About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.

What is a Centroid File?

Call it what you may, but surface mount assembly robots need a magic file to determine where to place your components and how to orient them. We call it a centroid. What is a centroid file and why is it important to your PCB assembler?

Many assemblers use automated equipment to place surface mount components on PCBs. One of the tools we use to rapidly program these machines is the centroid file (aka insertion, pick-and-place or XY file).

Some CAD packages automatically generate this file, some will not. Sometimes you may simply need to modify the file, and some assemblers can make minor changes to the file or create it for you for a small fee.

Ultimately, the centroid file describes the position and orientation of all surface mount components on the PCB. A centroid file includes: the reference designator, X and Y position, rotation and the side of board (top or bottom). Only SMT parts should be listed in the centroid file the basic format for the centroid file is a comma delimited (.csv) file with data in the following order: RefDes, Layer, LocationX, LocationY, Rotation.

Here’s a breakdown of the data:

RefDes
The reference designator that matches your BOM and PCB markation.

Layer
Either the word “top” or “bottom.” This is not necessarily the CAD layer designator. Just “top” for a part located on the top of the board and “bottom” for parts on the bottom side of the board. Top is often referred to as the component side and bottom the solder side by assemblers and fabricators.

Location
The “LocationX” and “LocationY” values describe the part’s offset from the board origin. The location values require that the part origin be centered in the part. The board XY origin of 0,0 is in the lower left corner of the board. The 0,0 origin for the bottom of the board is in the lower left corner, looking at the top of the board, though the board. Preferred units are in inches (0.0000″).

Rotation
Rotation goes counterclockwise for all parts on top and clockwise for parts on the bottom. In both cases, this is from the perspective of looking at the top of the board. For bottom side parts, it is looking through the board, still from the perspective of looking at the top of the board.

LED & Diode Markation Guidelines for PCBs

Have you ever had an LED or other diode placed backwards? PCB assemblers work hard to place every component from the largest, highest pin-count logic chip down to the smallest passive components and micro wafer-scale BGAs correctly every single time. A key element of that accuracy is our understanding of your board and the component markings.

If you use surface mount diodes or LEDs, you probably understand the challenges involved in correctly and consistently indicating diode polarity. LEDs are usually cathode negative, while zeners and uni-directional TVS diodes can be cathode positive. Barrier diodes can be either orientation. It all depends on whether the diode is a rectifier, an LED, a uni-directional TVS, part of a daisy-chain and a host of other considerations.

When you start looking at the CAD libraries, you not only have all the differences from that manufacturer, you may also have different markation schemes from each CAD package developer and from each library builder.

Guidelines for diode polarity mark silk-screening — the diode symbol, “K” for cathode or “A” for anode. To ensure the best accuracy, we recommend extra care in marking diodes to remove any ambiguity.

The preferred method is to place the diode schematic symbol in the silkscreen. You may also place a “K” for cathode adjacent to the cathode. “K” is used because “C” could imply that the spot wants a capacitor. An “A” adjacent to the anode on the board works too, though it’s less common. If you are producing a board without silkscreen, put the mark in the copper layer or submit a clear assembly drawing with the other board files.

Relying on +, – or _ are not definitive in what they indicate and are not recommended. For example, a “+” or “-“ sign isn’t good enough, because it’s not always true that current flows through a diode from the anode to the cathode. For the common barrier diode or rectifier, it’s a pretty safe bet. However, with a zener diode or TVS, it’s not necessarily true. That is why marking a diode on your PCB with the plus sign (+) is not good practice.

Ever Use a TI X2SON Packaged Part?

The name stands for extra small outline no-lead. It’s a newish package from Texas Instruments. In my experience, TI is one of the better companies insofar as testing and documenting manufacturability is concerned. The datasheet for this device is no exception.

The TI part is the five-lead thing above the grain of Jasmine rice, surrounded by a few 01005 ceramic capacitors. I’m selling the capacitors for $500 each. (Just kidding.)

