Short Cuts Don't Always Make Long Delays

The saga continues. I have my parts kit. The PCBs should be here from Sunstone tomorrow. When I placed the order on our website, I estimated that I'd have the parts and PCBs today, but I wasn't really sure. I knew there would be a chance that it would be tomorrow, but I thought it would be good to see how our communications goes when something is late. Obviously, an assembler can't start building until the parts have arrived, so the Industry standard is to start the turn-time once everything is in the shop.

If a box is late and the sender doesn't know it, unintended delays can be added into the process. Knowing this, we recently did a lot of work to improve our communications, on such issues as late parts, to help reduce delays. Sure enough, I dropped on over to the website and right on the top of the home page is a note that I have an issue (late parts) with my job. Tonight at midnight, I should receive an email telling me the same thing too.

On the subject of the PCBs, I sent Gerbers to Sunstone. That works just fine, but I'm always a bit nervous about the accuracy of my layer mapping. They double check, so I've never had problems, but I still get nervous.

If I'd waited a few days, like until today, I could have taken a short cut by just sending in my CAD board file - they just started accepting native CAD files. You can still use Gerbers, but if you use Altium, Eagle, OrCAD, National Instruments Circuit Design Suite, Ivex Winboard or PCB123, you can just send in the board file and save some time and hassle.

When I get the boards tomorrow, I'll pack everything up and deliver it to the receiving folks. Then I'll see how the rest of the build process goes from the other side of the fence, and I'll see how we deal with extra parts. I did that on purpose also. With a couple of parts, I'm delivering several hundred more than I need. With a few other, just the requisite 5% over. It will be interesting to see just how I get the extras back.

Yes. I know. I work here, so I shouldn't have any doubt about how all of this stuff works. I do know how it goes, but it's always a good thing to, every now and then, check and see how well practice matches up with theory.

Duane Benson
Grip, Fang, Wolf! Guard the mushrooms!

Centroid / XYRLS / Pick and Place

Call it what you may, but surface mount assembly robots need this magic file to determine where to place your components and how to orient them. We call it a Centroid. Others may call it something else, but it's all basically the same. In our case, the basic format is comma delimited, in mils:

Ref designator,     Layer,     LocationX,     LocationY,     Rotation
    C1 ,                       Top ,           0.5750  ,       2.1000  ,           90

That's not too difficult. Most CAD programs will automatically create this file for you. Eagle doesn't natively, but we have a ULP to do it for you in Eagle (Downloaded here). Again, no problems here. Mostly...

I say mostly because, at this point, you are at the mercy of the person who created the CAD library part. Provided they center the origin and follow the IPC for orientation, everything should come out just fine. Unfortunately, we do find parts that don't follow those rules. We'll do our best to catch and correct such things here, but for maiximum reliability, check you library components to make sure. We find the problem crops up most commonly with passives.

IPC says that zero orientation for two pin passives is horizontal, with pin one on the left. For polarized capacitors, pin one is (+). For diodes, pin one is the cathode. They note that pin one is always the polarity mark pin or cathode. Pin one is also on the left for resistors, inductors and non-polarized capacitors, but left vs right doesn't matter so much with non-polarized things. The most common orientation error we se is to have the "zero rotaion" 270 degrees off from the IPC standard.

Every now and then we'll find that someone assumes that since usually the anode on a diode tends to be on the positive side, that the anode should be pin one. Nope. Nope. Nope.

Duane Benson
Is it pulling electrons or pushing holes?

Cat Chow

I recently wrote a bit about eating my own dog food relative to a small design I've put together. Today, I'm moving along with that process and kitting it all up, so I thought I'd pass on some hints on making a good parts kit. I'm looking at this from the perspective of Screaming Circuits, but my guess is that it would fit for just about anyone assembling your prototypes. Even George's PCB Assembly and Dry Goods Emporium down past the railroad tracks.

