Tag Archives: PCB design

How Should You Mark Your Diodes?

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Current flows through a diode from the anode to the cathode – it will pass current only when the potential on the anode is greater than the potential on the cathode. This is mostly true, but not always.

For the common barrier diode, or rectifier, it’s a pretty safe bet. However, with a zener diode, or  TVS, it’s not true. And, that is why marking a diode, on your PC board, with the plus sign (+) is not good practice.

Take a look at the schematic clip below.

 

 

 

 

 

 

Once you put this circuit on to a PC board, you could legitimately place a plus sign on the anodes of D3 and D4, and another on their cathodes. In the next schematic clip, you could legitimately place both a plus sign, and a minus sign on the anode of D9.

We don’t know what you had in mind, and, we don’t have the schematic. If you use the practice of marking diodes with a (+) on the anode, we don’t have any more information than if you didn’t mark it at all. The same holds for using a minus (-) sign. It really doesn’t give us any information.

So how should you mark your diodes? The best method is to put the diode symbol next to the footprint. on the PC board, as shown below. You can also use “K” to indicate the Cathode, of “A”, to indicate the Anode. “K” is used because “C” could be mistaken for “capacitor.”

D5, in the illustration on the right, would be the preferred method. D7 will work as well. If you don’t have enough room on the board due to spacing constraints, you can put the same information in an assembly drawing.

Ambiguity is the enemy of manufacturers everywhere. Read a bit more on the subject here, or here.

Duane Benson
Help stamp out and eliminate redundancy, and maybe ambiguity, or maybe not

Those Danged LEDs Again

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I was caught by one of my own favorite “simple” traps last week: the dreaded LED footprint mess.

I designed a board based on the Microchip PIC32 — it’s a ChipKIT Arduino-compatible board — that has a number of RGB LEDs on it. I used RGB LED part number LTST-C19HE1WT, from Lite-On. The datasheet is easy to find, and the footprint information is right up front, just the way we like it.


Almost all is well, but I somehow missed taking my own advice and I didn’t double-check the footprint.The footprint I used is more or less 180 degrees off from this one. The common anode is still on pin 4, but the numbering is different. It’s got pin one in the same place, then pin two is in the lower left. Pin 3 is on the same place, and pin 4 is on the upper right. That’s the conventional pin numbering order.

Fortunately, the fix won’t require any mod wires. If I rotate the LEDs 180 degrees, the anode will be in the right spot. All I’ll need to do is adjust my software for the correct R, G and B pin locations.

Duane Benson
I’m dizzy with rotation

http://blog.screamingcircuits.com/

Freescale KL03 and PCB123 at 0.4mm Pitch

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Small component packages seem to be a recurring theme with me. It’s understandable, I guess. Super tiny packages are becoming more and more common and we build a lot of product with them.

The smallest we’ve built is 0.3mm pitch. Those aren’t common enough to be considered standard — they’re still an experimental assembly — but not many chips use them yet. 0.4mm, on the other hand, is something we see on a pretty regular basis.

What’s so tough about that? The biggest challenge with these form-factors seems to be footprint design and escape routing. I can see why. There really isn’t room to follow any of the standard BGA practices. You can’t fit escape vias between the pads and you can’t put vias in the pads, unless they are filled and plated over at the board house. Filled and plated vias are the easiest way to go, but it can make for an expensive board fab.

KL03 WLCSP20 on a US Lincoln penny. One of my side projects involves trying to make the smallest possible motor driver. For this project, I’ve chosen the Allegro A3903 driver. It’s a 3 x 3mm DFN (dual flatpack no leads) with 0.5mm pitch pads and a thermal pad in the middle. The microcontroller will be the new Freescale KL03 32-bit ARM in a 1.6 x 2.0mm WLCSP (wafer level chip scale) package. It also comes in a 3 x 3 x 0.5mm pitch 16 pin QFN. Without an expensive PCB, that may be my only option.

Pick your CAD package. I’m using the newest version (5.1) of Sunstone Circuit’s CAD package, PCB123, but the principles here will apply to any CAD software. If you don’t already have a copy, download PCB123 V5.1 here.

If you’ve got fast Internet, you’re done now, so go ahead and install it. You’ll need the manual too, which you can get here.

I need to eat now, so stay tuned for Part 2.

Duane Benson
Nerfvana – It’s like Nerdvana, but with more foam darts.

http://blog.screamingcircuits.com/

Wilk the Winner

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Congratulations to Edward Wilk of Facts Engineering, who won the incentive prize for filling out a recent survey of PCB West 2014 technical conference attendees.

Edward, a $50 AMEX gift card is headed your way. Thanks for your support, and thanks to everyone who took the time to respond!

Pads on Ground Plane

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Generally, small pads for passive parts are connected  with a single PCB trace of equal size to each pad. That’s the right way to do it.

Top pads solidly connected to copper pour

However, sometimes, circumstances dictate a little different approach. The illustration on the right shows something of a worst-case. This is for a snubber (resistor, capacitor pair) between two power planes.

A couple of things will likely happen. The power plane will act as a heat sink, preventing the solder paste on one side from melting, resulting in a poor connection. Or, the unequal melting could lead to surface tension pulling the part up, causing tombstoning.

Most designers are aware of that, but sometimes, thermals will be deliberately turned off to allow for better current capacity to and from the large power Mosfets (not shown).

Thermal pads on side connected to pour.

