10th Anniversary Top 10 Traps

A few folks requested my presentation from out 10th anniversary open house.So, without much adieu, here it is.

Download Top 10 traps 7-2013 (PowerPoint format)

Download Top 10 traps 7-2013 (PDF format)

Duane Benson
10 times 10 isn't necessarily equal to 10 times 10.
Especially if you mix bases.

Fiddling with Fiducials again

I recently posted a note about fiducials but I didn't have any images. Here's a couple of examples:

IPC acceptable fiducialsThis first example shows what IPC would like to see. If this is an individual board, this would be it. If it were part of a panel, you would follow the same pattern on the panel rails and also put it on each individual board in the panel.

As I wrote in the earlier post, we don't require these, but it's always a good idea. You'll need them once you go into volume manufacturing anyway.

The next example won't make IPC happy, but it will make Screaming Circuits happy:

Also acceptable fiducialsIt only uses two fiducial dots, but it isn't reversible. Reversibility is okay for jackets, but not for circuit boards. Since one of the dots is offset, it can't be placed on the machine and recognized as correct in any way except in the proper orientation.

The important aspect of both of these examples is that they remove ambiguity. There can't be any uncertainty, which is good because uncertainty is your enemy. It's a subtle enemy. It might not bother you 99.9 times out of a hundred, but then, when you're not looking, it can strike. So, give a hoot and stomp out ambiguity.

Duane Benson
False data can act only as a distraction. Therefore, I shall refuse to perceive.

Is it your fiduciary duty to use fiducials?

We don't require fiducials here. It's not mandatory. That's because we live in a prototype world and in that world, theory doesn't always match up with reality. That being said, there are things we can do and things we would prefer to do. Personally, I would prefer to get some ice cream, but my belt suggests otherwise.

So we do like fudicials, which begs the question: "how do we like them?" Well, the objective of a fiducial is to make machine registration of your PCB easy. That means the pattern should not be reversible. It should only have one correct orientation. Use three of them separated as far apart from each other on the board as you can. But don't put them any closer than 4.75mm from the edge. Ideally, it would be a 1mm copper area centered in a 3mm circle with no solder mask. Oh, and all of the fiducials on the board should look the same and be the same size.

Duane Benson
Over easy, yolk not broken.

Is your BOM a Smart BOM?

Please be aware that there is no "B" at the end of these BOMs. Still, without the "B", you can have a smart BOM or a dumb BOM. It is important to note, however, that a dumb BOM may not be a bad thing. It just depends on what you want to do with it.

No matter what you're doing, there are a few necessities. Let's start at the very minimum, for someone designing something to be self-built from mostly already owned parts; maybe just a few from a dealer.

Reference designator: R1, R2, R3, C1, C2, U1... You have to have this information.

Quantity is important so know know how many to pull. Although, with a small garage-built project, you can probably just as easily count how many you need for a given value. And what about the value? Actually, the value isn't always all that necessary if you have the correct part number information. A line item number is hand for big bills to keep things straight.

This is actually too basic and kind of pointless, so I'm going to jump ahead. Take an assembly house like Screaming Circuits. Screaming Circuits will either build your boards from your kit of parts or purchase them from your BOM (or a combination thereof).

Once you have the item number, quantity and reference designator, you need to tell your assembler or purchaser what it is. If you already have the parts kit, just add in the manufacturer's part number and a description / value. That should do it. Some assemblers, like Screaming Circuits, will take part numbers from a distributor in place of or in addition to the manufacturer's part number(e.g. Element14, Digikey...)

If the assembly house is going to buy the parts, then add in the manufacturer and double check that the part numbers are accurate with all suffixes and things of that sort. The distributor part number can be added, but when the assembler is going to build the boards, you really should include the manufacturer and manufacturer's part number to cover all basis.

BOM sample

That's cool, but your circumstances might require just a little more. You might need to list an approved substitute or two for parts that come in and out of stock frequently. You could also list multiple distributor part numbers for the same specific component, again, in case of lead-time or stock issues.

