Screaming Circuits: Our website and Process


What do you do if you can't have reference designators?

The first answer to that question is probably going to be along the lines of: "put them on the board."

But, sometimes you can't have reference designators on your board. Maybe it's too densely populated and there isn't room. Maybe, for aesthetic reasons, you've chosen lo leave them off. With some products, like development boards, it's sometimes necessary to use the space for instruction or functional identification and reference designators would confuse your customers.

It's always best to put reference designators as close to the part as possible, and on the same side as the part, but if that's not possible, you can still create an assembly drawing. When laying out the board, put the reference designators in a different layer than the text you want in silk screen. Then, create a PDF Assembly draowing examplethat has all of the component outlines in their place, with reference designators. Make one for the top and one for the bottom. Call this document your "assembly drawing" and include it in the files you send in to be manufactured.

The image here shows a good assembly drawing format. It's got reference designators and polarity marks.

You might ask why we need reference designators when all the surface mount parts are machine assembled. First, any thru-hole parts are hand assembled. Their locations and board side needs to be clear for the people stuffing them.

Second, CAD systems don't always have 100% accurate information. If the center point of the footprint is off, surface mount machines (ours and anyone else's) will center the part where file says to put it, which, in the case, would be the wrong spot.

The reference designators are also part of quality control. It's another opportunity to remove ambiguity. Ambiguity bad. Certainty good.

Duane Benson
Car 54, where are you?

Building boards for the Intel Edison

I've recently spent some time getting familiar with the Intel Edison. The Edison has a dual-core 500 MHZ 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 5 volt and 3 volt 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.

Stacked Edison

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.

SCmultiboard layout

After getting the files ready and placing a turn-key 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.

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In one of our MyData My500 solder paste printers

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

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The parts plate in the machine.

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With most of the components placed

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 Through the reflow oven, prior to final inspection

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The final product, top view

DSC_0002 (3)

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

Duane Benson
Happy birthday (month) Nikola Tesla

Top 5 Things to Know When Moving from Hand Assembly to Robotic Assembly

A lot of factors go into the decision to hand build or outsource circuit boards. I hand build my own sometimes, simply because I enjoy the challenge. Of course most of the projects I design are for my own use, so timeliness isn't that important. When I do design something that will go out to a customer, like my electronic business card holder, I will send the board through our shop. In those cases, quality is important, as is delivery, and the quantity is often too high TI TPS62601 front and backto hand build. Machine building also allows me to use smaller and more complex parts.

That same decision - hand build or outsource - takes place in the heads of designers all over the country. When the decision is to outsource, there are a few important things to consider. Some things that work fine when hand soldering may stand in the way of quality, repeatability, and reliability when machine assembling.

Here are five of the most important considerations when changing from hand built to outsourced at a place like Screaming Circuits

1. Use solder mask and silk screen

A good solder joint needs the right amount of solder in the right place. Solder will tend to flow down bare copper, bleeding outside of the area it belongs, and down exposed copper traces and vias.

The main purpose of solder mask is to keep the solder where it belongs. It also protects the traces, but that's a longevity issue. Solder bleeding is a manufacturing and reliability issue. This isn't a problem when hand soldering. In fact, it can even work to your advantage when hand soldering really small parts. It gives you more room for your soldering iron to hit metal.

Not so with solder paste and machine assembly. Use solder mask.

2. Avoid the pseudo panel

Keeping small boards in a panel is the recommended best practice in the manufacturing industry. We appreciate it and, while not always necessary, can reduce your costs. We sometimes see what we call a "pseudo panel." This is a board where multiples of the board are put in the same PCB, like a panel, but unlike a panel, the boards don't have routing or V-score between them. Sometimes the designer will put a bunch of vias to outline the board, or just ask that we use a band saw to separate them.

That's a time consuming, expensive, and potentially damaging process. The vibration of the saw can crack solder joints, and, you're unlikely to get boards that are all the same size. Have small boards panelized by your board house.

3. family panel (pseudo or not)

Similar to the pseudo panel is the family panel. A family panel is a case where a project is made up of several different PC boards, and they are all laid out together, as though they are one design. If the board isn't routed between to designs, you'll have the pseudo panel problem described above.

The bigger problem, though, comes with reference designators. We typically see family panels with duplicate reference designators. Each design, for example, will have its own C1, R1, Q1, etc. We use the reference designators as position identifiers/ If you have three different parts labeled R5, our machine programmers will have a problem with it. It's even worse if the values differ; on one design, C1 is a 0.1uf capacitor, while on another design, it's a 22pf cap.