If that’s the case, make sure that you can turn the thermals (see figure at bottom right) on or off by the part, rather than just by the plane.

Duane Benson
The rain falls mostly on the ground plane due to static attraction

 

http://blog.screamingcircuits.com/

 

Will a Via Fit Between?

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I don’t know that it would be accurate to say that BGAs have ever been easy, but with 0.4mm pitch being common and 0.3mm pitch showing up, some of the older size, like a WHOLE millimeter pitch seem 0.5mm pitch padspositively spacious. With 1mm and larger ball pitch, putting a via between the pads (not in the pads) is a no-brainer.

IPC-7095B classifies 0.8mm and smaller pitch as fine-pitch. It really starts to get complicated at around that point. For example, take a 0.5mm pitch BGA. Since we’re looking to put a via between the pads, the diagonal pitch is the critical measurement. In this case, it’s 0.71mm (17 mil). It might immediately seem like that’s plenty of room for a 6 mil via, but upon closer examination, not so much.

0.5mm pitch pads viasIPC states that a 0.5mm pitch BGA will have a nominal pad diameter of 0.3 mm. It should be a non-soldermask defined pad, which will add about 0.07 mm to the pad diameter. That gives 0.44 mm total pad diameter. The radius is 0.22 mm (8 mil). Take that out of the 0.35 mm (14 mil) you have to work with and you’re not left with much space.

If your fab house can do 3 mil trace and space, you will end up with enough room for a 0.06mm (5 mil) via, including annular ring. That’s not much space. Most designers, at that point, will seriously consider putting the via in the land pad and having it filled and plated over. You can’t leave the via open or un plated.

Duane Benson
All was in chaos, ’till Euclid arose and made order

http://blog.screamingcircuits.com/

Beagle CAD Paw Prints

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Unfortunately, I can’t generically hand out Eagle CAD QFN footprints without knowing the specific part, but I can illustrate the areas I initially had difficulty with. All of the traps that used to get me seem blindingly obvious now, but they weren’t when I first tried to make my own library parts.

The very first thing I would recommend is to make your own library file. When I started in with my own parts, I would just add them to an existing library. For example, I’d put a new Microchip PIC processor into the “microchip.lbr” library. It seemed the logical choice because there are other similar parts to start with. But, when it’s time to upgrade, migration of those custom parts becomes a nightmare. So, now all of my custom parts go into “dfb-parts.lbr.” Eagle footprint menu bar.

Speaking of modifying existing parts, another recommendation I have is, except for parts where the package footprint is exactly the same, start from scratch with the package footprint. The schematic symbol is easier to reuse – just make sure you have the right pins in the right place – but subtle differences in the copper footprint can have a big difference at the assembly stage. Datasheet footprint page land pattern.

I also don’t try to hand size and hand position the pads on the silk screen. Start by just putting a pad in the footprint area. The use the Properties/Info button (the big “i”) and use the dimensions given in the data sheet to enter the size and position by number. Look for the “recommended land pattern” or similar diagram toward the end of the component datasheet. Entering the numbers in the Properties/Info box will bypass any position precision issues. Just make sure that you use the right units (i.e., metric to metric).

Stay tuned for the next installment.

Duane Benson
World to end at 9:30. Details at 11:00

 

Why Not Nick?

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Nick Martin, the founder and, until last week, CEO of Altium, is fighting back against the board that tossed him out.

But the real question isn’t whether he will regain his spot atop the CAD tool developer. It’s why the board saw fit to relieve him of his duties in the first place.

Some contend privately that at least one board member wants to sell the company but that Martin, who is the company’s largest shareholder, has been reluctant to go along. If so, pushing him out would mean removing, in part, one big barrier. For its part, the board has publicly stated that the decision to leave was Martin’s — something he vehemently contests, and which seems unlikely on its surface — and that the company has not returned the type of shareholder value the board seeks.

So while it’s true the move to Shanghai coincided with an improved bottom line and a higher share price, it’s also true the stock hasn’t topped $1 in years (chart — the top line = $1; the current price is about 80 cents). No one is getting rich owning Altium right now. If the board is getting antsy, it’s understandable. Whether that merits replacing  Altium’s answer to Steve Jobs — a design visionary who, according to many we’ve spoken with, has always put the technology first — is for the historians to determine.

 

CAD Tool Survey

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PRINTED CIRCUIT DESIGN & FAB is undertaking a survey of how printed circuit board designers, design engineers and other layout specialists use and think about various CAD tools. Results will be published in an upcoming issue of PCD&F. Please be assured that the data collected are revealed only in the aggregate. No individual data will be revealed.

The survey will take about 3 minutes to complete. Please click below to begin: http://www.surveymonkey.com/s/NC35KK8

Because this is a survey, not a poll, the audience being surveyed is not selected or controlled. In short, the more responses we receive, the more representative the results will be. So feel free to share this link with other designers.

If you have any questions, please contact me at [email protected]. Thanks!

Bragging Rights

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I want to call attention today to a great new contest for designers being run by Sunstone Circuits.

The contest works like this: From now until Dec. 16, design engineers can share their PCB-related design success stories online at Sunstone’s website.

Friends and others to the site can vote for the best project. All entrants and voters are entered into a sweepstakes to win a series of prizes, ranging from gift cards to an iPad.

It’s a great way for designers talk (and yes, perhaps, brag) a bit about what they do. And while Sunstone certainly benefits from the exposure, I’m glad to see a company taking steps to highlight the remarkable things designers can do.