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

Let’s Talk about HAL – And Another Thing

A few days ago, I wrote about HASL PC board surfaces, explaining that it’s not an appropriate choice for small parts.

Look at the same PCB image I used the other day. You might not recognize it because before it was on the right, and today it’s on the left. Getting past the fact that I just insulted everyone’s intelligence, there is something else about this board that we don’t recommend.

I’ll give you 30 seconds to figure it out. I don’t have a stopwatch, so the 30 seconds is on the honor system.

This is a land for a 0.5mm pitch BGA. As I wrote before, HASL is not the right choice for BGAs, especially for those of the smaller pitch variety. The other problem with this board is in the pad layout.

These are solder mask defined (SMD) pads – the solder mask covers the outer part of the pad, so the solderable copper surface is determined by the size of the opening in solder mask, not by the area of the copper pad.

For BGAs 0.5 mm pitch and larger, we (and pretty much everyone else) recommend non-solder mask defined (NSMD). With a NSMD pad, the solder mask opening is larger than the pad. This leaves more copper area to adhere to, including the sides of the copper pad. It tends to be much more reliable.

The image to the right illustrates the difference. 

The left-most pad in the image illustrates an SMD pad, while on the right is an NSMD pad. The NSMD pad leaves a lot more surface area of the copper pad for the solder ball to grip on, including the sides.

BGAs with 0.4mm pitches might need either SMD or NSMD pads, depending on a number of circumstances. Read this blog information for a bit more on 0.4mm. When in doubt, look in the back of the datasheet.

Duane Benson
Question for physicists and mathematicians:
Should the last recursion in the Mandelbrot set land on Plank’s constant?
Show your work.

http://blog.screamingcircuits.com

 

Milling Madness

Sometimes, we find things that kind of defy explanation. Fortunately, this didn’t come from Sunstone, our normal board house.

Regardless of who it came from, I’m sure it was a one-off mistake, but, wow. How could anyone miss this?

 

 

 

 

 

 

 

 

 

It just goes to show, it’s always a good idea to take a look at what you get from your board house before sending it on to us.

Duane Benson
Termites, maybe?

http://blog.screamingcircuits.com

Designing for Movement

What is the difference between electronics in a robot vs., say, a stationary temperature monitor and control device? For one, if the temperature controller goes haywire, you can pull it off the wall and stomp on it, while you might have to chase the robot (or be chased) to deactivate it if it’s gone into world domination mode. More relevant, though, is vibration.

Fixed embedded electronics generally don’t need to worry about vibration induced reliability issues. Mobile robots, however, do. Unsecured connectors can work their way loose. Bolts can back off. wires can brush against stuff. A lot of practices that don’t cause problems in a fixed installation can bite in a mobile setting.

For example, a simple board-to-board ribbon cable. On the left is a common friction-retention cable connector. Fine for a development board, but not for an environment subject to vibration. Instead, use a mechanically captive connector, as shown on the right.

 

 

 

 

Free hanging cables are also a “no” for mobile devices. Cables hanging loose can get caught on edges, or tall or hot components. That can lead to worn or melted insulation and shorts. Instead, use cable ties, insulating grommets, and careful routing.

There are plenty of other considerations, but these are two of the biggest traps to avoid when movement is called for.

Duane Benson
Klaatu barada nikto. Translation: “Spaceman says what”

Surface Mount Power Component Footprints

There was a time when the bison ran free on the plains and power components were easy to design with. Everything, with the exception of an exotic few, used either the TO-220 or TO-3 packages. Even when surface mount came along and cut the bison off from their grazing lands, most power components came in some derivative of the TO-220, with bent leads.

That’s no longer the case. Today, power components come in those TO-220 derivatives, SO-8 packages, QFNs, and down to 0.3 mm pitch wafer scale micro-BGAs. It’s madness.

The advantage of all of that chaos is that it gives more flexibility for sourcing and sizing of components. Which, of course, brings in a few more potential issues. Take the example below:

 

 

 

 

 

 

 

The footprints were originally created for a package with four 1.27 mm (0.05″) pitch leads on one side and a big heat slug on the other. The component selected is a variant in an SO-8 package. It’s not an uncommon occurrence.

As long as pins 5 – 8 all share the same internal connection, there isn’t anything electrically wrong here. However, with that large open copper pad on top, it’s going to be very difficult to get a good solder joint.

The fix is pretty easy. Just add solder mask to separate the pins. Make the mask openings the same size as you would if the pins were on individual pads. You don’t need to cover the whole pad with solder mask — just surround the pins so solder will stay where it’s needed. The mock-up below illustrates what it would look like:

 

 

 

 

 

 

 

Do the same with your solder paste layer. Unless the component has a heat slug underneath, make the paste layer block the big open area.

Duane Benson
Would a bisontennial be a 100 year old, large grazing animal?

Fab Drawings or Assembly Drawing Standards?

It’s not always possible to have all of the information needed for a successful PCB assembly printed on the blank PC board. When this is the case, we ask for an assembly drawing – like I suggest here. But what about things that are important at the PCB fabrication stage rather than at assembly.

