a new competitor enters the market. My guess is that most people who say that are probably thinking to themselves: “Yeah. In a pig’s eye” while stating it.

I’m also not a big fan of the phrase so often heard in start-up companies: “We don’t have any competition.” To me, that’s a warning sign. You might not have much competition, but you always have some. At minimum, other companies (maybe even with non-competing products) are competing for the same dollars. If someone thinks they don’t have competition, I would suggest they look a little closer at what their customers need and are doing.

The number three statement that I’m not a big fan of: “Imitation is the fondest form of flattery.” I do understand it. If someone is copying you, that must mean that you’re doing something right (the possibility of the blind leading the blind not withstanding). In a business context I do believe that all three of those statements are a form of saving face. You can’t stop competition from showing up, but you can pretend to be noble and welcome it. It’s not always possible to stop people from copying you, but you can pretend it’s a complement.

Here’s what I think about competition: It’s my job to give you better value than our competition. Plain and simple. If you come to me for business and I give you better value: What you want, when you want it at a fair price, then I have earned your business. If a competitor gives you better value, it means that I’m not doing my job right. We are all in this to make money, but we’re in this to make money in such a way that we are the best value for you. Not the lowest price, but when you add up our reliability, quality and technical capabilities, doing business with us should save you time, aggravation and money.

So why the manifesto? It annoys me when competitors place comments on our blog linking to their website. Especially when they don’t identify themselves. Yes, it means that they believe that we are doing things right. Yes, it means they think we have enough customers that it’s worth trying to lure some away from us. So, in a sense, it is validation that they think we’re doing a good job. I don’t really see that form of “validation” as being worth much though. What I really care about is that the people who give us money think we’re doing a good job and that they get their money’s worth.

Duane Benson
We are with you, sire! For Sparta, for freedom, to the… to the… Um…
to the sucess of your project!

Ambiguity

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P3281577 smIt’s pretty important to have unambiguous polarity markings and pin one markings printed on your PCB. In theory, for SMT parts, it really shouldn’t matter; the centroid would take care of the placement orientation. But, you may have noticed that it’s not a perfect world. It took me a while to figure that out, but I have finally concluded such.

It’s not uncommon for the CAD library part to have the wrong zero degree rotation orientation. The IPC specified location for pin one orientation Quad and BGA for square chips like QFPs, QFNs and BGAs is either the upper left or middle top. Check out our Centroid guide for more detail. If it’s wrong in CAD, the centroid will be wrong as will everything downstream. That’s why markings on the board are still important.

What do you do if your part is ambiguous though? This particular chip has three markings that could be interpreted as pin one indicators. At first glance, I’d assume it’s the dot in the center top. It would match with the text. However, there is a white dot in the lower left that could be pin one indicator which would mean, in this case, the CAD library component had the incorrect zero rotation orientation.

Datasheets aren’t always easy to find. This one is behind a registration wall. If you have a part like this, it’s really helpful if you include some documentation (in electronic form) clarifying. I found the datasheet for this particular part and was able to confirm that it is correct as placed with pin one down in the lower left (90 degrees).

Duane Benson
Via via in the board,
what’s the top on my PCB?

www.blog.screamingcircuits.com

Connectors Kill

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Lots of types of components can cause footprint woes. QFNs have their center pad issues. BGAs have escape via issues. But the most common footprint issues seem to be with connectors. At least with chips Connector footprint 2smand discrete silicon and passive components most manufacturers pretty much follow IPC standard footprints. Sometimes they’ll create new ones for smaller parts, but generally they still stay reasonably close to in line.

Connector footprint 1smConnectors are another story, though. I’m not sure any manufacturer follows anything close to a standard. This pair of Ethernet jacks is a good example. Often the actual pin layout will match, but the mounting will vary widely. I’ve seen it on Ethernet, mini-USB, micro-USB and even the old, old RS232 connector.

It gets more frustrating when they’re almost the same. We see that a lot; the layout will almost, but not quite match a footprint in the library. The bottom line is never take a connector footprint for granted. Always double check before getting your boards fabbed.

Duane Benson
Carburetors man. That’s what life is all about.

http://blog.screamingcircuits.com/

More CAD Footprint Woes

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At this point, I really shouldn’t call them “woes.” More like business as usual. I’m talking about the need to make custom footprints, or at leas modify footprints. Back in the old days, the only thing needed to make footprints was some copper pads, maybe plated through, maybe not. It was pretty rare to even need to make a custom footprint. Other than the occasional odd switch or relay, it was all done.

I really need to just get over it, though. On the one hand, it seems like nonproductive time; like I should be able to get right to schematicing and layouting. On the other hand, it’s so common, I just need to see it as no different than any other routing task.

Starting at the top of my BOM, I have:

  • An MCU in QFN format – I modified a symbol and added a custom paste layer to the copper land
  • Two SOIC Mosfet drivers – I modified the symbol on an existing footprint
  • Some Mosfets in a PowerQFN package – Made a complete custom footprint
  • A Mosfet in SOT-23 package – Woo hoo! I found a workable part in the library
  • Some Power Schottky diodes – Custom copper land

Custom footprints

I have another Schottky, some TVS diodes, LEDs and a bunch of passives that came straight out of the library. It’s certainly not everything that needs footprint work, but with so many variations of the more complex parts these days, it safe to assume that any SMT project will require a fair amount of library work. It’s just the way it is.