If you're sending all of the parts, you can leave them in the original package. Just be sure to clearly mark the packages with your reference designators. If you've got to cut your strips down, you may need to Downsized_0418110808 re-package the parts, as I am. I got these little handy dandy anti static bags from Digi-Key (part number 16-1032-ND) for less than ten cents each in a pack of 100. You can use the little pink anti-static bags too.

I've labeled each bag with the reference designator, the component manufacturer and the manufacturer's part number. The more ambiguity that you can remove without adding excess clutter, the better. Making the labels was easy. I used Avery #5366 labels and mail merged from my BOM spreadsheet.

You can also put the component value on the label as well, if you can do it without clutter. Maybe line 1, reference designator; line 2, manufacturer and manufacturer part number; line 3, component value. Once you've got the bags labeled, go ahead and fill them up with the parts needed for your assembly. Add in 5% extra just in case (50% extra for 0201 passives).

If any of your parts are moisture sensitive either leave them in the original moisture barrier packaging or let us know that they need to be baked prior to assembly. That will prevent popcorn in the reflow oven.

Duane Benson
You'll like it better or my name isn't Orville Partenbacher

 

Dog Chow

I don't know how common the phrase "eating your own dog food" is. I know I've heard it before in some of those obnoxious business seminars. Not all business seminars are obnoxious. Some are quite helpful and actually, now that I think about it, I'm not really sure if I've heard the phrase in the obnoxious seminars or the useful ones. Maybe the so-so ones. Hmm.

Anyway, in case you haven't heard the phrase (it may be a regional thing), it means to use your own product, or in our case, service. I'm not an engineer, but I play one on the Internet. Still, I design and build little things. Since generally what I build is hobby related, I tend to solder them up myself leaving our capacity here at Screaming Circuits for the paying customers. But right now, I'm doing something a little different.

SC Promo 042011 top layer I've got a little design that I'm going to use to help some folks better understand how things work around here. At first, I'll just give it to some writers and editors (writers and editors, feel free to shoot me an email about it), but at some point, I hope to be able to have enough to send out to design engineers that want to get a feel for our process. It's quite a simple board; a PIC microcontroller (18F25K20 SSOP), some switches, resistors, a bunch of 0603 LEDs and some bypass caps. One schottky diode too. I'm putting together a sample kit just like the sort of kit we like to receive. The files will be on a Screaming Circuits USB drive. The PCB, fabbed over at Sunstone.com, will be in there. All the parts, purchased from Digi-Key will be in individual bags; one per BOM line item.

The idea is for someone to take the kit as though it were theirs, create an account on our website, quote the job, place the order (no payment will be needed), upload the files and send the kits in. Along the way, that person will see what we like to see in a parts kit and how the whole register, quote and order process goes. Once they receive the working board back, all they have to do is decode the secret message it displays.

Back to the dog food. From my side of the Interpipes, it's easy to say that things are easy. I sit back, drinking lattes and eating oatmeal while everyone else does the real work. But during this process, I'll get a refresher course on what it's really like to get a prototype built up.

The other day, I sent the GERBER files off to Sunstone to get the PCBs fabbed and the parts order off to Digi-Key. Tomorrow, I've got to kit everything up. Stay tuned. Details as events warrant.

Duane Benson
Is this the kind that makes gravy when you pour water on it?

Hello... Any Good Part Shipping Ideas?

Ever have one of those situations where there really aren't any good answers? There may be right answers, but not necessarily any that fit real well to the specific situation or are all that convenient. The question at hand relates to packaging and shipping small quantities of big ICs to your prototype house.

If you buy them in small quantities, go ahead and use that packaging to ship to Screaming Circuits. That's not a problem. The problem (and this question) comes in when you need to purchase a bunch of them but only need to use a few at a time. This is primarily an issue for parts that come in trays.

Let's say you have a 23 x 23mm BGA or QFP that came in a tray with 60 parts. You need to ship five of the parts for an assembly order at Screaming Circuits. We tell you not to ship them loose. You don't want to risk in-transit loss or damage to all 60 parts by shipping the full tray. What do you do?

