Coming Soon!

If  you happened by our booth at DesignEast, you may have gotten a personal preview of our new automated parts quoting system. If you didn't get to see it, you will shortly. It's in the final stages of beta.

Order cost - new

 

This sample shows what you might see when you order Screaming Circuits assembly along with Sunstone PCBs, and components from our website.

In the meantime, you can still quote your assembly and PCB prices online here, and you can have us quote your parts offline.

 

 

 

 

Duane Benson
Maybe not today. Maybe not tomorrow, but soon and for the rest of your life.
You won't regret it.

Electrolytic Ambiguity

I've written about ambiguity a few times before, like this post about fiducials. But I'm not talking about the PC board today. I'm talking parts. More specifically, I'm talking about silk screen markings for your parts on the PCB.

CapacitorsDiodes have a lot of opportunity for ambiguity, as you can read here. There are many ways to mark parts, but fewer ways to clearly mark them. Take a typical electrolytic capacitor. It can be thru-hole, smt metal can, tantalum, or a few other form factors. The capacitor manufacturers aren't doing any of us any favors where markation is concerned.

Check out this image. Yikes! In all cases shown here, I've oriented positive on the left, which, according to IPC is pin 1. This is also the zero degree rotation for the centroid value. But, isn't it nice of those component manufacturers to put the identification bar on the positive side for tantalum capacitors and on the negative side for metal can electrolytics? Not!

So, how should you mark this in the silk screen on your PCB? For an electrolytic capacitor, the best approach is to mark the positive sided with a (+), plus sign. If you mark pin 1, with the number 1, it can easily be mistaken for the minus sign. If you mark the negative side with a minus sign, it can easily be mistaken for pin 1.

For a metal can capacitor, it is also acceptable to put the notched outline in silk screen. We still recommend that you place the (+) plus sign on there too.

Duane Benson
I'm just positive I put the negative right on the left

Counting once, counting twice...

Panel single scLet's say you have two options: First, you could send in your PC boards for assembly as individuals. Second, you could send them in a panel. That's all fine and dandy. For a few, send individuals. For a bunch, panels might make more sense. But, when you do go to quote and order, how do you count the parts?

Let's take this example. As a single, this board has 32 line items on it's bill of materials. That's 32 unique parts. Counting all of the individual part placements, there are 56 total parts: 42 smt and 14 thru-hole. So, naturally, if you quoted the assembly of 20 of this board at Screaming Circuits, you would enter your desired board quantity as 20, Panel 4-up sc32 total unique parts, 42 smt and 14 thru-hole.

But what do you do if you send it in panel form? How do you count? It's actually not as difficult as it seems. In this example, it's in a panel of four. There are still only 32 BOM line items, but there are four times as many placements. That means that if you quoted this, as a panel, you would enter 32 total unique parts, 168 smt and 56 thru-hole parts. If you still need 20 of the final boards assembled, you would enter 5 as your desired board quantity.

In the end, you will have 20 assembled boards. In case you are wondering about the cost, there won't be a difference. As long as the final number of boards (after the panel is broken apart) are the same, your cost will be exactly the same for panel vs. one up. You don't save any money by sending in singles. However, if your board is panelized and all of your parts on on reels, full or partial, you can save money by ordering Short-Run production.

Duane Benson
50 Years ago today
Robert Rushworth flew the X-15 to Mach 5.03 at 100,400 feet altitude

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.

 

Zzzzzzzap!!! Static be Bad

Engineers these days have so many issues to worry about just in component handling alone:

  • Do my parts need baking to get the moisture out before reflow soldering?
  • Are my parts in stock?
  • Are my parts real or are they counterfeit or secretly remanufacturerd?
  • Are my parts really lead free?
  • Are my passive components small enough to make it out of the holes in my salt shaker so I can put them on the PCB?
  • Are my parts too small form my manufacturer to handle?
  • Are my parts too complex for my manufacturer to assemble?
  • Have my parts been zapped by static electricity either before or after assembly?

Static electricity is really something that no engineer should have to worry about these days. We know how it gets created. We know how to artificially create it and we know how to guard against it. There's really no excuse - especially from those that an engineer entrusts to build his or her designs.

Tesla_colorado_adjusted 500

People can carry around a static charge anywhere from several thousand volts to more than ten thousand volts; just by walking around. Joe Volta would be proud. Touching an electronic component or assembly the wrong way at the wrong time can discharge much of that through the electronics. Yes, most chips are better able to handle static electricity than the old 4000 series CMOS that could get zapped just by being looked at harshly, but pretty much any active component is susceptible to static damage to some degree. What makes it so Anti stat shoesinsidious is that the damage may be done in handling or in assembly but might not show up until the unit fails in the field.

The whole world knows how to keep electronics safe (that's an exaggeration, but at least most people in the Industry know how), and the whole industry understands the risks, so why would anti-static handling or packaging be an extra cost option? If it's you're own stuff, then fine. It's up to you. But someone you're paying? I don't get it.

