Surface Mount Power Component Footprints

Posted on January 16, 2017 by Duane

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?

Start the Year Right, Without PCB Placement Overlap

Posted on January 4, 2017 by Duane

Today’s illustration isn’t a super-bad problem. You can usually make this work — unless you’ve got to align with a hole in case. I’m talking about the venerable 3.5mm audio jack. They aren’t used all that often these days, but when they are, one of the most common formats has a design detail that makes edge alignment pretty critical.

The part of the connector that receives the jack is a short barrel, with an outside diameter larger than the height of the rest of the connector, as you can see in the image on the right. It comes in thru-hole and surface mount varieties.

This means that you have to have your solder pads or holes just the right distance from the board edge. Too close, and you can violate design rules. Too far inset, and you won’t be able to mount the connector flush.

This part can cause additional problems if the board is panelized. Like other overhanging connectors, the panel tabs, panel rails or other boards in the panel may make it impossible to mount the part, even if the spacing is correct.

The board shown below has both incorrect spacing, and another board in the panel blocking placement. The surface mount pads allow for more flexibility in positioning — it would have worked if not in a panel.

I’ve done this myself. Speaking from experience, I can say that it’s easy to avoid, and quite sad when discovered at assembly.

Duane Benson
Down at the edge, close by a panel rail
Close to the edge, round by the routing tab

Top 10 PCB Assembly Tips for 2016

Posted on December 27, 2016 by Duane

I’ve already written my top 10 predictions for the coming decade, in this blog post. But, while predictions might be fun to muse upon, they really won’t help you get your job done. My top-ten₈ pieces of PCB assembly advice for the coming year should make up for that.

000

Before you even start component selection, give thought to the design scale. What’s more important, board size, cost, or time to layout? A large board will be easier to route, but will cost more for the fab. A smaller board will cost less for the fab in terms of square inches, but may cost more due to extra layers, and may take longer to layout.

001

Factor in the cost of component size. For passives, roughly 0603 size parts will probably be the sweet spot in terms of lowest cost. The 0603 is also a good size for overall handling. We’ll assembly down to 0201 parts, but not all manufacturers will. 0603s are also easy to rework, and are manageable if you feel the need to hand solder a few.

010

Check out any exotic or very new parts. Some parts, these days, are only available in super small wafer scale BGA, or small QFN form factors. Take a look at your integrated circuits and make sure they come in packages that you’re comfortable working with.

011

Check for sole-source parts, or low-availability parts. The last thing you want is a completed design that’s sitting around waiting for one long-lead time, sole sourced part. If a sole-sourced part is at risk for availability, you might want to find something similar and more available.

100

Don’t forget manufacturing thermal concerns when laying out your board. Very large parts next to very small parts can cause problems. The large parts will act a bit like a heat sink and may prevent the solder for the small part from melting properly. The same thing can happen with internal copper planes that overlap on half of a small part, but not the other.

101

Give extra care to the clarity of reference designators and polarity markings. Make sure that it’s very clear which designator goes with which part, and that there isn’t any ambiguity in polarity markings. Take special care with LEDs, as manufacturers sometimes swap polarity markings between the anode and cathode – yes, the exact same mark can mean anode on one LED and cathode on another. Also, do your best to keep reference designators off of vias or any other spots that might break up the text.

110

When you’re ready to send your project our to be built, give your files a double check to makes sure you have the correct versions. bills of materials are especially susceptible to having bits of information out of date that might cause delays.

111

If you’re sending in a parts kit, double check that you have all of the parts, and that you have part number and reference designator on the individual part bags.

Manufacturing is just putting parts on boards, but it’s doing so with a whole lot of variables. A few extra checklist steps can go a long way toward removing variability of those variables.

Duane Benson
I am one with the net force. The net force is with me

Suspect Through-Hole Packaging

Posted on December 23, 2016 by Duane

Surface mount components are carefully packaged up in strips, tubes or trays, because they’re machine-assembled. The assembly robots need order and organization to properly do their job.

Through-hole parts, on the other hand, are almost always manually inserted by actual human-type people. That being the case, the manufacturers and distributors are sometimes more lax with their packaging. They assume that, since a human is picking the parts, a jumble is okay. Sometimes it is, but not always.

In the case of these through-hole DB-25 connectors, the jumble was too much and lead to a number of bent pins. Slightly bent pins usually aren’t a problem, but some of these ended up a lot more than “slightly” bent.