The part is 0.8 x 0.8mm, with the five leads. TI suggests either a 4 mil (0.102mm) trace coming out of the center pad, or a 4 mil via in the pad (the via must be filled and plated at the fabricator ) to escape the center pad. They also do a nice job of detailing out the solder paste stencil layer, as in the following image:

You’ll most likely need a custom CAD footprint for one of these. Either very carefully do it yourself, or go to a solid source like SnapEDA. If they don’t already have it in their library, they’ll make it for you.

These small packages aren’t going away. We’re only going to see more of them. They may seem intimidating, but with a good footprint and a competent manufacturer, they aren’t so bad.

Duane Benson
“A ruler of follows”? That makes no sense.
How about ” a rule of followers”?

http://blog.screamingcircuits.com

Do Bolts Go On Your PCB Bill of Materials?

The short answer: Yes. If you want prototype assemblers like Screaming Circuits to install it, it must go in the bill of materials.

For the most part, we solder through-hole and surface mount components on PCBs. As most everyone knows, all those parts need to be put in the bill of materials (BoM). The BoM is a list of all of the components to be placed on the PCB. The file typically includes an index number, the number of times a specific component will be used on the board, the reference designator from the schematic, the component manufacturer, and the manufacturer’s part number.

If a specific component is used more than once — a common bypass capacitor, for example — it will take only one line in the BoM. One field in the BoM will list the number of times the component is used, and another field will list all of  the reference designators for that part number.

You may also want to include alternate parts for components likely to go out of stock. Passives, like capacitors and resistors, are notorious for going out of stock without notice. Invariably, though, there will be a half-dozen nearly identical parts that will fit the bill just as well. Create an alternates list so your purchasing folks or manufacturer won’t get stuck not knowing if a substitute is valid or not.

But what about things that aren’t soldered, like nuts and bolts, double-stick tape, or display panels and such? Where do they go? The quick answer is they go in the BoM like all the other parts. Manufacturers build from the BoM. That means that if it’s not in the BoM, they won’t know to install it.

Some of these parts are nonstandard and can’t easily be quoted online, but they still need to be in the BoM. If you have such things, give your manufacturer a call to see how much it will cost and they can assemble it. Then either put the reference designator in the silkscreen or offer an assembly drawing with a reference designator for whatever it is.

That means a set of bolts might be BT1, BT2, BT3 …. Washers could be W1, and nuts N1. A glue dot could be G1. It doesn’t matter that much. Just make sure the reference designator in the BoM matches that on the silkscreen or in an assembly drawing.

If it requires hand operations like double-stick tape under a display, again check with your customer service rep first, but then put the display and tape in the BoM and provide any non-obvious information in an assembly drawing or special instructions.

Embedded Passives

I recently participated in an Altium podcast where I discussed the origins of Screaming Circuits, some DfM hints and a few other topics. I was discussing some of the challenges everyone is having these days in procuring components, and the host asked if we see many people using embedded passives as a way to mitigate supply difficulties.

I told her that I don’t think we’ve seen very many embedded passives come through our shop; too late realizing that given that embedded passives are — embedded — inside — the PCB, we wouldn’t actually see them.

Then, somewhat coincidentally, yesterday I was visiting our San Diego PCB Design division and that very subject came up. It seems that our SDPCB layout group does sometimes use embedded passives in some of the boards they lay out. I need to have a conversation with them about the layout and fab implications of embedding passives.

I’m kind of guessing here at what they look like, so don’t take this representation as literal fact.

Originally embedded passives were invented primarily for space savings. Now with 0201 and 01005 components available, that’s less of a need these days, but embeddeds can still be advantageous for reduction of parasitic effects or in areas where even 01005s are impractical, like termination of large numbers of transmission lines.

What I’m wondering, is if embedded passives could be a viable solution to some of the supply chain issues we’re seeing lately? If you need a few dozen 0.01 uF bypass capacitors on your PCB, but can’t find them*, would embedded capacitors be a practical solution?