Take a close look at the picture on the right. If you ever get a tour through Screaming Circuits, you'll see a lot of this. The floor is conductive. The bright green straps on the shoes are not a fashion statement. They're grounding straps. The blue jacket is conductive. Parts and PCBs are protected from static through these means and others all the way in and all the way back out to the customer. It's the right thing to do and the healthy way to do it and it doesn't cost extra. It shouldn't cost extra. Follow good static mitigation procedures yourself and make sure that whomever is assembling your parts does the same. That's my two cents worth.

Duane Benson
Frankenstein was grounded through his neck bolts, so he's okay.

 

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?

Major Major and Standard Standard

We ask for your bill of materials, Gerber and centroid files to assemble your pcbs. All of those pieces of information are necessary to properly program our machines to place your parts. That's pretty standard stuff, but did you know that when the Gerber format reference book was first published, Jimmy Carter was President of the United States, Russia was the "Soviet Union" and Voyager 1 was well inside the Solar System? Use of the format has been going on even longer. Yeah. It's been around a while. For some reason, it has been very difficult to get everyone to agree to and use a standard file format. Gerbers really don't have enough information in them to do the job properly, but it is the standard. Hopefully not for too much longer. How many of you reading this were even born when Gerber was new?

XKCD 15th standard
(Drawing courtesy of XKCD) <- worth spending time on

There are a number of formats around that are better than gerber and Screaming Circuits will accept many of them. First, your CAD software probably will export an "ASCII CAD file". This is a good format. Some export ODB++, which is one of the newer formats, again a good choice. One of the newest standards is the IPC-2581. It's been around a few years and is now getting a lot of attention. If you happen to use Eagle CAD, you can also send us the Eagle ".brd" file.

IPC-2581 includes the best of ODB++ and GenCAM. It has all of the fab data, assembly data, netlist and BOM. Everything needed in one convenient file. My understanding of the format is that you can exclude portions of the data set that you consider proprietary. You can learn more about the format here. There's more background information on the subject over at Circuits Assembly magazine too.

Duane Benson
Where's Henry? I need an inductor.

Day two. Custom parts

Moving on from where I left off a few days ago... I was planning on using the PIC18F2320, but in poking around, I found that the PIC18F2321 is about $3.00 less expensive in small quantities. I'm not entirely sure why. Their virtually identical. The 2320 does have two 8-bit timers instead of one in the 2321, but I haven't spotted any other differences that would matter to me in this case. The 2321 has lower sleep and idle currents but I don't think that matters in this application either.

PCB123 PIC partial sch PCB123 doesn't have the 2321 in its library. I could just use the 2320 part, but to get full use out of the pricing and availability features, I'll have to customize the part so that the BOM tool can find it at DigiKey.

I had the "place component" box up already, so I just clicked on "manage Parts" and started filling in the information in the middle column of the dialog. The I clicked the "Select Simple" button, searched on "2320" and selected the symbol for the PIC18F2320-I/SO. So far, so good.

PCB123 manage parts dialog Now, the question is: do I select "Apply Changes" or "Create a New Part"? This would be easier if I actually looked at the documentation or something, but am I doing that? Of course not. I'm going with "Create a New Part." Oops. Needed to select or generate the footprint first. Do that and search on "SOIC" and pick out an SOIC28, "Create New Part" and save it in a Library. I picked "Microchip."

Done. Now when I go back to the Insert / Add Part function, I search on PIC18F2321, and there it is. Apparently, I did it right, because the BOM tab will find it and show price and availability at DigiKey.

Duane Benson
And, today, it's not just a rain cloud, but a full one

 

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.

Done and Done.

LED scroller 005 trimmed And... Drum roll, please... It works. I put in a couple of batteries, programmed the MCU and turned it on. It works. I'm always surprised when something I design works on the first try. This being such a simple design, I probably shouldn't be surprised though. I should at least give myself a little more credit.

The unpopulated lands on the board in the photo are supposed to be unpopulated. I left a few things out because they aren't needed for what I'm doing with this piece now and leaving them off keeps the cost down.

So, what did I learn from the process?

  • If you have a lot of different parts laying around, it's pretty easy to grab the wrong one
  • I ran into some variability in the "zero rotaion" position in the CAD library land patterns
  • The whole process is pretty easy, but start to finish, there are quiet a few steps
  • It's a nerve wracking wait after sending a box of parts off
  • Good communications between designer and assembler are very important
  • Clear documentation from the designer is very important
  • This was a WHOLE LOT easier than hand soldering all the SMT parts (I've done that before)

That's a good set of educational results. Next time, I think it will be easier.

Note the large diode polarity indicators on either side of the long row of LEDs and by LED D25. D1, the Schottky on the upper right has the same polarity indicator, but it's in between the pads, under the part. In case you're interested, I have a 3 volt supply. The LEDs drop 1.8 volts and I've got a 150 ohm resistor for each. That gives me a theoretical 8 mA per port for a total maximum of 176 mA with all 22 lit up. That's within spec in the -40C to 85C temperature range but too much when above 85C. I'm not sticking this in an engine compartment or anything, so no worries there.

Duane Benson
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