For leadless parts like BGAs, LGAs and QFNs, you can VERY carefully pack each one in a small individual anti-static bag and then wrap them in bubble wrap. Make sure you don't damage any of the solder balls. You can't do that with leaded parts like QFPs. Don't do it. Bent leads don't solder well. And, don't put more than one BGA in an antic-static bag. Missing solder balls won't solder either.

For all types of chips, you can find someone that sells JEDEC Matrix IC Trays and see if they have any that meet your size and capacity requirements. www.Topline.tv or www.practicalcomponents.com are good places to start. That won't help much if it's Tuesday, noon and you need to make a shipping deadline of 3:00pm. But it will likely help for future projects. The other problem with this approach is that without the proper tools, it may be very difficult to get the parts out of one tray and into the other without bending a few pins.

The best option may very well be to just send in the full tray and have us send them back after we've assembled for that job. If you do choose to send in your full tray, we will treat all of the parts with care, bake if needed, properly reseal and pack them for the return trip to you.

Duane Benson
Three

Electronics Shelf Life

Do parts and PCBs have a shelf life? Well, yes and no. I have some 7400 series logic chips in DIP form 7400 TH that I bought back in 1980. Every now and then, I pull one out and put it into a proto board to test some circuit idea I've got. They still work thirty years later. I haven't taken any special care in storage either. Some are stuck into anti-static foam. Some are not. All are sitting in a mini-parts bin without any moisture protection. I guess they do get a little shielding from light, but basically, they're just hanging out. They've been, at various times, in the attic, in the basement, in the garage or in the house.

That may seem like good evidence refuting a shelf like for parts. And today's parts are even more robustly Bent pins in strip designed to start with. Still though, if I use any of those parts, it's generally in a proto board or a socket. Sometimes I have to straighten the leads a bit. A lot of things don't matter so much at low temperatures, low speeds, low volumes and large geometries.

It's different when you have fine pitch parts being picked up and placed by a robot and then run through a 10 stage reflow oven. Oxidation that doesn't matter for a socketed prototype can interfere with the solder adhesion. Bent pins or missing BGA balls can prevent the part from fitting. Moisture absorbed over time can make the chip act like a pop corn kernel when in the reflow oven.

That's not to say that you can't use old parts for a prototype these days. Just give them a good inspection before sending them off for assembly. And, if they're moisture sensitive parts or have been stored in high-humidity areas, consider having your assembly house bake them before assembly. The same goes for raw PCBs too. Overly moist PCBs can delaminate during reflow. Some PCB finishes such as immersion sliver and OSP can tarnish or degrade over time too.

Duane Benson
Archaeologists, we are not

Spam, Spam, Eggs and Spam

I normally expend most of my writing words on challenges our customers and other engineers might face in their day to day design and layout activities. But not today. Today, it's about a specific challenge faced by your typical blogger. Off and on for the last couple of weeks, I've come into work in the morning, opened up the blog and found three spammy comments. Here's today's three"

"Compare to the majority of the other blogs, your site tend to be so fantastic. Therefore nice to examining the post. If I've a probability, I would like to research along with you because I think that my potential haven't yet achieved the excellent amount."

"You may remember the three proverbs: Laugh, and the world laughs with you; Weep, and you weep lone. Life is measured by thought and action not by time. Long absent, soon forgotten."

"You may remenber [sic] the three proverbs: Laugh, and the world laughs with you; Weep, and you weep lone. Life is measured by thought and action not by time. Long absent, soon forgotten."

Now, it is a bit flattering to hear that my site tend to be so fantastic. Not just "fantastic", but "so fantastic"! But perhaps the subject matter could be a bit more on topic. The author noted that if he have probability, he would like to research along with me. I could always use some extra help, but I don't have any probability to pass on. Perhaps a call to Zaphod would be in order.

