What Makes a Good Fiducial?

Accountants may have a fiduciary responsibility, but that really has nothing to do with PCB assembly. Change the “ry” to a “ls,” however, and you get fiducials, which does have something to do with PCB assembly.

A fiducial is essentially an alignment mark for surface-mount assembly machines. High-volume assembly requires them to ensure accurate registration and parts placement. Low-volume assembly, like we do at Screaming Circuits, doesn’t necessarily require them. (Some low-volume shops do, so ask before assuming.) Even if they aren’t required, they still help and are always a pretty decent idea.

The basic idea, explained in this blog post here, is to create a non-reversal pattern with two or three fiducial marks on the board or panel. As you can see in the image above, the designer placed three fiducials around the board in a non-reversible pattern. (To protect the confidentiality of the board design, I obscured the circuit detail with this convenient robot head.)

In terms of the specific construction of a fiducial, two things are most important: contrast, and accuracy of position.

Contrast comes from it being bare copper – make it 1 to 2mm in diameter. Don’t cover it with solder mask. Make the mask opening 2 to 5mm larger than the copper.

The image on the left shows closeup detail. This particular fiducial mark uses a square cutout in the silk screen. Most use a round cutout, but the shape isn’t all that important. The copper pad should be round, though.

Making it out of copper gives the positioning accuracy. I’ve been asked why silk screen markings aren’t acceptable. Silk screen isn’t always registered consistently, and is therefore won’t ensure accurate alignment. Don’t use silk screen as a fudicial or positioning mark of any kind.

Again, they’re generally required for high-volume manufacturing. We (Screaming Circuits) don’t require them for low-volume, but some assembly houses do. Even when not required, they’re still a good idea.

Duane Benson
Fiducial on the roof is a long movie
But at least it stays in place

http://blog.screamingcircuits.com

Keeping Things on the Level

Sometimes, in PCB assembly, it’s not the layout of the SMT PCB that creates issues, but the design of the part itself, or the plan for the part’s location, given its dimensions. We have to ask ourselves, sometimes, “What were they thinking?”

In this case, a customer’s BoM called for a part (an RF200 module with through-hole pins) to be mounted onto a PCB. At one end is a bulky SMA connector that due to its size exceeds the thickness of the module. The SMA connector faces inward on the PCB; it’s not mounted to hang over the edge. As a result, the SMA connector bottom side touches the board and props one end of it up; it doesn’t permit the module’s pins to be properly inserted into their corresponding PTH barrels on the board. One end of the SMA is pointing upward on an angle like a missile-launcher.

Figure 1

Figure 1

Figure 2

Figure 2

This is obviously not acceptable in circuit board assembly; not only is customer access to the connector compromised, but the module cannot be mounted in a planar fashion and having some of the pins fully inserted and some halfway out of the barrels, with one end of the module elevated, is certainly not acceptable.

The fix was relatively easy; we recommended that the customer allow us to use two single-row socket pin adaptors to provide the standoff necessary to keep the SMA connector from touching the board while at the same time allowing easy and unobstructed access to the connector.

Figure 3

Figure 3

Figure 4

Figure 4

Two socket headers were used, corresponding to the module’s two rows of pins. Not only did this provide the needed standoff, without creating any other issues, but it also allowed the customer the potential for removing or replacing the module in the socket pin adaptor in the future without serious rework issues, since it’s a mechanical mounting. It’s also a robust electronic assembly connection in terms of strength and durability, and the module is completely planar with the surrounding PCB surface.

Roy

http://rushpcb.com/

 

Indicating Polarity on Diodes

Everyone knows which way current flows through a diode. Right? Of course they do. Diodes only permit current to flow in one direction.

Well, sort of.

In the case of your garden variety rectifier, barrier diode, or LED, that’s true. That line of thinking leads a lot of people to assume that you can indicate diode polarity by putting a plus sign “+” next to the anode.

Here’s why you can’t.

Zener and TVS diodes have a breakdown voltage. They are put in the circuit with their cathode on the positive side. In that configuration, they don’t conduct unless the voltage rises above their breakdown point. Zeners and TVSs are used for regulation, transient suppression, and things of that sort.

But wait! There’s more!

Regular diodes can be pointed backwards too. Anytime an inductive load is switched, like a solenoid or motor, you need a flyback diode to protect the switching logic. A MOSFET switching a solenoid on and off is a good case to look at.

When the MOSFET turns off, the current in the solenoid coil starts to drop. As it starts to drop, the magnetic field generated by the current flow starts to collapse. The collapsing magnetic field generates an opposite current, referred to as flyback, or back EMF.

