More Beagle CAD Paws

Continuing on from my last post

As I said, I do everything I can to avoid reusing the package footprint when adding the the parts library in Eagle CAD. The schematic symbol can be a different story though. It still takes a lot of caution, but it’s less risky (in my opinion) than reusing the package footprint.

Eagle v.  6 made some improvements in the way copy and paste works. It’s still a little different from your typical word processor, but it’s not that difficult.

Eagle footprint menu bar 3 buttonsBut before I get to that, I want to mention one item that caused me a fair amount of confusion early on. And that’s the way all of this fits together. There are three buttons you will need to worry about. From left to right in the green oval are; the device, the package footprint, and the schematic symbol. In my last post, I pointed out the package footprint and today I’m talking about the schematic symbol.

Really, you only build the footprint and the schematic symbol. Then you connect the two up to create the devices. And, you can build the footprint or schematic symbol in either order, but you have to have them both before the last step (the icon in the green oval with four little AND gates).

If you’re using a chip that comes in a couple of different packages (e.g., DIP28, SOIC28, TSSOP28) you most likely only need to make one schematic symbol. You can make the multiple footprints and connect them up in the device section as different variants of the same part.

There are a few exceptions though. Sometimes QFN, QFP or BGA parts will have a few extra pins. In those cases, it’s generally better to create a different schematic symbol.

Duane Benson
This solder paste stencil glows blue when goblins are around

Beagle CAD Paw Prints

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


An Unanswered Question

I’ve been reading through my Virtual PCB chat session transcript from Tuesday. It was a fun session and I have a much better idea of how the virtual shows work now. I think I may just be getting it.

The chat session had a lot of interesting questions and dialog. I did notice, however, that I missed one question and thus didn’t answer it. Oops.

Owen asked if I am of the opinion that all footprints should have rounded pads (probably stencil cutouts, too) to help with paste release. Sorry I missed your question.

I’m not of that opinion. There are a lot of factors that come out of stencil decisions. Paste release is one of them. There are others, some more important. For example, the shape of a pad and stencil cut out can either encourage or discourage solder balls. The size of the opening can put too much or too little paste on the pad. Wide open cut-outs over heat slugs can cause float.Bad QFN paste w caption

The pads themselves, should follow the part manufacturers recommendation for shape and size. Most are rectangular. BGAs have round pads. Unless you have a very good and very specific reason, I would not deviate far from the part manufacturer’s recommended footprint.

Some of the factors that influence paste release are the stencil thickness, whether it’s polished or not, the angle of the cut, ratio of thickness to width and paste properties. How long the paste has been exposed to air as well as the room’s temperature and humidity can also have an impact. Lots of permutations.

If you’re reading this Owen, sorry I missed your question in the chat. I hope this answers it for you.

Duane Benson
If it’s going to the EU, make sure it’s peanut butter-free.

Rework Foibles

Yesterday, I wrote about my foibles in ignoring my own advice. As SiliconFarmer pointed outRework 002 cropped over on Twitter, it’s not just something you need to do when you’re re-purposing a close land pattern. Sometimes even the “correct” pattern can have the wrong drill size or a few mixed up pins.

The bottom line is that if you want to reduce the chance of scrapping some expensive PCBs, or having spots that look like what I did (above), check your land patterns.

I couldn’t find my wire-wrap wire late last night, so instead, I used the leads from old though-hole resistors. It’s kind of a mess, but I do like the hatch-markish look that I gave it.

Not to shift any blame frommyself, but I do find it quite annoying when a part falls into such a common standard configuration, as in three-terminal regulator, but the manufacturer picks a different pin-out.

(Note that I did this rework myself at home. The folks here at Screaming Circuits do  much, much higher quality work.)

Duane Benson
The problem with unwritten rules is that they’re unwritten

Lesson Learned … or Not

MC39100 pin out I’ve written quite a number of times about the perils of CAD software land patterns. Especially if you don’t have an exact match and need to adapt something close.

Recently, I was looking in my Eagle library for a low-drop out regulator, MC39100 is SOT223. It’s just a standard, run of the mill 7805 replacement. Nothing special. A million other parts share the same pin-out. Shouldn’t be a problem. Shouldn’t …

If I were to follow my own advice, it wouldn’t have been a problem. But did I follow my own advice? Well, not this part of it. I took for granted that all three terminal regulators follow the 78XX pin-out. Most do, but the LD1117A (below) does not. This isn’t the first time I’ve used a non-standard regulator, so I really don’t have an excuse.LD1117A pin out

Naturally, I assumed that the pin-out matched what I needed and I didn’t hunt down an LD1117A data sheet to verify their pin-out. Well, at least I didn’t do so until trying to get my new PCB to power up. Very sad.

So, is there a moral to this story? Probably. Most likely it would be twofold. One, if you’re repurposing a land pattern from a part that’s close, but not exact, double-check your work. Get both datasheets out and compare the pin-out.

The second part of the moral is, if you give advice … follow it yourself. Duh.

Duane Benson
Help! I’m blinded by the obvious.

On Final For Landing on Runway 0805

S part pad shift in oven process Keep out areas can be a problem when adapting a CAD component land pattern, but that’s not the only potential problem. Sometimes the part may be close, but the footprint is different enough to cause problems, as in the picture on the right.