Sometimes I get myself into a bit of trouble by not specifying some part values at design time. I might just throw in things like bypass caps, RS232 driver charge pump caps or LED current limiting resistors assuming that I just know what the value is. It's not a big issue, but it would probably be less work to just do it at the start.

Duane Benson
What the Bureau of Meteorology has to do with your parts kit, I'm not sure.

 

Thru-Hole Parts

Screaming Circuits uses machines to place surface mount parts; even if it's just one board. Thru-hole are a different story though. Way back in the cobweby section of the building, we do have a thru-hole part sequencing and insertion machine. Our volume manufacturing division still uses it on occasion, but it's just not efficient for small quantities, which is why thru-hole parts get hand inserted at Screaming Circuits. We have three options for soldering the parts into your prototype. We can hand solder all of the parts, we can send the board through our selective solder machine or we can send it through the wave solder machine. We'll pick whichever route makes the most sense based on quantity and configuration.

It's good that we can solder the thru-hole parts, but how, you might wonder, do we know where to put the thru-hole parts? The SMT has the centroid file to tell our machines where to put them. Thru-hole Thru-hole locationsbeing more of a manual process, we rely on visual data. If your silk screen markings are readable, we can used that as a reference. If the parts will only fit one way into one footprint on the board, then  it's not much of a challenge. Regardless, make sure that the polarity is clear for any polarized components.

Sometimes, though, there isn't enough room on the PCB for clear silk screen and parts will fit in a number of different places. That's where the assembly drawing comes in. This illustrates an example of a suitable assembly drawing. It's got your web order number in the image and all of the parts are clearly pictured and their locations clearly identified. If any of the parts are polarized, make sure you include that information as well. Send the assembly drawing as a .JPG or PDF file format in your ZIP file with the BOM, Gerbers and Centroid.

Duane Benson
It just goes to the back side of the board. It's not a wormhole going to another galaxy.
Or is it?

How to Build a footprint

Well, not really how to build one in a technical sense, but some thoughts on how to better ensure that you get it right. In theory, it shouldn't be that difficult. You download the datasheet and build the land pattern based on the information in the datasheet. That usually works, but not always.

I had a thru-hole battery holder that didn't match up with any of the land patterns in my library, so I modified one that was close. That worked mostly okay, but there was one measurement in the data sheet that was a little ambiguous. I ended up with the mounting holes being off by a millimeter or so. Not too much, but enough to make the fit difficult.

I went in and shifted the leads over by the same amount, used it again, got another PCB fabbed and discovered that I had shifted the pins the wrong way! Then it hit me. In the first application, I had the battery holder on the bottom side of the PCB but I had looked at it through the mounting holes from the top side of the PCB. D'oh! One reason why I'm not a professional designer.

The other part was a little tiny SMT trim pot. Since there are pretty close to a million different little trim pots, the likelihood of me finding an exact match in my CAD library was precisely zero. I didn't want to Gieger VR mistake close re-invent the little zig zag resistor symbol, so I just found a part that looked the same. Well, it was almost the same. The footprint I found is for a 4mm x 4mm part and the part I ordered is 3mm x 3mm. That's a tiny trim pot. Somehow, when looking at the datasheet, I got the measurements wrong. Once the part came in the mail, it was quite obviously too small.

The pad pretty much ends right at the edge of the trim pot. We won't be able to reflow that part. No solder paste would be touching the pads on the trim pot. I'll see if our guys on the floor can figure out how to get the thing soldered on there. If they can't, I'll need to look for a larger part to put in it's place.

Fortunately, I physically looked at the part and the PCB before assembly. Unfortunately, I got the measurements wrong. If at all possible, get some sample parts before you order your PCBs. Then you can print out a 1:1 image of your PCB and lay the parts out on it. That would have saved me in both of the above cases.