I'm not sure I agree completely with the second supposition. I'd bet that a lot of unemployed or underemployed folks are weeping right now. Probably enough that they could be considered to have a world weeping with them. I know I would. I'm also not sure what the deal is with the third one. That author just copied from the one above it. How rude.

All is not always as it seems though. After reading these this morning, I did as I always do and fed the three comments into my netlist confabulator. It turns out that the text in these three comments is actually a turbo-encoded form of the design of the Constellation spacecraft. If I had checked the IP address prior to marking the comments as spam, I wonder if I would have found that this is a desperate rocket scientist tying to smuggle his decade of work home before the lights go out and the servers get recycled.

Duane Benson
Have you got anything without spam?

Dripping Wet Is Too Much

Well, that's obvious. But what's not so obvious is that some components may look perfectly fine but act like Orville Redenbacher when in the reflow oven. Well, they won't actually act like Orlville, but rather, like his pop corn. Sort of. With popcorn, you can tell when it's popped. With a popped chip, you can't always tell right away.

Moisture sensitivity is a bigger issue with RoHS-compliant components, but can be an issue in leaded components as well. Even though the parts look like water-tight plastic, they really aren't. They do absorb 
moisture and after improper storage, moisture-sensitive chips may popcorn or crack subtly underneath. This MSD logo 75 can create hard to find or intermittent defects. It is often more of an issue with prototypes because components are quite frequently consigned in partial lots. This may result in impaired moisture sensitive packaging or storage beyond recommended shelf life.

So, the message here is that if your parts are labeled as moisture sensitive, don't open the moisture barrier packaging before sending them to Screaming Circuits. Or, if you have to open the package, please let us know. We'll bake them at the proper temperature

Duane Benson
Easy-Bake Oven: $25.99 from Hasbro
No. You can't use an Easy-Bake oven for your parts

To Lead or not to Lead. That is the question

Back at the Embedded Systems Conference in September, I had a number of folks ask me about mixing leaded and lead-free components on a PCB. It's a difficult situation for some people - especially when using old and very new BGA form-factor components.

We generally tell people to follow the BGA. Since the BGA has those little solder balls on it, it's the most sensitive to temperature as far as soldering is concerned. Reflow a leaded BGA at no-lead temperatures and the flux may all burn off and the solder may sag down too far and bridge or dry and crack. Do the reverse and reflow a no-lead BGA at leaded temps and you won't get a good intermetalic mix and the solder joint will be prone to cracking and other bad stuff.

In most cases no-lead components, other than BGAs can be used on a leaded board. Going the other way isn't always so easy though because of the additional 20 degrees C in the no-lead process. Everything's more sensitive to moisture absorption so baking parts or keeping them sealed in moisture-free packaging is more important. Some components may melt, especially chip LEDs. And metal can capacitors can pop.

In a prototype world, where you just need to see if something works, you can sometimes get away with a lot more than you can in production, but it's still not an easy question to answer. Unfortunately if you're in the situation of one of the guys that asked about it and have one leaded BGA and one no-lead BGA, you may have to get one of the BGAs re-balled or you may just need to redesign on of them out. No easy answer there.

Duane Benson
My 24 hours is almost come
When I to sulphrous and tormenting flames
Must reflow up myself

Updated Centroid Documentation

Passives orientation r2 A little housecleaning is usually a good thing. Here at Screaming Circuits, we try to be as flexible as possible and we'll do a lot of different things - standard and non-standard. But we really should, when passing on documentation, give out the standard form of data. And that's what housecleaning has done for us today.

I got a comment on an old blog post calling out an error relating to our centroid (AKA XY rotation / pick & place file), so I went back and cleaned up the blog post and linked to a PDF we have describing our centroid file requirements. It matches IPC-7351A now. And that kind of match is a good thing.

Duane Benson
Matchmaker, Matchmaker,
Plan me no plans
I'm in no rush
- actually, we're always in a rush.