*It’s important to note, that in most cases, the term “embedded passives” refers to the process of using various resistive and dielectric materials to create the components within the layers of the PCBs. I’m not talking about embedding the currently hard-to-find discrete resistor and capacitors within the bare board.

Duane Benson
Sorry. I have nothing snarky to say, and as you know, if you don’t have anything snarky to say, you shouldn’t say anything.

What is Your Supply Chain Telling You about Packages?

Have you purchased any electronics components lately? Have you tried and failed to do so lately? Allocation is the word of the day and substitutions are your friend.

Many, many parts are in short supply, or unavailable with extraordinarily long lead times. Sure, that happens every now and then in this industry. It’s a periodic nuisance, but what should you do for the long term? We’re are getting some interesting stories from component suppliers that might help. 

What we’re hearing is that many passive manufacturers will be trying to move their customers to smaller sizes. They want to consolidate on as few packages as is possible. That means we may be seeing the end of 1206, 0805, and maybe even 0603 form factors for many passive values.

It kind of makes sense. Right now, there might be several dozen different varieties of 0.1uF, 16V capacitor. Does the industry need that? And if there isn’t enough fab capacity to make all of the variations, why not consolidate and run more of fewer variations? It won’t surprise me if we start seeing fewer voltage ranges as well. In most cases, a 16V part will be just fine if you’re calling for a 6V or 10V part.

The chip industry has been doing this for a while. Many of the newer components just come in BGA or QFN packages. Fewer and few leading edge parts come in large through-hole or SOIC packages.

Consider using smaller components, like standardizing on 0402 parts. I know it can be a pain to use smaller parts, but any potential for future proofing your design now can prevent delays or otherwise unnecessary redesign cycles. You might just be able shrink your board size and save some money on the board fab too.

Keep approved substitutions close by, and look for newer chips that are more likely to stay in production. For microcontrollers, pick parts that have multiple memory capacity or speed range variants all in the same package.

This looks to be a pretty extreme allocation cycle, and I have a feeling that the industry will be different when we come out of it.

Duane Benson
Which is worse
Being the missing link or the weakest link?

 

A Question on Schematic Style

Just a quick question here…

I’m placing the components for my TinyFPGA based stepper motor controller board (see the prior articles here firsthere next, and here last). Specifically, I’m finding places for all of the capacitors. That’s where the question of schematic style comes in. 

When I first started using CAD software, sometime shortly before the stegosaurus roamed the land, I would position component symbols on the schematic near the chips they belonged with. Doing so made it easy, come layout time, to remember which capacitors go where.

Since then, though — and I don’t remember when I started this — I’ve started following the practice of grouping capacitors on the schematic, as I’ve done on this sheet to the right. All of the capacitors are up at the top, shown connected to V+ and ground rails.

That’s not a problem when they’re all 0.1uf bypass capacitors and each chip just takes one. However, with higher speed and more complex chips, multiple bypass caps or different values are often required on each chip.

Now, when I go to layout, I need to go find my component data sheets again (they really should never be very far away) and re-figure out what combinations of bypass capacitors go to which pins on what chips.

I like the cleaner schematic that results from grouping bypass caps, but it’s adds pain and a bit of opportunity for error during layout.

What style do you use and why? Am I an idiot for doing it this way? Wait. Don’t answer that last question.

Duane Benson
Six of one and 12 × 5 × 10-1 of another

5 Traps to Avoid When Changing from Hand to Machine Assembly

In the past, it was usually pretty easy to find chips in both surface mount and through-hole packages. Somewhere in the past decade or so, component manufacturers stopped introducing through-hole versions of their newest chips as standard practice. In many cases, new components can only be found in tiny QFN (quad flatpack, no leads), or wafer-scale BGA (ball grid array) packages.

The maker community, never shying away from a good hack, found ways to work with many of these parts while still hand building. There are very few components used in the pro-design world that are still unusable by a creative DIY maker.