Duane Benson
Is it "datasheets" or "data sheets"?

Loopy Ground Loops

A while back, I posed a question about using flood fill (AKA copper pours). I've been reading a lot about ground loops lately which brought me back to that original question.

LED scroll ground plane Some people suggest segmenting your ground plane between analog and digital sections. Some people suggest segmenting the ground plane for individual critical ground return paths. The follow on to my original question is: On non-exotic designs does segmenting ground planes really help? There's actually two questions, with the second being: At what clock speed does it make sense to start worrying about issues caused by ground return paths / ground loops? There are probably more questions. Those are just the two rattling around in my head at the moment.

Interestingly, though, when I wrote the original post, there didn't seem to be a clear "most common" between pour and no pour PCBs. Today, I'd have to say that the majority of designs we see here at Screaming Circuits do use flood-fill ground planes, either internal or external.

Duane Benson
You can solve ground and noise problems by just not hooking up power

Rain, Rain, Go Away

It's almost June her in the Pacific Northwest. At least, that's what the calendar says. I'm not sure I beleive it at the moment. The weather is acting more like October. It's a bit warmer than January, but every bit as wet. That pretty much equals October. We'll just call in Junetober.

And what does Junetober have to do with electronic assembly?

MSD logo Moisture. That's what it has to do with electronics assembly. Most of the parts running around int he world today have some level of moisture sensitivity. Despite my lament of the rain here, you have to consider component moisture no matter what your climate may be.

Looking at IPC-M-109, you can see the there are sensitivy levels MSL-1 though MSL-6. There are acutally eight levels: 2A and 5A make up the extra two. If you've got an MSL-1 part, you really don't have to worry about. I wouldn't store it in your fish bowl, but the standard says you don't have to bake it. Up at MSL-6, you have to bake the parts before use no matter what.

When you buy your moisture sensitive components, the should come in a moisture barrier antistatic bag with an indicator card and a little baggy of moisture absorbing dessicant. The best approach with these compontnts is to leave them in the original un-opened bag. We'll use what we need and properly seal up the rest just the way IPC-M-109 wants us to.

If you do need to open the bag and ship parts to us without the moisture protection, we may need to bake them for a while to make sure they are properly dried out before putting them in the reflow oven.

Duane Benson
Gore-Tex is a registered trademark of W. L. Gore & Associates.

An Unanswered Question

I've been reading through my Virtual-PCB chat session transcript from yesterday. 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. Lot's 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.

Drip. Drip. Drip. Drip. Drip...

MSD logo Okay, water doesn't have to actually be dripping down on to your parts to be a problem. I don't know if you've heard or not, but water also comes in vapor form. Weird. I wonder if there's a way to harness water in that vapor stage and do something useful with it. Hmmm. Ponder material for another day.

Here is something not so useful about vaporous water: It can get into your parts and make them unhappy. I was recently asked about opening and resealing moisture barrier packages for moisture sensitive parts. This old post has a link to the JDEC standards document (J-STD-033B.1) covering this subject. It's can be a complicated subject and the document is worth the read.

In sort, the best thing to do is just leave the parts sealed in their original moisture barrier packaging. If that's not practical, look at the MSD classification. There are eight levels with one being least sensitive and six being most. (Levels are 1, 2, 2A, 3, 4, 5, 5A and 6). Most parts seem to be level 3, which can be open and exposed for a cumulative time of 168 hours. The actual safe time may vary based on your local humidity.

If you want to open and reseal, you'll need the humidity indicator card that came with the parts, desiccant and a thermal sealing gizmo. Open the package, take out the parts you need, put the remainders, desiccant and card back in the package and reseal it. Sealing it with tape won't do the trick. Then you would count the time that the components were in the open air toward the cumulative open time.

If there's any doubt, just let the assembly house know that the parts need to be baked. It will probably add some time to your job, but it's better to add a bit of time than have bad parts.

Duane Benson
Add walnuts and chocolate chips.
Two