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:
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.
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.
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.
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”?
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.
Most of the electronics design world is by now aware that we’re in a very serious period of components shortages. Hardest hit seem to be ceramic capacitors, but other passives as well as a variety of connectors and silicon parts are also caught up in the shortage storm. Allocation and shortages hit every few years, but this one seems to be the worst in recent memory. It could be a problem until 2020, and the supply chain and world of components manufacturers will likely be a different animal coming out of it.
So, you might ask, isn’t that just a problem for high volume producers? No, I would answer. It affects anyone regardless of volume. The exact way that it hits you and what you can do about it may differ, but it has or soon will hit all of us.
Here’s five things you can do to minimize the effects. I’m going to go backwards and starting with the most important thing for people who need low volumes manufactured:
1. Check the availability of all of your parts immediately before sending us your bill of materials.
The. very. last. thing. before sending us your BoM. It’s not uncommon for a part to be in stock one day and out the next. We’ve even seen cases where the part’s in stock in the morning and out by the afternoon. If you’re having us quote and order your parts, verify they are in stock as the last thing you do before sending your files to us.
Almost every BoM we see these days has one or more parts that are out of stock. We send you an email about the parts being out of stock. We can’t do anything else until we hear back from you. We can’t build without parts and we don’t know your design like you do, so we can’t guess at substitutions. A last-minute check can save days of delay.
2. Put one or two alternate part numbers in your BoM, especially for passives.
As I said above, we don’t know your project so we can’t pick a sub for you. Give us some alternates. Put them on the same line as the original part, to the right. And be sure to tell us in the special instructions that you’ve put alternates in the BoM.
3. Consider your parts values carefully. You may be able to pick something with better availability.
The 0.01?F capacitor is the hardest hit component. It’s the most commonly used bypass capacitor. Some designs need exactly that value, but many don’t. It may be easier to find a 0.022?F, a 0.0047?F, or something else close enough. If that’s the case, choose a close enough value that has better supply, or put one in as an alternate.
4. You might need a slight redesign to use a smaller package.
Since smaller packages can be used in more applications, many suppliers will be allocating more of their foundry capacity to smaller form factors like 0402 and 0201 sizes. Some component manufacturers have said they’ll be permanently discontinuing anything bigger than 0402 parts except when absolutely necessary.
Stick with 0402 size passives. It may be easier to find the parts you need in that package, and those size parts will be the first ones to come back in stock.
5. If we send you a message about a part we can’t find, respond as quickly as possible.
We do our best to avoid any delays in this process, but we can only do so much. Help us out by getting back to us as soon as possible, and don’t be afraid to give us more than one part number to try.
This can be a pretty annoying problem and it can cause delays and other problems. The good news is we’re having this problem because the design world is booming and technology is advancing. It will get better, and following these five tips can help prevent delays. Don’t forget to check your parts for availability right before sending your BoM in to us. I mean it!
Duane Benson Parts, parts everywhere, but not an 0805 to solder
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.
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
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.
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
Not long ago, I designed an Arduino compatible clock board. The board has 12 NeoPixel (digital addressed RGB LEDs) arranged around the board to act as hour hands. The minutes and seconds are represented by an external ring of 60 NeoPixels.
How did I go about positioning the 12 NeoPixels, and what does it matter? For aesthetic reasons, I do want each NeoPixel in the proper place. If any are off a bit, I’ll notice every time I look at the clock.
I created a triangle, with all of the correct distances, and drew in in my CAD software’s Document layer. The Document layer looks just like a silk screen layer, when visible, but it won’t be printed on the board. You can use this layer to put in extra information for yourself, or for the manufacturer.
You’ll notice that I also wrote in the document layer “No tabs here.” That’s an instruction to the board fabricator to not put a panel tab where the micro USB connector goes. If it did, the board wouldn’t be buildable when panelized.
Some create a fabrication document layer and an assembly document layer. An example might pertain to reference designators. If the board is too compact for reference designators, of if, for aesthetic reasons, you want to leave them off the finished board, You can put the reference designators in an Assembly Documentation layer. Then be sure to let your assembler know what you’ve done.
The other things I did here is to keep all the LEDs aligned with the baseline of the PCB. In theory, you can place a component at any rotation angle you want. But, like any system, manufacturing works better when there are fewer variables.
You reduce the probability of error if you keep components aligned at factors of 90 degrees. It also helps to keep polarities oriented the same way, as much as possible. For example, if you can, have all the diode polarities facing the same direction.
Duane Benson Time flies like an arrow; fruit flies like a banana
I recently ran a batch of my Neo Pixel clock boards through the factory here. It’s an Arduino UNO-based design that I made for myself not long ago. It sports an Atmega328P, with bootloader, an FT231X USB chip, and a DS3231 real time clock (RTC) chip. Pretty standard stuff. It doesn’t even use small parts. All the passives are 0805 size. There’s nothing exotic here. So, where did I go wrong?
I also used my 3D printer to make a clock frame to hold this board and a 60-pixel ring of NeoPixels, from Adafruit. I found that with the micro USB connector on the top of the board, it’s a little awkward to plug in the USB cable, so I put pads for the connector on the back side of the board. Depending on exactly where and how the board will be used, the micro-USB, button switches, and clock backup battery can all go on either the front or back surface of the board.
Programming the bootloader worked as expected, so I assumed it was just a job well done. Except it wasn’t. When I plugged in the micro USB cable, the RX and TX LEDs flickered briefly, but the board wasn’t recognized by my PC.
Take a look at the back side of the PCB and see if you can find my mistake (spoilers after the photo).
I ran a 24 mil trace around the back side of the board to supply power to the NeoPixels. That’s not a problem, except that I closed the loop on that trace, and didn’t put a path for the ground to get across the trace.
Follow it around, and notice that the ground connections to the u-USB connector don’t go anywhere except to this part of the plane. Ugh.