That’s where the fab drawing comes in.

One of the problems with this system is that the “standards” for fab and assembly drawings are only loosely adhered to, if you can call them standards at all. If in doubt, label the fab drawing “Fab drawing” and the assembly drawing, “Assembly drawing.” That may seem obvious, but in the wide world of technology, obvious too often is anything but.

benson-oct25

(image from xkcd.com)

Clearly label anything that the fab shop needs that isn’t obvious from the Gerber files, make a PDF, and label it “Fab drawing.pdf.” Do the same for any assembly information and instructions and label it “Assembly drawing.pdf.” If information is needed by both the fab shop and us, the assembler, put it in both drawings.

We recently had a case where a component polarity wasn’t marked on the board or in the assembly drawing, but was in the fab drawing. We do our best to catch such things, but it ads a bit of ambiguity to the process. If you’ve been reading this blog before, you’ve likely picked up that I do not like ambiguity. I do not like it one bit.

Duane Benson
Vote for clarity! Kick ambiguity out to the street

http://blog.screamingcircuits.com

Happy St. Patrick’s Day!

SC4And some trivia. You may have noticed that the soldermask used on most PC boards is green, as is the paint used on most steel truss bridges. Why is that? And what do the two things have in common? Why green PCBs and why green bridges?

To answer, I brought in color expert expert Patty O’Patrick O’Dell, who stated: “Many bridges and PCBs are green because they absorb red and blue light, only reflecting the green.”

That wasn’t quite what I was getting at, but close enough. The important thing, is that, generally, in commercial products, the PC boards are hidden, so the color doesn’t matter that much. With prototypes and a lot of the hobby or development boards, that is not the case, so many companies have chosen to use a different color as a part of their identity.

Arduino products are blue, as are most boards from Adafruit. SparkFun makes theirs red. Ti Launchpads are red as well. The Beaglebone uses color, essentially, as a model number; Beaglebone black, Beaglebone green. This is possible because major PC board fab houses have made different colors more economical than they used to be.

I’ve been asked if the color makes any difference electrically. In general, no. If you’re dealing with super high speeds, RF, or other exotic conditions, it’s always best to ask your board house. In those fringe areas, a lot of things have the potential to make a difference. Other than that, if you can afford it, and want to make a statement, go for it. You can often get different color silk screen legend too. Just make sure there’s contrast between the two. White silkscreen on white soldermask would not be the best choice.

Duane Benson
Beware the monsters from Id

blog.screamingcircuits.com

Proper PCB Storage — The Top 3 Hazards

It’s late. Do you know where your printed circuit boards are? Let me rephrase that: Can unused PCBs be stored for future use?

Yes, they can – if stored properly. Keep them wrapped up, or sealed in a bag. Anti-static isn’t necessary in this case, but it won’t hurt. Keep them in a cool, dark place. Keep them clean. Do your best to avoid dropping them on the floor and stepping on them.

The board in this photo was left out on a desk for a while, and then shoved into a desk drawer. The environment took its toll on the immersion sliver finish, making it very much unusable.

PCBstorage

What can go wrong:

1. Fingerprints. The oils on your finger can etch fingerprints into ENIG or immersion silver board surfaces. If you plan on committing a crime go ahead and do this so we can catch you. If you aren’t going to start a life of crime be careful to not get your fingerprints on the board surface. Handle on the edges, or at least, don’t touch any exposed metal.

2. Moisture. Moisture is good for your skin but not for your PCBs. Over time, PCBs can absorb moisture, especially in a humid location, or the ocean. If thrown into a reflow oven they then might laminate. Store boards in a dry environment. If stored for a long time, you may want to pre-bake them prior to use.

3. Atmosphere. Sometimes dirty air can contribute to tarnish or corrosion on the exposed land pads. Dust can settle onto the boards as well. Tarnish and dust can usually be cleaned off, but corrosion can’t. Wrap up your boards for long-term storage.

Treat your boards well and you can likely use them at a later date. Don’t treat them well and you may need to replace them, wasting a bunch of money. Often, the damage isn’t as clear as in the above photo, but could still lead to poor solderability.

Duane Benson
Don’t surf on your silver

http://blog.screamingcircuits.com/

Comings and Goings

Big news: IBM will “sell” its chip operations to GlobalFoundries according to a joint statement by the companies. IBM will pay “the buyer,” owned by the government of Abu Dhabi, a reported $1.5 billion over the next three years to take the chip manufacturing business “off its hands.”

The world’s dozen or so leading chip foundries that account for more than 90% of global production (including IBM) will all be foreign-owned when this deal is completed.

Who benefits the most? Who will protect the innocent (chip users)? Is leaving America “foundry-less” good for the United States?