To make matters worse, these pins are small thin-wall tubes, which are more susceptible to breakage when bent than are solid wire pins. For the connector on the bottom of the image, some of the most horribly bent pins may not be straightened without breaking. If they are, they’ll certainly be weakened.

The moral of the story is that through-hole parts need care too. We can’t toss them around just because they aren’t the latest technology.

Duane Benson
Spider-Pin, Spider-Pin,
Does whatever a Spider-Pin does

7 Cost Reduction Design Tips For Makers

Posted on November 10, 2016 by Duane

As a maker, you really need a decent price, with good quality and good service. Contrary to what many people think, you don’t need to look outside of North America for this. You can keep your gaze west of the Atlantic and east of the Pacific.

Like everything else in the modern world, design decisions can have a pretty big impact on your cost. So, let’s take a look at seven design decisions that can make your manufacturing more affordable.

Accept longer lead times. Lead times are one of the biggest factors in electronics manufacturing. Businesses can turn a kitted assembly job overnight, but it costs a lot of money to do that. When you can, a 20-day turnaround that is much more affordable. Accepting longer lead times on PCB fab will drop your cost as well.

Avoid leadless packages Like QFNs and BGAs. Screaming Circuits builds tons of QFN and BGA boards, even down to 0.3 mm pitch micro-BGAs. That’s great if you need those packages. However, since all of the leads are underneath, we have to x-ray every part. That adds a bit of cost to the process. If you can, stick with TSSOPs and other parts with visible leads.

Use reels and continuous strips. To save costs, use full or partial reels or continuous strips of at least 12″ long.

Stick with surface mount. These days, through-hole components tend to be hand soldered. That costs more than machine assembly, so use surface mount wherever possible. Surface mount components tend to be less expensive than through-hole, too. If you do need a few through-hole parts, this is an opportunity to put in a little sweat equity by soldering the through-hole yourself and save a bit of money.

Keep surface mount parts on one side. Putting surface mount parts on both sides of the PCB is a great way to better utilize space. However, if cost is more of a concern, and you only have a few parts to put on the back side, it may be more cost effective to move them to the top side. If you’ve got a lot of parts, the additional cost for assembling both sides may be less than the cost for the extra board size, but with a small number of parts that’s probably not the case. Quote it both ways and see which is less expensive.

Panelize small boards. Sticking with a larger size makes the job easier, and, again, creates extra savings. If your board is smaller than 16 sq. in., panelize it.

Save on start-up costs. Just the act of starting out can pretty much break the bank. Software like PCB123 offers full-featured PCB CAD systems you can get free of cost.

By following these guidelines, you can get a decent price and quality service.

Duane Benson

Fab Drawings or Assembly Drawing Standards?

Posted on October 26, 2016 by Duane

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.

(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

Accursed Diode Marking

Posted on August 25, 2016 by Duane

Am I a broken record? Pretty much — especially when it comes to confusing diode marking.

For example, everyone knows what the diode symbol looks like, and pretty much everyone knows which side is the anode and which is the cathode. Right? It’s just like in the following picture:

Is that big enough?

Normally, the clearest way to indicate polarity on an LED is to put something like this diode symbol in silk screen next to, or between, the copper pads. In theory, that should remove ambiguity.

Ambiguity in marking is the enemy of polarized parts. Unfortunately, as I cover in this, and many other blog articles, LED manufacturers seem to conspire against us all when marking is concerned.

We recently ran across a case of built-in ambiguity. The PCB had, what looked like, a very clear marking. The image on the right is from the assembly drawing, which is just a blow-up of the board silk screen and documentation layer.

With that marking, I’d quickly come to the conclusion that the anode is on the right and the cathode is on the left. I’d even confidently state that it’s a sure thing and extremely unlikely to cause any problems. But …

Here’s where I’d be very wrong, and why it’s so important to always check the datasheet when dealing with diodes. Take a look at the following clip from the component’s datasheet. Scroll down to the bottom of the image for the punch line.

Wow. I can’t even …,

The board designer was just following the datasheet. That’s a perfectly proper thing to do, except when the manufacturer flips a coin, as it appears to have happened here. In this case, dispense with the symbol altogether and use “A” for anode and/or “K” for cathode in the silk screen. (Use “K” because “C” looks too much like a reference designator for a capacitor.)

Duane Benson
In the land of the insane, only the sane are crazy

http://blog.screamingcircuits.com/

What Route Do You Take?

Posted on July 22, 2016 by Duane

There are a lot of polar opposites in the “what is my philosophy” world: Mac vs. PC, on shore vs. off shore manufacturing, Ford vs. Chevy, Atmel vs. Microchip (well, maybe not that one so much any more), auto router vs. hand route…. Yes, I’m specifically avoiding political opposites.