To save your silicon switching device, you put a flyback diode across the coil, or motor, terminals, pointing backwards from normal current flow – with the cathode pointed toward +V. Doind so shorts the flyback current back into the coil, preventing damage to the MOSFET. These are typically Schottky diodes, but can be ordinary rectifier diodes.

A “+” plus sign alone, doesn’t tell anyone anything. For more information on what to do, read this post. Just for fun, read this post too.

Duane Benson
Diodes. Not just for breakfast anymore

Do You Need that Part, or is It Just Habit?

At the moment, I’m working on an Arduino compatible clock. Like most of my Arduino compatible boards, this one uses an Atmega32U4, with USB built in. With the Atmega32U4, I sacrifice a little in program memory and SRAM, but gain a bit in reduced parts count.

A USB capable Arduino-compatible is, of course, programmed via USB, and can be powered by the USB port. Most Arduino boards also have a 5V regulator to be used when being powered by a wall-bug power supply. Naturally, I put the USB connector on the clock board, as well as the 5V regulator. With the two different supplies, I also put in circuitry to auto switch sources and protect the USB host when both supplies are connected at the same time.

My first PCB revision required6a00d8341c008a53ef01b8d0aae30b970c-800wi a few hand-mods, but not many. Still, I decided to respin the board and remove the two mod wires. While doing so, it suddenly occurred to me — a blinding flash of the obvious — that most cellphones and other small devices are charged with a USB-connector 5V wall-bug power supply. Why then, would I also need a separate power supply and on-board 5V regulator?

By pulling the regulator off of the board, I could eliminate a few capacitors and the supply auto-select / protection circuitry. Not only did I save in component cost, but I was able to reduce the PCB size, and thus cost, by about a third.

  1. I had the 5V regulator in the design because Arduinos can be powered by either USB or a non-regulated power supply.
  2. The reverse power protection is necessary to prevent damage to the USB host if the other power is also connected.
  3. The auto-power switching circuit is necessary so that a user doesn’t need to flip a switch or change a jumper when changing power sources.
  4. I had two extra LEDs to indicate which supply was powering the clock.

I questioned my original assumptions, found a “because it’s always done that way” and eliminated it. Assumptions are meant to be challenged.

Duane Benson
Question authority!
And then get squashed
(or, squash extra space out of your PCB)

Warped PCBs

You just got a nice big PCB back from the fab shop. You set one on your desk to admire only to discover that it’s warped. What do you do?

There are two primary types of causes of board warping: process related at the fab or assembly shop, and layout-related issues. If it’s warped before assembly, it’s between fab and layout. If it’s flat before assembly and warped, after, it’s most likely between layout and assembly — although sometimes a fab problem won’t show up until a pass through the reflow oven at your assembly partner.

Determining the root cause is generally a bit of an iterative process. It’s tempting to start right off with your fab or assembly partner, but you need some information before giving them a call. You’ll need such things as the amount of warpage per inch, board size and thickness. With that, you need to take a good look at your design and consider copper pours, component size and component placement.

With that information in hand you can make your phone call. If the board is warped before assembly, call your fab shop. If it’s flat pre-assembly and warped post-assembly, call your assembly house.

The shop you call will want to talk over your design to help you pinpoint the cause. If you can rule out a design issue,then you need to talk with your partner to determine whether it’s a fab or assembly issue and next steps to take care of you.

 Here are a few design issues that could contribute to warping:

  • Uneven copper pour. Copper and FR-4 are a good match relative to thermal expansion, but they aren’t exact. A large pour on one side or corner of your board can lead to warping due to dissimilar expansion characteristics. This could cause warpage either at the fab shop or the assembly house.
  • Components with large thermal mass grouped together on the board. This would be more likely to cause problems during assembly than during fab. The thermal mass will act as a heat sink for that area on the board, which can lead to uneven expansion and uneven soldering.
  • A board that’s too thin for the size or number of components could lead to warping at any stage.
  • Odd shapes or large cutouts could also lead to warping at any point.

There may be other, more obscure causes, but those are the main design related causes. If it’s none of those talk with your partner.

Occasionally, design requirements lead to a board that is essentially non-manufacturable. Hopefully you never have this situation, but if you do, make sure that thickness, component location, pours, or cut outs really, really, really, need to be the way they are.

If you absolutely, positively can’t change anything, go back and try again. Then you can to look for heroic means to get the board fabbed and built.

Slight warpage might go away when the board is mounted. Just be careful with that. Some components may not stay securely soldered when you flatten it.

The board may need a special fixture during assembly to prevent warping. This will likely cost extra, but if you can’t change your design, and still need it built, it may be your best option.

Finally, if nothing works, you may need to look harder at the design, or look for a new fab or assembly house. We all like to think we can do just about anything, but every shop has its limits, and on rare occasion those limits can be difficult to spot.