Back to the future. Lockheed is reported to have three captive PWB shops in the US. Northrup Grumman has an in-house board operation. Whelan is establishing a new highly automated in-house PWB operation. Intel is said to be planning a new captive board facility in Arizona and said to be offering a bounty for “successful” job referrals. Is it to develop new technology that independent shops cannot afford? Is it for secrecy? Is it to shorten supply lines? Is it to gain time to market or some other competitive advantage? Is it the start of a trend?

Is it still just the price? After visiting the Design-2-Part Show  (D2P) we revisited the concept of using value propositions to offset cheaper prices. We were astounded at the number of people that effectively stated that they would the cheapest system rather than the lowest cost equipment. Yes, there is a difference, and sometimes the added cost of using the cheapest system is substantial. We also found that those that bought the newer system with greater productivity, ease of use, updated software, and smaller footprint often ordered more of the units of the newer system after a short (several months) of running production along side of the “older” competitive models who tried to protect their business by simply lowering the price and promising future improvements.

Some things just never seem to change. When one visits some of the leading board makers making advanced substrates and assemblies in Asia today, one usually sees the latest production equipment. Then I thought of the New Jersey manufacturer that I met at D2P show, and wondered what I will see when I visit a new “highly automated” board maker in the U.S. next month.

Move over Amazon. Dragon Circuits in Texas announced that it successfully completed 14 test runs of delivery by drone of packages weighing up to several pounds. It did not state the range of the drones used in the test runs. Dragon has a drone division that builds a wide variety of these systems.

We need more industry participation and on campuses collaborations like the new Raytheon-UMass Lowell Research Institute (RURI). Plans for the center were announced in August. It officially opened on the campus on Oct. 10 in the school’s Mark and Elisia Saab Emerging Technologies and Innovation Center, the school’s new $80 million research center.

Kyle Homan, a doctoral student in electrical engineering, gave a presentation on printable electronics and nanotechnology at the opening. Raytheon has already embedded employees on site and plans to commit $3 to $5 million over 10 years to support the collaborative operation. It’s a great way to move critical technology forward while simultaneously training candidates for the company.*

Have you seen the TPCA’s (Taiwan Printed Circuit Association) first class promotional video on its interconnect industry? Take nine minutes and watch the video on the following link. ??????? https://www.youtube.com/watch?v=8w_jIpYu54A

 

*Raytheon currently employs about 1,000 UMass Lowell alumni.

 

The End of the Viasystems Era

At long last, the hunter became the hunted.

TTM Technologies today announced its pending acquisition of Viasystems. The deal, expected to close in early 2015, will vault TTM to second place among the world’s largest PCB fabricators.

No matter how the deal turns out for TTM, Viasystems will remain one of the most talked about PCB companies in the industry’s history, held in awe for its audacity and blamed on multiple continents for nearly single-handedly devastating local supply chains.

For the entirety of its 18 years, Viasystems was worth 10 times its revenue in industry controversy and chatter. It sprung on the scene in fall 1996, the brainchild of New York investment firm Hicks, Muse, which in quick order bought up AT&T’s board shop in Virginia, Circo Craft, Kalex, Forward Group, ISL, Mommers and Zinocelere, plus several EMS and peripheral businesses. They were the Yankees of the PCB world, albeit without the same level of success.

Then came the Tech Recession of 2001, and Viasystems’ debt ballooned to over $1 billion. Two Chapter 11 restructurings and countless lawsuits later, the company stabilized and managed to spend the better part of the rest of the decade simply managing the business.

In 2010 the veil was lifted. Viasystems resumed its buying ways, snatching up Merix and then, two years later acquiring DDi (which in turn had gobbled up Coretec). Yet consolidation didn’t bring happiness. Over the years Viasystems found it nearly impossible to turn a consistent profit. Debt, a persistent problem dating to its Hicks, Muse days, now sits at $561 million.

TTM is getting Viasystems for $16.46 per share, or about 6.8 times adjusted EBITDA. You tell me if that’s worth it.

I would expect TTM will sell off Viasystems’ wire harness business, which is small ($174.6 million in 2013) relative to the rest of the business and has shown operating losses in five of the past seven quarters. Viasystems has already consolidated its China manufacturing base, so I would not expect much change there. TTM is running at 75% capacity in China but only 60% in North America. TTM has seven sites in North America and Viasystems has nine, including a combined three in the Silicon Valley and two in Orange County. Perhaps they will seek to consolidate here in order to boost rates.

Viasystems changed the way the world viewed the industry. It forced Wall Street to take notice. It laid waste to the regional landscape, ultimately closing millions of sq. ft. of some of the once-best shops in the world. Some will say this was inevitable. Viasystems bought plants that were obsolete or quickly headed that way, whose workforces could not change even while the technology was quickly shifting away from them. And the firm tied up enormous amounts of capital in dubious debt deals that may have enriched a few but certainly did not leave their business units with the balance sheets necessary to operate in such a cyclical market.

There’s still time for the deal to fall through, and it took about 18 seconds before shareholder lawsuits began piling up. No matter what happens on the ground, come next spring, Viasystems will again occupy the rarest air of the PCB world. It just won’t be as Viasystems.