If you're making a family panel, give each and every placement a different reference designator. One way would be to us extra digits. For example on one design on the family panel could have C100, C101, C102... The next would be C200, C201, C203, and so on.

And - don't forget the routing or V-score between the designs.

4. QFN - hole  in the middle

A common technique in the hand soldering world, for soldering QFNs and other parts with thermal pads underneath is to put a big via in the middle of the center pad. By doing so, you can stick a soldering iron and some solder down through the hole and get a good solder connection on the bottom pad.

This doesn't work with machine assembly. the solder paste will flow down and out the hole in the reflow oven. You'll end up with a poor connection (or no connection) to the thermal pad, and solder slop on the back side of the board.

BOM line items 0055. Parts and the Bill of Materials (BOM)

When I build my hobby projects, I often get a bit carefree with the bill of materials. It's not good practice, but I do. I'll put a part in the BOM that I used before, and not check to see if it's still in stock. I'll put parts in the BOM with just the values and not any part numbers. Things of that sort require tribal knowledge, which only the designer has.

When building, sometimes I'll just grab a part that's close. If I need an 0805 1uf, 10 volt capacitor, I can grab a 16 volt, 25 volt, etc. I can even make an 0603 part work. You as the designer may know that something close will work, but an outside house can't know. You need to tell them exactly what the part is.

Before sending anything through our shop, I do clean up the BOM. In order for us, or any manufacturer, to build the boards, the BOM needs:

  • A unique reference designator for each part placement
  • The quantity of each part used on the board
  • The manufacturer
  • The manufacturer's part number
  • Digikey part numbers can be used as well

Here's our web site page explaining the BOM format in more detail.

The transition from hand building to outsourced machine building can be an intimidating one. But, with a few considerations, it can be an easy and rewarding transition.

Duane Benson
Put the right part in
Put the wrong part out
Put the right part in
But please don't shake it all about

 

How Should You Mark Your Diodes?

Current flows through a diode from the anode to the cathode - it will pass current only when the potential on the anode is greater than the potential on the cathode. This is mostly true, but not always.

For the common barrier diode, or rectifier, it's a pretty safe bet. However, with a zener diode, or  TVS, it's not true. And, that is why marking a diode, on your PC board, with the plus sign (+) is not good practice.

Take a look at the schematic clip below.

P-Mosfet and barrier diodes

Once you put this circuit on to a PC board, you could legitimately place a plus sign on the anodes of D3 and D4, and another on their cathodes. In the next schematic clip, you could legitimately place both a plus sign, and a minus sign on the anode of D9.

Flyback diode configuration

We don't know what you had in mind, and, we don't have the schematic. If you use the practice of marking diodes with a (+) on the anode, we don't have any more information than if you didn't mark it at all. The same holds for using a minus (-) sign. It really doesn't give us any information.

Diode markingSo how should you mark your diodes? The best method is to put the diode symbol next to the footprint. on the PC board, as shown below. You can also use "K" to indicate the Cathode, of "A", to indicate the Anode. "K" is used because "C" could be mistaken for "capacitor."

D5, in the illustration on the right would be the preferred method. D7 will work as well. If you don't have enough room on the board due to spacing constraints, you can put the same information in an assembly drawing.

Ambiguity is the enemy of manufacturers everywhere. Read a bit more on the subject here, or here.

Duane Benson
Help stamp out and eliminate redundancy, and maybe ambiguity, or maybe not

Continuous Strips of Parts

Not long ago (No Need To Waste Parts), I wrote about the use of tiny cut strips here at Screaming Circuits. That's good information, but only part of the story. (When has anything ever been the whole story???).

IC cut stripThere may be some times when you don't want to buy a reel, full or partial, but still want your parts all in one continuous strip. Everyone wants to use up their odds and ends, but once those are gone, it's easier for you if each part is just in one strip. If you want to save a little money here and can use a longer turn-time, you might want to pick our Short-Run production service. If that's the case, you will need to send in continuous strips. Short-Run requires reels or continuous strip of at least 12" in length.

There are two questions you'll probably ask:

1.    How can I make sure my parts add up to at least 12 inches (305 mm)?

There are two ways to do this. First, if you have some of the parts, measure the length and calculate the number of components per inch. Use that to determine the number of parts you need to get 12 inches.. Make sure to calculate the length based on the quantity you need + 10% (+50%) with 0201 passives).