Duane Benson
What if Godot was late because he was waiting for John Galt?

http://blog.screamingcircuits.com

Where’d Design East Go?

As recently as February, United Business Media staff were promoting the then-upcoming Design East trade show.

Big changes and improvements were promised, and given last year’s flat turnout and buzz, they would have been a welcome shot in the arm.

We saw “would have been” because the show has been canceled, with the understated  message on the website telling visitors only, “Thank you for visiting the Design East site. Unfortunately, Design East will not be held in 2013.”

The fact is, it’s really tough to do a great show in certain markets. Even Boston, which has a thriving tech community, doesn’t go out of its way to support these events. Show producers wrestle with the question over whether it makes sense to undertake the expense, effort and risk involved, especially when it might cannibalize other convention offerings.

As an event organizer that has put on shows in the Boston area in the past, we can empathize with UBM’s decision. But we also understand the fragile nature of supplier loyalty, and when you open the door for customers to go somewhere else for their needs, they usually walk through it.

All Quiet on the Wilsonville Front

A timely piece from the hometown paper of Mentor Graphics looks at how Carl Icahn has calmed down now that Mentor’s stock price has doubled since he started accumulating shares of the company a couple years ago.

The legendary investor is Mentor’s largest shareholder, at just under 15% of the company. Since he starting buying up shares, Icahn has been vocal about the need for the software company to shed its country club culture. He forced the issue in 2011, successfully getting three of his nominees elected to the company’s board. Last year, Mentor only nominated one of the three, which drew fire from Icahn, but with the stock price up 50% over the past 12 months, all is quiet in Wilsonville.

 

LEDs: Seeing Double

Like I do so often, I’m being a bit redundant. While I’m all for stamping out and eliminating redundancy, this is redundancy with a purpose (not a porpoise). Not long ago, in a galaxy not far away, I blogged about annoyances in surface mount diode polarity markings. You can read that here.

I’ll wait.

Messy isn’t it? Well, after reading that blog, someone asked me about dual diodes. For some reason, I can’t seem to find the page covering dual diodes in my IPC book, but that’s not the important part. What is important is the way the diodes are marked on the PC board.

We do ask for centroid data which, in theory, contains the component rotation. That would be cool except that we find that far too often, the zero degree orientation (and the rotation from that) differs from the standard. That, and there are seemingly half a dozen or so standards.

Since LEDs don’t work too well backwards, we really would like to see everything marked in a non-ambiguous way in silk screen (or in an assembly drawing if you don’t have silk screen). A “cathode bar” won’t work because it could be a bar indicating the cathode or negative. The cathode isn’t always negative, especially when looking at TVS or Zener diodes.

Mimicking the diode markation pattern printed on the part may not be secure either. Read that article I linked to right at the start of this blog. What if you put silkscreen down to match one of those LEDs but ended up buying the other one? That’s exactly what I did myself. Trust me. It just leads to disappointment and possible soldering iron induced finger burns.

So what is the answer, and why am I talking about single LEDs and TVS diodes when the blog is about dual LEDs? Well, the answer is the same. The best way to communicate the desired polarity of an LED or any kind of diode is with a mini version of the schematic symbol. It doesn’t matter if it’s a single LED, dual LED, Schottky, Zener or whatever kind of diode. The schematic symbol is the clearest way to go.

Led marking

The diode footprint has the manufacturer’s polarity marking, but I don’t care. I still put the diode schematic symbol next to it. If you don’t have room for silk screen, put it in an assembly drawing. You won’t regret it.

Duane Benson
And they called him Flipper…

http://blog.screamingcircuits.com/

‘Board Talk’

Today we launched Board Talk, the bulletin board for the Printed Circuit Board industry, a service brought to you by UP Media Group, Printed Circuit Design & Fab and CIRCUITS ASSEMBLY magazines.

The bulletin board — the URL is www.theprintedcircuitboard.com — is open to anyone in the industry. We’ve set up categories for PCB design, fabrication, assembly, market data, trade shows and press releases. Members are invited to create their own topics (threads) for discussing anything industry related that they have on their mind.

We also are happy to announce an agreement with the IPC Designers Council to offer Board Talk as a communications center for news, announcements and meetings, plus information on the Designers Certification program.

Please check it out!

Riding High on Design

The herd is riding on the EDA vendors, almost all of which are at or near 52-week high share prices.

In the past week, Cadence, Mentor and Synopsys hit or were trading just pennies off their yearlong highs. National Instruments and Ansys both traded much closer to their highs than their lows. Even Altium closed in on a high, but that’s a bit deceiving because it’s a penny stock and lightly traded on the Australian exchange.

So, is it the investor herd driving up an industry? Or is it a sign that the EDA market, which topped $5 billion for the first time in 2011, is geared up for a sustained run?