But what happens when a maker has a great design and wants to mass-produce it?

Sometimes the techniques that make things work when hand soldering, will completely break a machine assembly process. To cure that ailment, I’ve compiled five common traps to avoid when moving from hand to robotic assembly.


5. Consider moisture sensitivity. 
It may not seem logical, but plastic does absorb moisture. And, it doesn’t have to be dropped in the sink for it to happen. Just sitting around exposed to the air, plastic chips will absorb humidity. In a reflow oven, these parts can end up acting a bit like popcorn. The moisture turns to steam, and if it can’t outgas fast enough, may split the chips open. Often, the damage isn’t visible to the naked eye, but with show up as an unreliable product in the field.

When we DIY folks hand-build boards, we tend to open the component packages and then just let the parts lie around without giving thought to proper storage. If you are going to send your project off to be machine assembled, you can do two things with moisture sensitive parts.

First, order the parts when needed, not before, and keep the packages sealed. Alternately, you can send in parts that have been exposed to the air; if you inform your assembly house that the parts are moisture sensitive, and ask that they be baked prior to assembly. Prebaking will remove the moisture safely.

4. Don’t skimp on solder mask. Some board fabricators offer reduced prices if you order your boards without soldermask or silkscreen. That’s not a problem when you’re hand building — you can regulate the amount of solder by eyeball.

However, when a stencil is used to apply solder paste and the board is run through a reflow oven, the solder will spread back on the exposed copper traces. This may leave your parts without enough solder on the pins to create a reliable connection.

Solder mask may add a bit of cost up front, but will increase reliability and reduce cost in the long run. Creative choice of solder mask color can also add some personality to your boards.

3. Silkscreen is important too. Lack of sIlkscreen isn’t a reliability issue, but it can make accuracy of assembly more difficult to achieve. In a perfect word, the CAD files would tell the assembly machines exactly where each part is supposed to go and what angle and orientation it needs.

Unfortunately, we don’t live in a perfect world (who knew?). It’s far too common to have footprints with errors in them, or components with ambiguous marking, to depend on the CAD files alone. Clear silkscreen will help to ensure that any errors in the data are caught visually.

If you don’t want to clutter your PCB with reference designators and polarity markings, put the designators and any other important markings in the document layer in your layout software. Then, tell your assembly house to look on that layer for the information.

2. No need to fear surface mount. One of the easiest ways to ensure that a board can be hand-built is to stick with through-hole parts. But doing so puts many limits on a design, and rules out a lot of new technologies.

Little breakout boards — a small surface mount chip pre-mounted on a PCB, with hand-solderable headers — are available for a lot of new parts, but not all. That’s helpful, but they take up a lot of extra board real estate and cost more that the part alone.

If you’re hand building a prototype, or a small number of boards for your own use, go ahead use a breakout board. But, when it’s time to get a thousand built up to sell, re-layout your PC board to use the chip without the breakout board. Just don’t forget the bypass capacitors or any other required support components.

As a bonus, many breakout boards are open source, so you may be able study and use a proven schematic and layout for that part of your design.

1. No open vias in pads. QFNs and BGAs have pins/pads under the part, often completely inaccessible. That’s fine for a reflow oven, but what if you’re soldering it by hand?

A common hand-soldering practice is to put large vias in the pad. Fix the part onto the board with tape. Then, turn the board over and stick solder and a small tipped soldering iron through the via. By doing this, you can hand solder almost any leadless surface mount part.

You can probably guess that I’m going to tell you open vias in pads will not work with automated assembly. The solder will flow down the via and end up on the back side of the board. You may end up with shorts on the back side, and parts that fall off of the front side, or just don’t connect with all their pads.

If you use the open via hand solder technique, you’ll need to re-layout your board without any open vias in the pads before sending it for manufacture.

0. Go for it! It wasn’t that many years ago when the tools and services necessary to get an electronic product manufactured were so complex and expensive as to pretty much make it impossible for DIYers to turn a hobby project into a small business. Times have changed, and with those changes, the hardware startup is back — and within just about anyone’s reach.