You can also run into issues that don’t necessarily cause PCB assembly problems, but can be expensive nonetheless.

Say you are designing with a small microcontroller and the schematic symbol and land pattern don’t exist for the one you’re using, but something close does. Even though the two parts may look like pin for pin replacements, they may have a few differences.

The PIC family has a number of examples of this. For example, the PIC18F2321 and the PIC18F2455 have enough similarities that they look like pin for pin replacements. However, upon closer inspection, you’ll find that RC3 exists on the 18F2321, but doesn’t on the 18F2455. SCK/SCL and SDI/SDA are in differnt places on the two processors. You could end up with a bunch of jumpers and a PCB re-spin if you just used one land pattern for the other. It pays to check for those little details.

Duane Benson
Turn left at the big tree, and go until you see the creek.

Monsters Under the Bed

TO-263 land with mistakes Monsters, metaphorically speaking, that is. Take a look at this little land pattern for a TO-263 part. Can you tell me the two main things wrong with this land pattern?

I’ll give you a hint. One of the problems is an absolute no-no. The other one could be justified with a low-current application. But then, wouldn’t you use a smaller package?
Duane Benson
Green Grow the Traces Ho

0.4mm Pitch BGA Land Patterns

We’ve been getting more questions about laying out the 0.4mm pitch Ti OMAP BGA, as is used in the Beagleboard. As I’ve written before, some of the rules change at these tiny geometries. The Beagleboard folks discovered that non soldermask defined pads (NSMD) can lead to bridging and poor yields and therefor they recommend soldermask defined (SMD) pads. Check out page 10 in their design guide. If in doubt, or if you’re concerned that your set up might be different enough to warrant NSMD pads, I’d suggest you give a Ti Applications engineer a call.

And speaking of the Beagleboard, they just recently reduced the price on their pre-built Beagleboards. Like $125 for the original and $149 for the new xM version. Very nice.

If you’ve got a 0.4mm BGA part from a different manufacturer, check with that particular part manufacturer for the final say. Some 0.4mm pitch parts have a staggered arrangement and in that case, there is enough room center to center to successfully use NSMD pads.

Duane Benson
Joe Cool here.

Land Patterns – Equal and Not Equal

I was recently asked a question about QFN package varieties. The questioner wanted to know if different package variants of 16 contact QFN packages, such as HUQFN, DHVQFN, SQFN and such, all shared the same footprint.

If they did, the CAD work would be much easier. There would be one land pattern to worry about and that would be that. Unfortunately, that is not that and in this case, that, in fact, that may never be that.

Many different varieties of QFN packages could use the same land pattern, but they don’t always do. Some will have the same pitch, but more distance between the outside contacts and the corner, thus a greater overall dimension. That can happen even with the same labeled variety of QFN package. Some will have different dimensions, differnt pitch, different pad sizes or different thermal pad sizes. Sorry. No easy answer here.

I popped on over to the NXP website, one of our Circuit Design ECOsystem partners, for some examples. NXP lists two 60 contact HUQFN part packages. One is 5 x 5 mm. The other is 6mm x 4mm. Same with the HVQFN. There is a 0.65 mm pitch, 4 x 4mm package and two 0.5 mm pitch, 3 x 3mm parts with a different overall package outline.

In general, generalzations aren’t going to work here. You’re going to have to go dig out that datasheet and quite possibly create a new land pattern.

Duane Benson
One pattern to rule them all and in the solder bind them

Extra-Fine Pitch BGA Pads

One of the annoyances of the world of trade secrets and proprietariness is that we can’t all learn from each others’ experiences. That is important, and even generally necessary, in a competitive world. If you put in some hard work, you should get the first right to profit from it. Otherwise what incentive would you have to put in that hard work?

There are times, however, when it would be helpful for the industry or the economy in general if we can all learn from someone else’s challenges. Times when, for example, the entire auto industry and therefor the safety of the general public would benefit if all companies shared what they have learned about the reliability of electronic throttle systems.

Bb Good SMD pads Here’s another chance for open source hardware to shine. Take the Beagleboard. The TI folks who designed it pushed technology in a number of areas and by presenting what they have created as open source, we can all benefit from it. Even stepping outside of the great work in the schematic, they have done great service in the areas of manufacturing complex devices as well.

A while back, I wrote about soldermask defined (SMD pads) vs non-soldermask defined (NSMD) pads on 0.4 mm pitch BGAs. The basic idea is that while with most Bb Bad NSMD pads BGAs, you want NSMD pads for better mechanical strength, with the really small BGAs, like the 0.4 mm pitch OMAP processor, you want SMD pads to prevent shorts.

The messages that the Beagleboard team learned here are, first, it’s true that you want SMD pads and second, make sure that your PCB fab house follows your instructions in that regard.

Many fab houses have their own rules and will set the soldermask up based on what they feel is best. They may have never used your part though. Make sure the board house does what you need. By insisting on closing up the soldermask, the Beagleboard team went from 90% failure with the NSMD pads to 96% good and no BGA shorts with the SMD pads. (This info and the photos come from the Beagleboard ESC presentation by Gerald Coley.)

Duane Benson
The worms do.