If you don't have any of the parts already on hand, you can download the datasheet for the component. In the pack, you'll usually be able to 

If the length comes out to less than 12 inches, use the parts per inch to figure out the total you need. If it comes out to 12 inches or more, you're set. Passives commonly come in 2 mm pitch tape or 4 mm pitch tape.

2.    How can I make sure that my supplier sends my parts in continuous strips?

It's not uncommon for DigiKey or Mouser to send 50 parts in 5 different strips of tape. I've even received parts in strips of one. Each part was still in the tape, cut from the reel. They were just cut into strips of one. We can use those in our Full-Proto service, but it's a hassle for you and you can't use them with Short-Run.

Neither distributor has a setting for "continuous strip", however, they do have a place to enter special instructions. Simply state in the special instructions that you need uncut, continuous tape for all parts that come in strips of tape.

Duane Benson
What is the airspeed velocity of an unladen swallow?

No need to waste parts

We love parts on reels. Who doesn't? But reels aren't always practical - and it's not just about cost. Cost is, of course, important, but there may be other factors to consider.

Say, for example, you need 20 2.2K Ohm, 5% 0805 resistors. You could buy a small strip of 25 from Digikey for $0.32. That gives the 20 you need, plus a few spares just in case.

Alternately, you could buy a digi-reel ( a custom quantity reel). On the reel, you'll probably want more parts to keep the strip long enough for the feeder. Let's go with 250 parts for $1.39. Digikey charges $7.00 extra to create a custom reel, so that's a total of $8.39. Still peanuts.

For a third choice, you could just buy a full reel of 5,000 for $10.64. Still peanuts. If you're gong to need the same part for a lot of designs, this might make sense. But, there's more than just cost to consider. You need to store and ship it. Shipping two dozen reels gets pretty expense. Storing and inventorying several dozen reels can become a hassle too.

Cut strips on plateThe beauty of Digikey, Mouser, and other places that sell cut strips is that they essentially become your parts warehouse. You pay the $0.32 cents and never have to worry about whether the part is in your inventory, how many are in your inventory, digging it out of wherever you stuffed the reel when you last needed it...

If you do buy and store the whole reel, you don't need to ship the entire reel to us. Just cut a strip with the number you need, plus about 10% for that "just in case" (50% extra for tiny 0201 parts).

Of course, if you need a few thousand of the parts, go ahead and send us the reel. It would make sense then.

Duane Benson
Reel, reel your part
Solder it, solder it, solder it, solder it
Cost is but a factor

Packing Parts for Personal Manufacturing

Manufacturing, especially small volume one-time-only builds (like a prototype) is hard. It's not wise for most people to actively seek out chaos, but that's what we do, and we do it wisely. That's what we've been doing since 2003. 

We do it because it's hard and because it's necessary.

A big part of quality manufacturing involves risk reduction. Prototyping and quick-turns inherently add in a lot of risk. While we've designed our processes and systems around turning that risk into a quality product, there are a few things that you, the customer, can do to help reduce risk even further.

One of the best things you can do to reduce risk is to prepare a well organized kit, as shown in this video.

 

You can send us your parts in short, cut strips, like you get from Digikey or Mouser, long continuous strips, full or partial reels, tubes or trays. We machine place from all of those types of packages. What's important is clear labeling and organization.

Individual, or mixed/loose components are not good, though. Pins get bent, leads get contaminated, values get mixed... Leave them in the strip, even if it's short. If you've got multiple short strips of the same part, we can still machine place. Don't tape them together. We can deal with them as is.

Duane Benson
Peter Piper Picked a Peck of Pickeled Manufacturing

What is Personal Manufacturing?

There's a lot of buzz floating around these days, about "Personal Manufacturing." Screaming Circuits has more than a decade of bringing personal manufacturing to engineers. We pretty much started the category in the electronics industry, so we're quite familiar - but not everyone knows what personal manufacturing is. I'll do my best to describe it, and what it can do for you.

The short answer, is that personal manufacturing is building your boards on your terms, not on the terms of some nameless, faceless factory.

Vertical_markets
The longer answer is probably more useful. 

Traditional manufacturing is all about statistics and fractions of a penny. Those factors are important; especially if you're manufacturing millions. But, when you just need a few boards, or a few hundred boards, those factors can make your job nearly impossible.

With personal manufacturing, you can decide when you want or need assembled boards on your workbench. You won't need to beg for time on a busy volume manufacturing line. In the case of Screaming Circuits, it's cloud-based manufacturing so you can order online from your desktop, when you're ready, rather than waiting for someone to pick up a telephone.