Duane Benson
Breaker, breaker, one nine, clear the line, we’ve got boards to build

Electronics Manufacturing Files: What We Need

Manufacturing is all about taking data from you and delivering some good working circuit boards. Well, it can be just data — as in full turnkey — or data plus some parts and or PCBs, as in a partial turnkey or a kitted job. Regardless of whether you’re sending parts and boards, or having the us as the EMS buy everything, we need good data, and a lot of it.

That data are the difference between the working boards you want and need and a random jumble of expensive paperweights.

We need a bill of materials (BoM), the job specifications (which you give us by ordering and describing any special instructions on our website), and the CAD design files. Fab and assembly drawings are always a good idea too. A little extra time spent on the files sent reduces risk, and that’s a very good thing.

The CAD design files include Gerbers, a centroid (aka pick-and-place or XYRLS file), and intelligent CAD files, such as ODB++ or IPC-2581. In some cases, such as Eagle CAD, we can use the native CAD board file.

The ODB++ and IPC-2581 file formats are the future of electronics manufacturing. They come with more data, and more accurate data, than do Gerbers. If you can send either of these two, please do so. Even if you have those, still send us the Gerber files. Gerbers are the lowest common denominator, and provide a base that we and PCB fabricators can work from.


The Gerbers are a set of files used to create the various layers of the board. Each layer requires an individual file, so a six-layer board (six copper layers) will typically require at least 13 distinct files: one for each copper layer, top solder mask, bottom solder mask, top silkscreen, bottom silkscreen, the drills holes, and solder paste for the top, and bottom if the board has SMT parts on both sides.

The drill file is combined with the Gerber files to line up the via and through-hole component holes with the appropriate spots in the PCB. Then the pick-and-place file will tell the assembler where to put each component, what angle to place it at, and which side of the board it goes in.

Fab drawings hold a human-readable, often in PDF format, description of the board and any special instructions needed by the fabricator. The assembly drawing would be the same, but for the assembler.

Sometimes the parts are too densely packed for the reference designators and polarity marks to show up on the actual board, or for aesthetic reasons, the designer doesn’t want them on the board. In such cases, all that information would be put into a set of assembly drawings; PDF files showing all of the necessary reference information.

As of this writing, the ODB++ and IPC-2581 file formats aren’t universally accepted, but are getting more so all the time. Use of these new intelligent CAD output file formats helps to reduce the number of manual steps and human interpretation, and will eventually lead to better quality and faster manufacturing times.

Duane Benson
What do we need?
What does it really matter?
Matter converts to energy
E=(what we need)C2

Power Distribution – To Route, or to Plane PCBs

Power distribution on a PCB can come in a number of forms. The three most common methods are:

  • Route power and ground.
  • Use surface layer floods.
  • Use internal planes.

After component positioning, you’ll need to look at power and ground distribution. With a two-layer board, your options are limited to individually routing power and ground, or using a polygon fill, also called a flood or pour.

 

 

 

 

For simple low-speed layouts, it’s common to route power just like any other signal. You’ll typically use a wider trace, which you can set manually, or with design rules. Drawing a polygon in the board shape, and giving it the same name as your power or ground signals may make the job easier. Keep in mind though, that you can end up with parts of a ground plane disconnected from the rest of the board. This is called an orphan. Some CAD error checks will spot such a problem and some won’t.

I made that mistake not long ago, as describe in this blog post.

If you have a four (or more) layer board, common practice is to designate one of the internal layers for ground, and one for power.

 

 

 

 

 

 

Doing so can leave more room for signal routing, can reduce EMI, and can leave a cleaner-looking, easier-to-debug board. It also reduces the chances of having orphan ground or power areas, as I warned against in the prior post.

Duane Benson
Chocolate layer cake with coconut frosting will not help with EMI

http://blog.screamingcircuits.com/