Duane Benson
It’s a pain but at least it’s not as bad as 11811 has it

http://blog.screamingcircuits.com/

Beagle-Not-Board

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I wandered over to the TI booth at the Embedded Systems Conference to check out the Beaglebone and the MSP430 Launchpad. We haven’t built any Beaglebones yet, but a few years back we took the Open source Beagleboard files and built up a couple to show our package-on-package (PoP) workmanship. The Beaglebone doesn’t use PoP, but it is open source. As are the “Capes.” They call their expansion cards capes. I get a vision of Underdog when I hear that name.

What caught my eye was their selection of Capes available now.
0328121210
All of those are open source. They provide functionality such as displays, battery power, CAN, prototyping and more.

The other product I looked at was the MSP430 Launchpad. I’ve been getting to know the launchpad myself and wanted to see what’s new with it. 0328121213The unit shown here has a DSP and a little display driven by the MSP430. It’s a MP3 player with the purpose of introducing the DSP and MSP430 / DSP combination.

TI is doing an amazing job of making evaluation of its chips easy and inexpensive. I’ve seen a few other companies providing more complete and less expensive than traditional dev boards as well. NXP with their LPC series to name one. It’s a good idea and a good trend as far as I’m concerned.

Duane Benson
Robots good. Neurotic thermonuclear devices bad.

http://blog.screamingcircuits.com/

 

Shrouded vs. Non-Shrouded

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A connector isn’t a connector isn’t a connector. In the photo below, the original PCB was designed to have an unshrouded breakaway header, as shown in the inset on the right. I measured it. The entire header fits within the silkscreen outline.

However, as you can see, a shrouded header was used in that spot. While as designed, there was plenty of clearance between the header and the two capacitors and resister, the shroud for the substituted header covers all of the resistor and half of the capacitors.

You can prototype it this way, but it will never fly in production.

Duane Benson
Find the ghosts of Dawnstar

http://blog.screamingcircuits.com/

Shrinky Dink

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I had some Shrinky Dinks when I was a kid. Amazingly, you can still buy them. You can also use that concept in your prototyping. I did that recently. I have a robot board design that I’d like to shrink about in half and add in a LiPoly charger chip. Most of the design came from something I had built previously, but the charger chip was new to me, as was the compression needed to meet my size goals. Sadly, you can’t just put your PCB in the oven and have it shrink like a Shrinky Dink. Maybe if you could put stretchy copper traces on it so they wouldn’t peel off while the substrate shrinks…

The charger comes in both DFN-10 and MSOP-10 packages, and the MCU comes in SOIC and QFN packages. The QFN is the 44 pin version, while the SOIC is the 28 pin version of the chip. Same core. Just more I/O.

Rather than test my ability to shrink and use the LiPoly charger at the same time, I added it into the original design without changing the size. There’s much more room for probing or even for adding test points if I needed them. Once that design checked out okay (which it did), I just went into the schematic editor, changed the SOIC to the QFN package, the MSOP to the DFN and most of the passives to 0402 packages. I really didn’t have to make any changes to the schematic.

That almost worked perfectly. The 28 pin MCU doesn’t come in a variant with a QFN package, so I couldn’t just change the package type in the schematic editor. I had to delete the SOIC version, place and wire in the 44 pin QFN variant. I made a few other changes too. I added in a QFN packaged RS232 driver and a hard power switch. In the original, I had envisioned a soft power switch, but I changed my mind. I also had to modify the library parts to make sure that the solder paste layer on the QFN and DFN parts fit our guidelines. Lastly, I removed some LEDs that I only had on the board for debugging purposes.

The most important two considerations were watching out for physical part interference and getting the paste layer correct on the QFN/DFN parts.

Duane Benson
It’s the size of a small walnut

Oh MSOP, My MSOP

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In the land of prototypes, sometimes “close enough” is good enough. That can save money on PCBs and assembly when a particular package version of your part is out of stock. But, it’s not universal. Sometimes you can’t go that way.

 

I’ve got an MCP78338 Li Poly charger chip. It comes in 10-DFN and 10-MSOP packages. I originally used the MSOP version on my first PCB pass. Everything worked just fine, so I re-laid out the board to be about half the area. That meant that wherever possible, passives went from 0603 to 0402 and chips went from whatever to QFN/DFN packages.

Unfortunately, the DFN package Li Poly charger seems to be out of stock with long lead times. That got me looking at my options. Option 1 would of course be to just wait. Option 2 would be to lay out the board for the MSOP part in that space. Option three is to use the “we’ll make it fit” mantra. There are no guarantees at this point, but sometimes it’s worth a try.

But… Twas not to be. If you look at the second image, you can see that the footprint of the MSOP part leads is wider than the land pads for the DFN. I suppose there are still a few really messy and potentially expensive options You could solder a small wire on to the pads, sticking out from the pads, effectively making them big enough to accommodate the chip. Very ugly, but might work. Probably too spendy, though.

Duane Benson
Carpe DFN

http://blog.screamingcircuits.com/