With personal manufacturing; you design it, get some prototypes, make a few mods, lather, rinse, repeat. Then, you'll get a few dozen, few hundred, or few thousand, and start selling. You'll get what your budget allows and don't need to commit to minimum volumes, or long-term business. You can polish your design faster, with less hassle, and you can get to market faster, with less hassle. Faster to market and less hassle both mean more time and money for you.

NPI (new product introduction) has never been easier than it is with personal manufacturing. Years ago, I was a product manager at a start-up. The entire NPI process was a nightmare. Our engineers couldn't get anything built without half a dozen support staff. Someone had to make the documentation usable. Someone had to hunt down sample quantities of parts. Someone had to make sure the board would fit on the volume manufacturers' assembly line. It went on and on like that, taking up months of the design cycle. We were at the mercy or people who only cared about making their part of the process easier.

Rather than producing the quality product we wanted, our new products would be shipped to customers with mod wires. I recall one board that needed 64 mod operations before it could be shipped. Yes, that was on a released, shipping product.

With personal manufacturing, as Screaming Circuits provides, you can get a few prototypes built right away. If need be, you can modify, and get a few more built at your convenience. When the mode wires are gone, you can build up a hundred and get them out to customers without delay. It's not about what works best for Screaming Circuits; it's about what works best for you.

Duane Benson
Right now a personal pan pizza delivered to my desktop would work for me.

 

 

Manufacturability Index in practice

My prior blog covered the Screaming Circuits Manufacturability Index. It's something I'll be using from time to time when discussing new components I run across. I've got a few examples to put the numbers into context.

On the low side of the index, we have:

7400 TH1: Just about anyone could hand solder the part
Examples: Thru-hole parts

The SN7400 quad NAND Gate, shown on the right, is a good example. It's big, it's thru-hole, and if someone has trouble hand soldering it, they really need a few more classes.

Closer to the other end, is a new chip I've run across. The Silego GPAK4 is a small FPGA-like mixed signal device. It's got a number of analog peripherals, a bank of programmable logic, and the ability to configure it up the way you want. Take a look at it below:

GreenPAK4 cropped

This little thing is housed in a 2 mm X 3 mm QFN package. That's pretty tiny by the standards of my giant fumble-fingers. I've given it a rating of 4.b, on the Screaming Circuits manufacturability index. The number ranking "4" means: "Needs advanced automated assembly technique", and the letter suffix "b" means: "Typical level of challenge within the number rank." In other words, right up our alley.

Unless you posses super-human abilities, and maybe lasers in your eyes, you won't be hand soldering these. You'll have them assembled by us (or someone with the same technical capabilities as us), where it will be a standard process.

If you do want to put one or more of these in your design, you will want to make (or find) a custom library footprint for your CAD software. Due to the variable length pads, a standard one-size-pad footprint might lead to solder joint reliability issues.

Duane Benson
The chips go marching one by one, hurrah, hurrah
The chips go marching one by one,
The little one stops to suck her thumb
Just to see if the solder is lead-free

 

Screaming Circuits Component Manufacturability Index

Screaming Circuits Manufacturability Index

Ranks the difficulty of assembling a component. Index is one to five, with one being easiest, and five being the most complex

Sub index: a, b, c

    a: Not a challenge within the number rank
    b: Typical level of challenge within the number rank 
    c: Fits in the ranking, but likely needs special process, fixtures or attention

1: Just about anyone could hand solder the part
Examples: Thru-hole parts

2: Surface mount. Should be machine placed, but big enough to hand solder
Examples: 0805 or larger surface mount passives, SOIC packages

3: Pretty much any grade of surface mount equipment can handle this component
Examples: TSSOP or larger, 0.8mm pitch BGAs

4: Needs advanced automated assembly techniques
Examples: 0.4mm pitch BGAs or QFNs, CSP (chip scale package) or WSP (wafer scale package) BGAs, 0201 size passives, Package on Package (POP)

5: More or less R&D at this point. Few companies have or will assemble this part
Examples: 0.3mm pitch micro BGA, 1,700+ ball BGAs, 01005 passives

Just about everything 4b, and below are routinely within Screaming Circuits standard (guaranteed) process. 4c, 5a, 5b, 5c, are becoming more common here. These are special process (falling outside of our guarantee), but we can usually do a good job with them. You'll need to speak with a manufacturing engineer before placing the order.

Duane Benson
a colossal negative space wedgie of great power coming right at us at warp speed
Readings are off the scale, captain