Routing is what I’m really interested in today. The conventional debate is hand vs. auto route. CAD companies spend a lot of time and money on autorouters, but there’s definitely a line of thought that says it’s not ready for prime time yet. This shirt designed by Chris Gammel, on Teespring pretty much says it all.

But, it’s more complex than that. Most auto-routes end up requiring some hand work, either to finish routes that can’t be found automatically, or to clean up a few less than efficient choices. There are differing techniques for complete hand-routing as well.

I often find myself looking at a layout project a bit like a chess game. I don’t just start at one end of the board and work my way to the other side. I tend to focus on specific parts or critical requirements first, like signal paths that need to be short, or sections with more critical grounding requirements. (The image above isn’t mine. It’s from the Beagleboard.)

When it gets to the mass, I tend to try and think ahead, projecting moves out, as though it were a chess game. When I’m looking for the best route for signal path A, I try and think ahead to how it will impact B, D, D… as far ahead as I can go.

I’m not sure if doing it this way is easier, of if it would be better to just start routing and then re-route as I run into roadblacks. What about you? How do you approach a complex layout?

Duane Benson
Holy cow. I Googled “Trust no one” to get some ideas for my signature
Never do that. It’s going to take a week to shake off all the negativity

blog.screamingcircuits.com

Building Boards for the Intel Edison

Posted on July 18, 2016 by Duane

I’ve recently spent some time getting familiar with the Intel Edison. The Edison has a dual-core 500MHZ Intel Atom processor, with built-in Wi-Fi and Bluetooth. It comes with 1GB of RAM, 4GB of eMMC internal storage, and a USB 2.0 OTG controller. It doesn’t bring any of the connectors (power or signal) out in a usable form. Rather, it’s designed to be plugged onto another board through a 70-pin high density connector from Hirose.

I designed a small board with I2C (both 5V and 3V connectors) and a micro-SD card slot. My board still doesn’t have the power or console connectors. For that, I’m using a base board from Sparkfun.

Figure 1

Step one of the assembly process, is, of course, to design and layout the board. Using the Sparkfun open source designs as a jumping off point, I ended up with the nice, compact layout (1.2″ x 1.75″) shown below in Figure 2.

Figure 2

After getting the files ready and placing a turnkey order on our website, I followed the board through with my camera. Here it is after offline setup, with the parts ready for robot pick-and-place:

Figure 3

In one of our Mydata My500 solder paste printers:

Figure 4

On the pick-and-place machine, with solder paste, but before any components are placed:

Figure 5

The parts plate in the machine:

Figure 6

With most of the components placed:

Figure 7

Through the reflow oven, prior to final inspection:

Figure 8

The final product, top view:

Figure 9

I abbreviated the process a bit, but those are the major process steps along the way.

Duane Benson
Happy birthday (month) Nikola Tesla

Where to Put Panel Tabs

Posted on July 13, 2016 by Duane

Many small quantity PCBs are ordered individually cut. They come to us as a set of unconnected boards. For small quantities of reasonable size boards, it makes the most sense to order them this way. However, for really small boards, and larger quantities (50 or more), purchasing boards in a panel (also called an array) is more appropriate. It reduces errors and assembly time.

There are a few additional factors to consider with panelized boards.

First, don’t try to create a panel in your CAD software. Just lay it out as a single board and have the fabricator put it in a panel. You’ll get the most efficient use of PCB space that way, and the fabricator will create the files in the format that the assembly shop (Screaming Circuits) needs.
Avoid family panels. A family panel is when several different boards are put onto the same panel. The boards in family panels often repeat reference designators, which causes problems at assembly. See this blog article on how to properly assign reference designators on a family panel.
If you have overhanging parts, like the increasingly common micro USB connector, make sure that the panel tabs aren’t placed near the overhanging them.

This blog article gives some background on the connectors.

Some components, such as the connector in the link above, have protrusions that will keep them from laying flat on a panel tab. In all cases, even without the protrusions, the operation of separating the panels with a component on the tab can weaken the component solder joints, or even pop it off the board completely.

How not to do it:

Figure 1

Instead, make sure that the tabs don’t end up under your overhanging component. Have the tab moved like this:

Figure 2

You can put this instruction in the document layer of your CAD file, or in a separate document covering fab instructions. In the CAD image below, the overhanging component has a keepout area. The document layer has instructions to keep panel tabs out of the area.